Circulatory System

B.N. Ruck, G.J. Villares, C.M. Welch, D.E. Facey

(presented by Saint Michael's College, Colchester, VT, USA.)

All animals must be able to circulate nutrients, fluids, and gases in order to properly function under specified conditions. There are three ways that animals may do this.   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.  More complex animals have either an open or closed circulatory system.

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.

 

What are the differences between open and closed circulatory systems and what are the advantages and disadvantages of each?

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.

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.

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.

 

Describe blood flow through the mammalian 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.

 

What are heart rate and stroke volume and how are 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.

 

What are the 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.

 

What 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 the function of pacemaker cells and tell what makes them automatic.

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.

 

What are the four main functions of arteries?

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.

 

Explain the importance of blood pressure in the arterial system and how it is regulated.

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.

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 teh 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.

 

What are 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.

 

What adaptations in fish have evolved to better suit air breathing fish and water breathing fish in relation to heart function?

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.

 

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.

 

What is the lymphatic system and what are 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.