Fisiologi Hewan

SISTEMA SIRKULATORIA

Diposting Dari Tugas Kuliah Fisiologi Hewan

 Dosen Pengasuh: DR. H. A. Karim Gaffar, SU.

Living things must be capable of transporting nutrients, wastes and gases to and from cells, in order to properly function under specified conditions.

The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes.

Types of Circulatory Systems .There are three ways that animals may do this.  

Single-celled organisms use their cell surface as a point of exchange with the outside environment.

Multicellular organisms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes. Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide and metabolic wastes.

Simple, sac-like animal, such as jellyfish and flatworms, have a gastrovascular cavity that serves as an area for digestion and helps bring the nutrients from digested foods into close proximity to many cells in the animal's simple body.

Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by ciliary action.

Simple animals, such as the hydra and planaria (shown in Figure 1), lack specialized organs such as hearts and blood vessels, instead using their skin as an exchange point for materials. This, however, limits the size an animal can attain. To become larger, they need specialized organs and organ systems.

Figure 1. Structures that serve some of the functions of the circulatory system in animals that lack the   system.

Components of the circulatory system include

• blood: a connective tissue of liquid plasma and cells
• heart: a muscular pump to move the blood
• blood vessels: arteries, capillaries and veins that deliver blood to all tissues

There are several types of circulatory systems.

The open circulatory system, examples of which are diagrammed in Figure 2, is common to molluscs and arthropods. Open circulatory systems (evolved in insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. The resulting blood flow is sluggish.

An open circulatory system is a system in which the heart pumps blood into the hemocoel which is positioned in between the ectoderm and endoderm. The fluid described in the definition is called hemolymph, or blood. Hemolymph flows into an interconnected system of sinuses so that the tissues receive nutrients, fluid and oxygen directly. In animals that have an open circulatory system, there is a high percentage of the body that is blood volume. These animals have a tendency to have low blood pressure, with some exceptions. In some animals, the contractions of some species’ hearts or the muscles surrounding the heart can attain higher pressures.

Figure 2. Circulatory systems of an insect (top) and mollusc (middle). Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates  and WH Freeman.

In a closed circulatory system, blood flows from arteries to capillaries and through veins, but the tissues surrounding the vessels are not directly bathed by blood. Some invertebrates and all vertebrates have closed circulatory systems. A closed circulatory system allows more of a complete separation of function than an open circulatory system does. The blood volume in these animals is considerably lower than that of animals with open circulatory systems. In animals with closed circulatory systems, the heart is the chambered organ that pushes the blood into the arterial system. The heart also sustains the high pressure necessary for the blood to reach all of the extremities of the body.

Vertebrates, and a few invertebrates, have a closed circulatory system,.

Closed circulatory systems (evolved in echinoderms and vertebrates) have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Blood flow is not sluggish. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly hemoglobin molecules in blood cells transport oxygen.

In the closed circulatory system of mammals, there are two subdivisions—the systemic circulation and the pulmonary circulation. The pulmonary circulation involves circulation of deoxygenated blood from the heart to the lungs, so that it may be properly oxygenated. Systemic circulation takes care of sending blood to the rest of the body. Once the blood flows through the system of capillaries at the body’s tissues, it returns through the venous system. The pressure in the venous system is considerably lower than the pressure in the arterial system. It contains a larger portion of blood than the arterial system does, for the venous system is thought to be the blood reservoir of the body.As we see it, there are more disadvantages to having an open circulatory system but having an open circulatory system suits those animals well. There is a limited capability for such animals to increase or decrease distribution and velocity of blood flow. There is not a lot of variability to oxygen uptake because changes in such are very slow.

Because of the limits to diffusion, animals with open circulatory systems usually have relatively low metabolic rates.

There are a variety of advantages to having a closed circulatory system. Every cell of the body is, at maximum, only two or three cells’ distance from a capillary. There is the ability for such animals to have incredible control over oxygen delivery to tissues. A unique characteristic to closed circulatory systems is that capability for a closed circulation to include the process of ultrafiltration in blood circulation. Since the lymphatic system is included as part of the circulatory system because of its circulation of excess fluid and large molecules, it decreases the pressure in tissues that extra fluid increases. One of the most important advantages of the setup of the closed circulatory system is that the systemic and pulmonary branches of the system can maintain their respective pressures. 

Differences between open and closed circulatory systems and the advantages and disadvantages of each

The human closed circulatory system is sometimes called the cardiovascular system. A secondary circulatory system, the lymphatic circulation, collects fluid and cells and returns them to the cardiovascular system.

Vertebrate Cardiovascular System

The vertebrate cardiovascular system includes a heart, which is a muscular pump that contracts to propel blood out to the body through arteries, and a series of blood vessels.

The upper chamber of the heart, the atrium (pl. atria), is where the blood enters the heart. Passing through a valve, blood enters the lower chamber, the ventricle. Contraction of the ventricle forces blood from the heart through an artery. The heart muscle is composed of cardiac muscle cells.

Arteries are blood vessels that carry blood away from heart. Arterial walls are able to expand and contract. Arteries have three layers of thick walls. Smooth muscle fibers contract, another layer of connective tissue is quite elastic, allowing the arteries to carry blood under high pressure.

The four main functions of arteries are that they:

1. act as a conduit for blood between the heart and capillaries.
2. act as a pressure reservoir for forcing blood into the small-diameter arterioles. The further away the arteries are to the heart, the smaller and stiffer they get. Thus pressure is need in order for blood to travel into smaller arterioles.
3. dampen the oscillation pressure and flow generated by the heart and produces a more even flow of blood into the capillaries. The arteries range in size and are able to decrease the pressure of blood that flows into the capillaries.  If this did not happen, the capillaries would probably burst for they would be receiving a high pressure blood inside their sensitive thin layer capillaries.
4. control distribution of blood to different capillary networks via selective constriction of the terminal branches of the arterial tree. This is an important feature of arteries (and a benefit of a closed circulatory system), for if more blood is needed in one area, (as when there is a wound or infection) more blood flow is supplied there and less in areas where much blood is not needed.

A diagram of arterial structure is shown in Figure 3.

Figure 3. Structure of an artery. Image from Purves et al., Life: The Science of Biology, 4th Edition,.

The aorta is the main artery leaving the heart. The pulmonary artery is the only artery that carries oxygen-poor blood. The pulmonary artery carries deoxygenated blood to the lungs. In the lungs, gas exchange occurs, carbon dioxide diffuses out, oxygen diffuses in.

Arterioles are small arteries that connect larger arteries with capillaries. Small arterioles branch into collections of capillaries known as capillary beds, an exampe of one is shown in Figure 4.

Figure 4. Structure and blood flow through a vein..

Figure 5. Capillary with Red Blood Cell (TEM x32,830)..

Capillaries, are thin-walled blood vessels in which gas exchange occurs. In the capillary, the wall is only one cell layer thick.

Capillaries are concentrated into capillary beds. Some capillaries have small pores between the cells of the capillary wall, allowing materials to flow in and out of capillaries as well as the passage of white blood cells.

Changes in blood pressure also occur in the various vessels of the circulatory system, as shown in Figure xxx. Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries. Capillaries are microscopic in size, although blushing is one manifestation of blood flow into capillaries. Control of blood flow into capillary beds is done by nerve-controlled sphincters.

Figure xxx. Changes in blood pressure, velocity, and the area of the arteries, capillaries, and veins of the circulatory system. Image from Purves et al., Life: The Science of Biology, 4th Edition.

Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary, as shown in Figure 7.

Figure 7. Capillary structure, and relationships of capillaries to arteries and veins. Image from Purves et al., Life: The Science of Biology, 4th Edition,.

The extensive network of capillaries in the human body is estimated at between 50,000 and 60,000 miles long. Thoroughfare channels allow blood to bypass a capillary bed. These channels can open and close by the action of muscles that control blood flow through the channels, as shown in Figure 8.

Figure 8. Capillary beds and their feeder vessels. Image from Purves et al., Life: The Science of Biology, 4th Edition, .

Blood leaving the capillary beds flows into a progressively larger series of venules that in turn join to form veins.

Veins carry blood from capillaries to the heart.

With the exception of the pulmonary veins, blood in veins is oxygen-poor. The pulmonary veins carry oxygenated blood from lungs back to the heart.

Venules are smaller veins that gather blood from capillary beds into veins. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along.

The veins have valves that prevent back-flow of blood, as shown in Figure 9.

Figure 9. Structure of a vein (top) and the actions of muscles to propel blood through the veins. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman.

Blood is carried to all parts of the body via the arteries so it is important that enough blood is carried to the fingers and the gut even though the first is much farther from the heart.

To do this the arterial system is designed to keep a precise blood pressure to ensure that blood can travel through the body.

The heart produces a certain blood pressure by ejecting the blood into the arteries at a certain pressure. An artery that has blood ejected into it will expand slightly and allow the pressure to increase, however the heart also has a relaxed state where the pressure drops. When this happens the artery must have a way to stay somewhat pressurized to keep the blood moving although the heart is not pushing any.

This is done by the artery contracting along with the blood pressure. The less blood in the artery, the smaller it becomes to keep the pressure on the blood. If the arteries were just to relax and allow the pressure to drop, the blood will stop flowing and will not have enough pressure to make it to the entire body.

Ventricular contraction propels blood into arteries under great pressure. Blood pressure is measured in mm of mercury; healthy young adults should have pressure of ventricular systole of 120mm, and 80 mm at ventricular diastole. Higher pressures (human 120/80 as compared to a 12/1 in lobsters) mean the volume of blood circulates faster (20 seconds in humans, 8 minutes in lobsters).

As blood gets farther from the heart, the pressure likewise decreases. Each contraction of the ventricles sends pressure through the arteries. Elasticity of lungs helps keep pulmonary pressures low.

Systemic pressure is sensed by receptors in the arteries and atria. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure.

Another control that the arteries are designed for is to keep the blood flowing evenly in all parts of the body. For example a human, when lying down, has its heart at the same level as the rest of the body so the arteries can all produce the same pressure. However, when we stand up the heart is now above most of the body and therefore the lower arteries have to produce a higher pressure to keep the blood flowing evenly. If the arteries did not do this, then there would be no blood pressure in the legs and that would cause several problems. This is all controlled by the arteries’ ability to expand and contract as to keep an even blood pressure and flow.

One very important function of blood pressure is to ensure the exchange of interstitial fluids that contimually bathe the cells of the body. 

Because blood pressure on the arterial end of capillaries is greater than the colloid osmotic pressure of the surrounding tissues, water leaves the capillary and flows into the interstitial space among the surrounding cells.  At the venous end of the capillary, the colloid osmotic pressure exceeds blood pressure, and fluid is drawn back into the plasma from the surrounding extracellular space.  This helps to exchange the fluids surrounding the cells and remove metabolic wastes.

Another very important function of blood pressure is to maintain proper kidney function.  The kidneys are important in filtering wastes and removing some potentially dangerous chemicals from the blood.  The filtering process in the kidneys is driven by blood pressure in the renal arteries.  If blood pressure drops too low, the kidneys can no longer function.

The kidneys and the heart both play important roles in regulating blood pressure.  When blood pressure is low, renal filtrate moves slowly through the nephron, resulting in a low sodium concentration in the filtrate in the distal convoluted tubule.  This is sensed by the cells of the macula densa, and results in the release of renin from the secretory cells of the juxtaglomerular apparatus.  This begins a chain of physiological events that includes the formation of angiotensin II.  Angiotensin II helps bring blood pressure back up by (1) causing vasoconstriction in arterioles throughout much of the body, and (2) promoting increased synthesis of antidiiuretic hormone (ADH), which increases resorption of water in the collecting ducts of the kidney, thereby increasing blood volume.  Angiotensin II also promotes the release of aldosterone from adrenal cortex, which promotes retention of both sodium and water, thereby helping to bring blood pressure back up. 

Stretch receptors in the heart monitor the volume of blood returning to the atria.   If blood volume and blood pressure get a bit too high, the atria release atrial natriuretic peptide (ANP), which inhibits the release of renin, ADH, and aldosterone.   This reduces water resorption in the kidneys, thereby increasing urine production and reducing blood volume and blood pressure.

The three types of capillaries

Capillaries are used to transport gases, nutrients and waste products into the blood. They are normally about 1 mm long and 3-10 micrometers in diameter. There is not one cell in the entire body that is more than three or four cells away from a capillary. This is very important because every cell needs to be able to absorb oxygen and nutrients and get rid of metabolic wastes. In a mammal there are three types of capillaries: continuous, fenestrated and sinusoidal.

Continuous capillaries are made up of an endothelium that is 0.2-0.4 micrometers thick that has a basement membrane. The endothelium cells are separated by clefts. Each cell has many vesicles that can be used for transporting substances in and out of the capillary. The transfer of products through the membrane is done either through or between the endothelium cells. Lipid soluble substances can be transferred through the cells while water and ions have to be transported in between cells in the clefts. The vesicles are still being studied to see what role they play in transporting materials but certain studies have shown that the brain uses vesicles as a mechanism of transport.

Fenestrated capillaries are found in the glomerulus and the gut. They consist of an endothelium cell wall with vesicles. However the difference is that the fenestrated capillaries have pores that perforate the cells instead of clefts. There is still a basement membrane. Transport occurs through the pores, which can handle all types of materials except for large proteins and red blood cells. The basement membrane is complete and this enables the cell to move certain substances across. There has been no evidence that vesicles play a role in the transportation in these cells.

Sinusoidal capillaries have paracellular gaps in the endothelial cells. This, combined with the basement membrane not being complete, allows many materials to be transported across. There are no vesicles in the cells so the paracellular gaps are the only area where substances can be transported. These capillaries are found in the liver and bone. 

The structure of hearts varies in different vertebrates although the basic function remains the same: to get pressurized blood to the body.

The heart of air-breathing fishes differ from those of water breathing fishes in order to be efficient in their environmental conditions.

In water breathing fishes, such as elasmobranchs, the heart consists of four chambers in a series, all of which are contractile. These chambers are the sinus venosus, atrium, ventricle and conus (bulbus in some fishes). The blood flows uni-directionally through the heart. This is maintained by valves at the sinoatrial and atrioventricular junctions and the exit of the ventricle. In elasmobranchs for example, the four chambers are interconnected but have many valves between them. Blood flows through the sinus venosus into the atrium when it is at rest. When the atrium contracts, the atrioventricular valves open as well as the conus valve, thus blood flows into the ventricle and conus. The valve in the conus most distal to the ventricle is closed so that blood does not go into the aorta before enough pressure is gained. Once enough pressure builds up, ventricular contraction occurs and the atrioventricular valves close. At the onset of ventral contraction, conal contraction starts to let the blood flow into the ventral aorta. The valves proximal to the heart in the conus close to prevent backflow into the ventricle. Then the still deoxygenated blood travels through the ventral aorta into the gills. This flow to the gills in order for gas and ionic exchange to occur is known as gill circulation. After this, the oxygenated blood goes through the dorsal aorta to the rest of the body; this is known as systemic circulation. The gill circulation is under higher pressure than the systemic circulation. However the consequences of a higher blood pressure here is not clear.

In contrast, air breathing fishes do not use their gills as the only method of oxygen intake and gas exchange. They must rise to the surface to take in air bubbles to supplement the intake of oxygen. In some species, the gills of air-breathing fishes are so small that only 20% of the oxygen is obtained through the gills. Thus the gills’ main purpose is not for oxygen intake but for carbon dioxide excretion, ammonia excretion, and ion exchange. Fishes will use structures such as part of the gut or mouth, skin surface or even gas bladder to take up oxygen from the air, but cannot use gills when they are exposed to air because they collapse and stick together and thus cannot function. Because these fishes must use other structures for respiration, oxygenated and deoxygenated blood has to be directed to obtain maximum oxygen intake. In order to do this, oxygenated and deoxygenated blood must be separated so that the deoxygenated blood can be directed to the correct part; either the gills or the air-breathing organ. An example is in Channa argus, a fish which has a division in the ventral aorta. The anterior ventral aorta supplies blood to the first two gill arches and the air breathing organ, while the posterior ventral aorta supplies blood to the posterior arches. Thus deoxygenated blood can go to the first arches and air breathing organ, while oxygenated blood goes to the posterior arches and then the rest of the body. The blood does not mix thanks to some features such as arrangement of veins bringing blood to the heart, and muscular ridges of the bulbus. Other fish have a more divided heart to prevent mixing of blood. The lungfish has a partial septum in both the atrium and ventricle and spiral folds in the bulbus that allows this to take place. Thus deoxygenated blood will flow into the gills then into the lungs, back to the heart again and then into the dorsal aorta. However if the lung is not being used then the blood will flow from the gills through the ductus into the dorsal aorta without passing through the lung or going back to the heart.

All of these have been adaptations that air breathing fish have in order to better suit them for their low oxygen aquatic environment.

A pacemaker is an excitable cell or group of cells whose firing is spontaneous and rhythmic. Electrical activity begins in the pacemaker portion of the heart and spreads from cell to cell through membrane junctions over the rest of the heart. It synchronizes systole (contractions) and diastole (relaxations).
In vertebrates, the pacemaker is the sinoatrial (SA) node, a remaining part of the sinus venosus. It contains contractile specialized muscle cells that do not require constant stimulation. These muscle cells are considered to be myogenic (of muscle cells) as opposed to neurogenic (of neurons). All of these cells have an unstable resting potential and can therefore steadily depolarize to its threshold voltage, at which time an action potential is generated and the muscle contracts. Many cells have the ability of such activity because the capacity lies within all cardiac cells. Therefore, more than one pacemaker can exist in the heart but only one group of cells can determine the rate of heart contraction. Those cells have the fastest inherent activity. Slower pacemakers allow the heart to continue functioning properly if the main pacemakers malfunction. An ectopic pacemaker develops if a slower (latent) pacemaker is rendered out of sync with the rest of the pacemakers and leads one chamber to beat irregularly.

In invertebrates, it is not always clear whether an animal’s heart is myogenic or neurogenic.

The hearts of decapod crustaceans are neurogenic and the pacemaker within their hearts is called the cardiac ganglion. If the ganglion is removed from the heart, it ceases to beat but does show some activity. This goes to show that if a part of the heart were damaged, the pacemaker could function around that. The ganglion itself does not alter its function but some nerves in the central nervous system do. These nerves can change the pattern of the firing of the pacemaker which therefore allows a change in the rate of the heart.

The human heart is a four-chambered double pump, which creates sufficient blood pressure to push the blood in vessels to all the cells in the body. The heart has a route which the blood takes in order to achieve this blood pressure, and to become oxygenated. Systemic venous blood is brought to the heart from the superior vena cava and the inferior vena cava into the right atrium. From the right atrium, the blood passes through the tricuspid valve and into the right ventricle. When the ventricle contracts, the tricuspid closes to prevent a flow of blood back into the atrium. At the same time, the pulmonary semilunar valve opens and blood passes into the left and right pulmonary arteries. These arteries lead the blood into the left and right lungs where the blood gives off its carbon dioxide and picks up oxygen. The oxygenated blood returns to the heart through pulmonary veins, two from each lung and enters the left atrium. The blood then flows from the left atrium to the left ventricle through the bicuspid valve (also known as mitral valve). This valve is open when the left ventricle is relaxed. When the left ventricle contracts, the bicuspid valve closes preventing backflow into the atrium. At the same time, the aortic semilunar valve opens letting blood pass through from the left ventricle into the aorta. Once the blood passes, the left ventricle relaxes and the aortic semilunar valve closes thus preventing backflow from the aorta into the left ventricle. 

Heart rate and stroke volume and they affected by exercise
Stroke volume is the volume of blood ejected by each beat of the heart, or more precisely, the difference between the volume of the ventricle before contraction (end-diastolic volume) and the volume of the ventricle at the end of a contraction (end-systolic volume). A change in the end diastolic or systolic volume can cause differences in stroke volume and also cardiac output, which is the volume of blood pumped per unit time for a ventricle. For example an increase in venous filling pressure will cause an increase in the end diastolic volume, and an increase in stroke volume. However, during some circumstances, the heart rate might increase while the stroke volume remains the same. This is due to the fact that pacemaker cells are stimulated causing an increase in heart rate. The rate of production of ATP and other factors in the ventricular cell increases as well, so as to quicken the pace of ventricular work. This makes the rate of ventricular emptying increase during systole in order for there to be the same stroke volume at a higher heart rate. One of these circumstances is exercise where it is associated with large increases in heart rate with little change in stroke volume. This happens because an increase in sympathetic activity ensures more rapid ventricular emptying while the elevated venous pressure makes filling the heart quicker as heart rate increases.
One of the most important things which help regulate the stroke volume during exercise are the sympathetic nerves which raise the heart rate and maintain stroke volume, keeping the heart operating at or near its optimal stroke volume for efficiency of contraction.
Changes in blood pressure and blood flow that occur during contraction of the mammalian heart?
The contraction of the mammalian heart causes fluctuations in the cardiac pressure and the volume of blood in the heart. It is very important for the heart to maintain very specific blood pressure to ensure that the blood is being transferred all over the body and that the heart can repeat both stages of relaxation and contraction. If the blood pressure in the heart never dropped, there would be no relaxed state and the heart would not fill with blood returning from the body and the lungs. Therefore every change in blood pressure and flow is designed to pump blood.
A quick review of the diastole stage, or relaxed state, of the heart will give you a better understanding of what is happening in the systole stage, or contracted state. In the diastole phase of the heartbeat, the aortic valve will be closed. This will cause a difference of pressure in the ventricles compared to the pulmonary arteries and aorta, which will enable the atrioventricular valves to open allowing blood to be flushed through the venous system.
Once this has happened the heart will begin its contraction by increasing the pressure in the atria so that the blood flows into the ventricles from the inferior vena cava, superior vena cava and the left pulmonary veins. Then the ventricles will contract to exceed the pressure of the atria, which closes the atrioventricular valves (the tricuspid valve on the right and the bicuspid, or mitral, valve on the left side of the heart). This prevents the backflow of blood into the atria and allows the pressure to build up in the ventricles. The aortic valves are also closed to ensure that the volume of blood is not changed. Once this has happened, the ventricular contraction can be considered isometric. The pressure in the ventricles goes up rapidly and exceeds the pressure in the systemic and pulmonary aortas. The aortic valves will open and the blood will be ejected into the aortas, causing a drop in pressure and volume in the ventricles leading to another relaxed phase. 
Changes occur to the mammalian fetus after birth
In the fetus, the lungs have no air in them and there is a high resistance for blood flow.  In addition, blood returning to the heart has oxygen because it is coming from the placenta.  Therefore, there is no reason for blood to go to the lungs for gas exchange.  Two features of the fetal heart help to direct oxygenated blood returning from the placenta to the systemic circulation.   These are the foramen ovale and the ductus arteriosus. 
The foramen ovale is a hole in the interatrial septum that is covered by a flap valve that allows blood to flow from the inferior vena cava through the right atrium and into the left atrium. Therefore, much of the oxygenated blood returning from the placenta goes from the right atrium to the left atrium (via the foramen ovale) to the left ventricle and finally to the body via the aorta.  The ductus arteriosus shunts blood from the pulmonary artery to the aorta, thereby bypassing the lungs and sending the oxygenated blood to the rest of the body.  In the fetus, most of the blood flow is pumped by the right ventricle to the body and is returned to the systemic pathways through the ductus arteriosus.
At birth, the lungs inflate and there is a sudden increase in pulmonary blood flow.   This increases pressure in the left atrium, closing the flap over the foramen ovale.  Eventually, this flap seals shut.  In addition, the ductus arteriosus closes off, thereby preventing further shunting of blood from the pulmonary artery to the aorta.  These changes make sense, because the blood returning to the right atrium of the heart is now deoxygenated, and must be sent to the lungs for gas exchange.  If the foramen ovale fails to close after birth, there will be some leaking of deoxygenated blood from the right atrium into the left atrium.  This may correct itself in time, or may require surgery.  If the ductus arteriosus fails to close off at birth, deoxygenated blood is shunted from the pulmonary artery to the aorta, where it mixes with oxygenated blood.  This would decrease the amount of oxygen delivered to the the body, thereby decreasing the capacity for exercise, or any other strenuous activity.   If the condition is not corrected by surgery, the left ventricle of the heart must work harder to pump blood to the body and brain.  Over time, the left ventricle can become enlarged due to this additional strain.  In addition, the increased blood pressure in the lungs due to the left ventricle working harder can increase the amount of fluid leaving the capillaries in the lungs and lead to pulmonary congestion from fluid build-up. 
What is an electrocardiogram and what are its visible components when one is printed out?
An electrocardiogram is a reflection of the electrical activity of the heart. All of the components of an electrocardiogram vary for the hearts of different species of animals and the most information is known about the human heart. The changes in the duration of the plateau of the action potential and the rates of depolarization and repolarization of the heart are recorded as an electrocardiogram. The duration of the action potential in animals is directly related to the maximum frequency of an animal’s heartbeat. Atrial cells have shorter action potentials than ventricular cells. In smaller mammals, the duration of the ventricular action potential is shorter and the heart rates are higher.
All of these electrically-generated controls of the heart can be recorded in an electrocardiogram. Electrodes are placed on a patient so that the view that appears on the screen is an electric view across the heart. Each of the peaks on an electrocardiogram is given one or more initials. The first wave is the P wave, which represents atrial depolarization. It is a small wave that is slow to rise and fall. The QRS complex comes next and is the summation of two waves, ventricular depolarization and atrial repolarization. The T wave comes after the QRS complex and represents ventricular repolarization. The P-R interval is the time between the beginning of the P wave and the beginning of the R wave. This time interval represents the time that the electricity takes to leave the sinoatrial  node and reach the bundle of His. Any changes in this time can be a sign that things are becoming dangerous for the patient.  For example, an increase in the P-R interval, which could be due to damage to the AV node, might cause too long of a delay between the contraction of the atria and the ventricles, resulting in decreased efficiency in pumping blood to the lungs and the rest of the body. 
What are the effects of sodium and potassium influx on cardiac action potential and how does cardiac action potential differ from action potential of other muscles or nerves?
The cardiac action potential begins with a depolarization due to the influx of sodium ions, followed by rapid depolarization from an influx of calcium ions. This depolarization spreads rapidly across the heart through the interconnected cardiac muscle cells, causing the heart to contract. To ensure that the contraction pushes all of the blood from the heart, the action potential remains at a plateau phase, sustained by delaying the efflux of potassium. This "pause" in a fully contracted state before relaxation ensures that most of the blood that was in the heart is pumped out, thereby increasing the efficiency of each heart beat. The plateau phase seen in cardiac muscles is not seen in other muscles or nerves, both of which repolarize much more quickly due to the rapid efflux of potassium immediately after the sodium influx has stopped. This permits these cells to be ready to generate a second action potential almost immediately after the first.
Another difference between cardiac cells and the cells of other muscles or nerves is that cardiac cells of vertebrates exhibit a pacemaker potential. This is characterized by a slow depolarization toward threshold due to a constant leaking of sodium into the cells. Therefore, cardiac cells do not exhibit a true resting potential as seen in resting skeletal muscles or nerve cells. For this reason, cardiac cells generate action potentials on their own, whereas skeletal muscle cells require nervous stimulation.
Excess extracellular K+ will depolarize the cardiac cell membranes.  An increase in extracellular K+ concentration will decrease the rate at which K+ diffuses out of a cell.  This loss of K+ from inside the cell to outside the cell is a significant factor in establishing a negative resting potential.  If this rate of K+ efflux is decreased, normal membrane potential will not be established and the cell may lose the ability to generate an action potential.  Therefore, excess extracellular K+ can inhibit heart function, and could be fatal.
Cardiac cells also have a relatively long refractory period after contraction which prevents another contraction before the heart fully relaxes. This ensures that the heart chambers fill with blood before the heart contracts. Skeletal muscle cells, however, can be stimulated again immediately after contraction, resulting in summation of contractions from repetitive nervous stimulation.
Action potentials in nerves are characterized by rapid depolartization followed by rapid repolarization and a very brief refractory period, which ensures that the nerve can quickly produce another action potential if stimulated. 
Describe some adaptations of air-breathing diving animals which allow them to stay submerged for long periods of time.
Diving air-breathing animals have adapted some features or ways of being able to remain submerged for extended periods of time. All diving animals rely on oxygen stores in the blood for the animal stops breathing while in a dive. The cardiovascular system will thus give off the stored oxygen supply to the brain, heart, and some endocrine tissues that cannot withstand lack of oxygen. When an animal dives, the continued utilization of oxygen, causes a decrease in blood oxygen levels and a rise in carbon dioxide levels. This in turn causes stimulation of arterial chemoreceptors which cause peripheral vasoconstriction, bradycardia and cardiac output. Thus blood flow to many tissues such as muscles and kidneys is reduced and consequently more blood and oxygen is conserved for the brain and heart. Sometimes during a dive, arterial pressure increases and in order for bradycardia to be maintained, an increase in chemoreceptor and baroreceptor discharge frequency occurs. The decrease of blood oxygen levels, rise in carbon dioxide levels and a decrease in pH cause the discharge of these receptors which maintains the brain and heart with sufficient oxygen during the dive. The effect of a discharge of arterial chemoreceptors is different in diving animals when compared to non-diving animals. Stimulation of arterial chemoreceptors in non-diving animals, results in an increase in lung ventilation. When this occurs high carbon dioxide levels and low oxygen levels cause vasodilation which leads to an increase in cardiac output. Now that the body has more oxygen, vasodilation helps oxygen reach all parts of the body faster. As we have seen, low oxygen or hypoxia caused by cessation of breathing (as in a dive) is associated with bradycardia and a decrease in cardiac output while hypoxia caused by breathing is associated with increase heart rate and cardiac output.

The lymphatic system and its uses

The lymphatic system is a system of vessels that returns excess fluid and proteins to the blood and transports large molecules to the blood. Lymph vessels also absorb the end products of fat digestion in the small intestinig.  Lymph is a transparent yellowish fluid that is gathered from interstitial fluid and returned to the blood via the lymph system. Lymph contains many white blood cells which makes lymph vessels quite hard to see. The lymphatic capillaries drain the fluid in the interstitial spaces and come together like blood capillaries. The larger lymphatic vessels are somewhat like veins; they empty into the blood circulation at low pressure via a duct (near the heart), which in mammals is called the thoracic duct.

Fluid flows easily into lymph vessels because there is a lower pressure in those vessels. The vessels are valved to prevent backflow into the capillaries. Pressure in the vessels can become higher if they are surrounded by autonomic smooth muscle cells. All movements of the body promote lymph flow. If lymph production exceeds lymph flow, edema (swelling) is produced. If the edema is severe, elephantiasis develops which swells and hardens tissues.

In reptiles and amphibians, there are lymph hearts which help in movement of lymphatic fluid. In this case, lymph output is more similar to cardiac output than in animals. In fish, it appears that there is either no lymphatic system or it exists but it is very rudimentary.

The lymphatic system also participates in circulation and the body’s immune response. Leukocytes (white blood cells) are in blood and lymph. Lymphocytes are prevalent in lymph nodes (along lymph vessels) and these nodes filter lymph and bring antigens in contact with lymphocytes. Leukocytes leave the lymphatic and circulatory systems by extravasation at sites of infection. They roll past infected tissues, adhere to cells and are able to pass between them.

There is some degree of correlation between circulation and immune response. All animals must be able to circulate antibodies to areas of the body that need such assistance. The means of circulation for such things is the circulatory system, but in particular, the lymphatic system. 

 

 

Vertebrate Vascular Systems

Humans, birds, and mammals have a four-chambered heart that completely separates oxygen-rich and oxygen-depleted blood, as is shown in Figure 10. Fish have a two-chambered heart in which a single-loop circulatory pattern takes blood from the heart to the gills and then to the body. Amphibians have a three-chambered heart with two atria and one ventricle. A loop from the heart goes to the pulmonary capillary beds, where gas exchange occurs. Blood then is returned to the heart. Blood exiting the ventricle is diverted, some to the pulmonary circuit, some to systemic circuit. The disadvantage of the three-chambered heart is the mixing of genated and deoxygenated blood. Some reptiles have partial separation of the ventricle. Other reptiles, plus, all birds and mammals, have a four-chambered heart, with complete separation of both systemic and pulmonary circuits.

Figure 10. Circulatory systems of several vertebrates showing the progressive evolution of the four-chambered heart and pulmonary and systemic circulatory circuits. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Heart |

The heart, shown in Figure 11, is a muscular structure that contracts in a rhythmic pattern to pump blood. Hearts have a variety of forms: chambered hearts in mollusks and vertebrates, tubular hearts of arthropods, and aortic arches of annelids. Accessory hearts are used by insects to boost or supplement the main heart's actions. Fish, reptiles, and amphibians have lymph hearts that help pump lymph back into veins.

The basic vertebrate heart, such as occurs in fish, has two chambers. An auricle is the chamber of the heart where blood is received from the body. A ventricle pumps the blood it gets through a valve from the auricle out to the gills through an artery.

Amphibians have a three-chambered heart: two atria emptying into a single common ventricle. Some species have a partial separation of the ventricle to reduce the mixing of oxygenated (coming back from the lungs) and deoxygenated blood (coming in from the body). Two sided or two chambered hearts permit pumping at higher pressures and the addition of the pulmonary loop permits blood to go to the lungs at lower pressure yet still go to the systemic loop at higher pressures.

Figure 11. The relationship of the heart and circulatory system to major visceral organs. Below: the structure of the heart. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Establishment of the four-chambered heart, along with the pulmonary and systemic circuits, completely separates oxygenated from deoxygenated blood. This allows higher the metabolic rates needed by warm-blooded birds and mammals.

 The human heart, as seen in Figure 11, is a two-sided, four-chambered structure with muscular walls. An atrioventricular (AV) valve separates each auricle from ventricle. A semilunar (also known as arterial) valve separates each ventricle from its connecting artery.

The heart beats or contracts approximately 70 times per minute. The human heart will undergo over 3 billion contraction cycles, as shown in Figure 12, during a normal lifetime.

The cardiac cycle consists of two parts: systole (contraction of the heart muscle) and diastole (relaxation of the heart muscle). Atria contract while ventricles relax. The pulse is a wave of contraction transmitted along the arteries. Valves in the heart open and close during the cardiac cycle. Heart muscle contraction is due to the presence of nodal tissue in two regions of the heart. The SA node (sinoatrial node) initiates heartbeat. The AV node (atrioventricular node) causes ventricles to contract. The AV node is sometimes called the pacemaker since it keeps heartbeat regular. Heartbeat is also controlled by nerve messages originating from the autonomic nervous system.

Figure 12. The cardiac cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Blood flows through the heart from veins to atria to ventricles out by arteries. Heart valves limit flow to a single direction. One heartbeat, or cardiac cycle, includes atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause. Normal cardiac cycles (at rest) take 0.8 seconds. Blood from the body flows into the vena cava, which empties into the right atrium. At the same time, oxygenated blood from the lungs flows from the pulmonary vein into the left atrium. The muscles of both atria contract, forcing blood downward through each AV valve into each ventricle.

Diastole is the filling of the ventricles with blood. Ventricular systole opens the SL valves, forcing blood out of the ventricles through the pulmonary artery or aorta. The sound of the heart contracting and the valves opening and closing produces a characteristic "lub-dub" sound. Lub is associated with closure of the AV valves, dub is the closing of the SL valves.

Human heartbeats originate from the sinoatrial node (SA node) near the right atrium. Modified muscle cells contract, sending a signal to other muscle cells in the heart to contract. The signal spreads to the atrioventricular node (AV node). Signals carried from the AV node, slightly delayed, through bundle of His fibers and Purkinjie fibers cause the ventricles to contract simultaneously. Figure 13 illustrates several aspects of this.

Figure 13. The contraction of the heart and the action of the nerve nodes located on the heart. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Heartbeats are coordinated contractions of heart cardiac cells, shown in an animate GIF image in Figure 14. When two or more of such cells are in proximity to each other their contractions synch up and they beat as one.

Figure 14. Animated GIF image of a single human heart muscle cell beating. Image from http://www.turbulence.org/Works/genresponse/heartbeat.gif.

An electrocardiogram (ECG) measures changes in electrical potential across the heart, and can detect the contraction pulses that pass over the surface of the heart. There are three slow, negative changes, known as P, R, and T as shown in Figure 15 . Positive deflections are the Q and S waves. The P wave represents the contraction impulse of the atria, the T wave the ventricular contraction. ECGs are useful in diagnosing heart abnormalities.

Figure 15. Normal cardiac pattern (top) and some abnormal patterns (bottom). Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Diseases of the Heart and Cardiovascular System

Cardiac muscle cells are serviced by a system of coronary arteries. During exercise the flow through these arteries is up to five times normal flow. Blocked flow in coronary arteries can result in death of heart muscle, leading to a heart attack.

Blockage of coronary arteries, shown in Figure 16, is usually the result of gradual buildup of lipids and cholesterol in the inner wall of the coronary artery. Occasional chest pain, angina pectoralis, can result during periods of stress or physical exertion. Angina indicates oxygen demands are greater than capacity to deliver it and that a heart attack may occur in the future. Heart muscle cells that die are not replaced since heart muscle cells do not divide. Heart disease and coronary artery disease are the leading causes of death in the United States.

Figure 16. Development of arterial plaque. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Hypertension, high blood pressure (the silent killer), occurs when blood pressure is consistently above 140/90. Causes in most cases are unknown, although stress, obesity, high salt intake, and smoking can add to a genetic predisposition. Luckily, when diagnosed, the condition is usually treatable with medicines and diet/exercise.

The Vascular System

Two main routes for circulation are the pulmonary (to and from the lungs) and the systemic (to and from the body). Pulmonary arteries carry blood from the heart to the lungs. In the lungs gas exchange occurs. Pulmonary veins carry blood from lungs to heart. The aorta is the main artery of systemic circuit. The vena cavae are the main veins of the systemic circuit. Coronary arteries deliver oxygenated blood, food, etc. to the heart. Animals often have a portal system, which begins and ends in capillaries, such as between the digestive tract and the liver.
Fish pump blood from the heart to their gills, where gas exchange occurs, and then on to the rest of the body. Mammals pump blood to the lungs for gas exchange, then back to the heart for pumping out to the systemic circulation. Blood flows in only one direction.

 

Blood

Plasma is the liquid component of the blood. Mammalian blood consists of a liquid (plasma) and a number of cellular and cell fragment components as shown in Figure 21. Plasma is about 60 % of a volume of blood; cells and fragments are 40%. Plasma has 90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and gases. It acts as a buffer, maintaining pH near 7.4. Plasma contains nutrients, wastes, salts, proteins, etc. Proteins in the blood aid in transport of large molecules such as cholesterol.

Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7 µm in diameter that carry oxygen associated in the cell's hemoglobin. Mature erythrocytes lack a nucleus. They are small, 4 to 6 million cells per cubic millimeter of blood, and have 200 million hemoglobin molecules per cell. Humans have a total of 25 trillion red blood cells (about 1/3 of all the cells in the body). Red blood cells are continuously manufactured in red marrow of long bones, ribs, skull, and vertebrae. Life-span of an erythrocyte is only 120 days, after which they are destroyed in liver and spleen. Iron from hemoglobin is recovered and reused by red marrow.

The liver degrades the heme units and secretes them as pigment in the bile, responsible for the color of feces. Each second two million red blood cells are produced to replace those thus taken out of circulation.

White blood cells, also known as leukocytes, are larger than erythrocytes, have a nucleus, and lack hemoglobin. They function in the cellular immune response. White blood cells (leukocytes) are less than 1% of the blood's volume. They are made from stem cells in bone marrow.

There are five types of leukocytes, important components of the immune system. Neutrophils enter the tissue fluid by squeezing through capillary walls and phagocytozing foreign substances. Macrophages release white blood cell growth factors, causing a population increase for white blood cells. Lymphocytes fight infection. T-cells attack cells containing viruses. B-cells produce antibodies. Antigen-antibody complexes are phagocytized by a macrophage. White blood cells can squeeze through pores in the capillaries and fight infectious diseases in interstitial areas

Thrombocyts (Platelets) result from cell fragmentation and are involved with clotting, as is shown by Figures 17 and 18. Platelets are cell fragments that bud off megakaryocytes in bone marrow. They carry chemicals essential to blood clotting. Platelets survive for 10 days before being removed by the liver and spleen.

There are 150,000 to 300,000 platelets in each milliliter of blood. Platelets stick and adhere to tears in blood vessels; they also release clotting factors. A hemophiliac's blood cannot clot. Providing correct proteins (clotting factors) has been a common method of treating hemophiliacs. It has also led to HIV transmission due to the use of transfusions and use of contaminated blood products.

Figure 17. Human Red Blood Cells, Platelets and T-lymphocyte (erythocytes = red; platelets = yellow; T-lymphocyte = light green) (SEM x 9,900)..

Figure 18. The formation and actions of blood clots. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates.

Figure 19. Blood Clot Formation (blood cells, platelets, fibrin clot) (SEM x10,980). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

 

The Lymphatic System

Water and plasma are forced from the capillaries into intracellular spaces. This interstitial fluid transports materials between cells. Most of this fluid is collected in the capillaries of a secondary circulatory system, the lymphatic system. Fluid in this system is known as lymph.

Lymph flows from small lymph capillaries into lymph vessels that are similar to veins in having valves that prevent backflow. Lymph vessels connect to lymph nodes, lymph organs, or to the cardiovascular system at the thoracic duct and right lymphatic duct.

Lymph nodes are small irregularly shaped masses through which lymph vessels flow. Clusters of nodes occur in the armpits, groin, and neck. Cells of the immune system line channels through the nodes and attack bacteria and viruses traveling in the lymph.

Learning Objectives

• List three functions of blood.
• Distinguish between open and closed circulatory systems.
• Describe the composition and functions of blood.
• Trace the path of blood in the human body. Begin with the aorta and name all major components of the circulatory system through which the blood passes before it returns to the aorta.