19 Cardiovascular - The Circulation and Blood Flow

 

The heart pumps blood through a series of channels termed blood vessels. Exchange between the blood and the tissues occurs across the walls of the smallest of these vessels, the capillaries. This section examines the vessels, the dynamics of flow, and exchange with the tissues.

 

A. General plan of the circulation

 

1. Closed system - The blood never directly bathes the tissues. It is confined to the circulatory vessels. A small amount of fluid is forced across the walls of the capillaries which then forms the tissue fluid and bathes the tissues.

 

2. Dual circulatory system - As previously discussed, the heart is a dual pump that distributes blood into two different circulatory pathways.

 

a. Pulmonary circuit - The right side of the heart pumps blood into this circuit. The blood courses through the lungs and returns to the left side of the heart.

 

b. Systemic circuit - The left side of the heart pumps blood into this circuit which supplies the rest of the body. The blood ultimately returns to the right side of the heart.

 

3. Lymphatic system - This is technically not part of the cardiovascular system but serves as an adjunct to it. It returns tissue fluid to the general circulation and plays an important role in the body defense.

 

4. Pattern of flow - Blood vessels which carry blood away from the heart are termed arteries. Large arteries subdivide into smaller arteries until the smallest arteries, the arterioles, are reached. The arterioles supply the capillary beds across which exchange of materials with the tissues occurs. Capillaries are drained by vennules which join to form larger vessels known as veins, which ultimately return the blood to the heart.

 

B. Vascular system anatomy

 

1. Structure of vessels - Blood vessels can consist of as many as three layers. From the inside to the outside, these layers are as follows.

 

a. Tunica intima (interna) - This is a simple layer of squamous epithelium plus the supporting connective tissue. It is sometimes termed the endothelium and is the only layer found in all blood vessels. It is continuous with the endocardium of the heart and makes up the entire capillary wall.

 

b. Tunica media - This is a layer of smooth muscle which is frequently mixed with elastic fibers. It varies in thickness from vessel to vessel and in the relative amount of elastic tissue present.

 

c. Tunica adventitia (externa) - This a thin outer layer of connective tissue that contains both collagenous and elastic fibers.

 

2. Arteries - These are usually classified based upon size and wall composition.

 

a. Elastic arteries - These are the largest of the arteries. They are characterized by

 

(1) a thick tunica media which is rich in elastic fibers.

 

(2) elastic sheaths that surround the tunica intima and tunica media. They have more elastic tissue than muscle. These are the aorta and pulmonary trunk plus their major branches.

b. Muscular arteries - These arteries are medium sized and distribute blood to the organs. They have a tunica media which has much more smooth muscle than elastic tissue. They range in diameter from that of the little finger down to about the diameter of a pencil lead. They constitute the bulk of the named arteries.

 

c. Arterioles - These are the smallest arteries (less than 0.3 mm in diameter). The tunica media is thick and elastic free. They have the ability to constrict (decrease the diameter of the lumen) and dilate (increase the diameter of the lumen). They are principally responsible for regulating blood flow into the capillaries which they supply.

 

3. Capillaries - These vessels consist of nothing more than an endothelium and therefore it is possible for exchange of materials to occur across their walls. The capillaries are so extensive that no cell in the body is more than a few micrometers from a capillary bed. There are three types of capillaries.

 

a. Continuous capillaries - These are found in most parts of the body. The epithelial cells are connected by tight junctions.

 

b. Fenestrated - These have pores that span the endothelium. They occur where extensive filtration occurs such as in the kidney.

 

c. Sinusoids - These are wide, flat channels, through which blood flows very slowly. They represent a special class of fenestrated capillary in which there are actual spaces between adjacent endothelial cells. They are found in organs such as the liver where an extensive amount of exchange occurs between the tissues and the blood.

 

d. Organization of capillary beds - Capillary beds connect arterioles to venules.

 

(l) Each capillary bed has a preferential shunt from the arteriole to the venule. This is termed the metarteriole.

 

(2) True capillary vessels exit the arteriole and the metarteriole. At the junction of each capillary vessel with the arteriole or metarteriole is a band of smooth muscle known as the precapillary sphincter. These sphincters regulate the amount of blood that enters into the capillary bed. If all of the sphincters are constricted then blood will flow directly from the arteriole to the venule.

 

e. Vasomotion - Precapillary sphincters go through a series of relaxations and constrictions averaging about 12 per minute. As a result blood flow through each capillary is a series of pulses instead of a continuous flow. This alteration of blood flow through the capillaries is termed vasomotion. As there is far more volume in the capillary beds than there is volume of blood, at rest, only about 25% of the vessels within a capillary bed will have blood flowing through them.

 

4. Venules - These consist of an endothelium surrounded by a small amount of connective tissue. The larger vennules may have a tunica media made up of a few smooth muscle cells.

 

5. Veins - These have all three layers but the tunic media is very thin with only a few muscle cells and the tunica adventitia is the thickest layer. The thin walled veins are much more distensible than are the arteries. Many have folds in the tunica intima which form valves which prevent the back flow of blood.

 

6. Unique or special areas - These are areas of the circulation that differ in some significant way from the general pattern.

 

a. Hepatic portal system - In a portal system the normal pattern of artery - capillary bed - vein is replaced by

 

artery - capillary bed - portal vessel - capillary bed - vein.

 

Portal systems by definition connect two capillary beds together. As a rule, in the portal system arrangement the first bed brings something into the blood and the second bed takes it out and processes it. In the hepatic portal system blood picks up nutrients from the digestive tract in the first capillary bed and the liver processes these nutrients in the second bed.

 

(1) Blood is supplied to the digestive tract by three major arteries: celiac, superior mesenteric, and inferior mesenteric.

(2) Drainage of blood from the tract is as follows.

 

(a) The inferior mesenteric vein drains the large intestine. It joins with the splenic vein.

 

(b) The splenic vein drains the spleen, part of the stomach, and other organs. It joins with the superior mesenteric vein.

 

(c) The superior mesenteric vein drains the small intestine. Its fusion with the splenic vein forms the hepatic portal vein.

 

(d) The hepatic portal vein runs to the stomach where it breaks out into sinusoids. These sinusoids are drained by the hepatic veins into the inferior vena cava.

 

b. Cerebral circulation - The brain is an organ that must have a continuous supply of blood. A circular feed system with inputs from four vessels has evolved.

 

(1) The major portion of the cerebrum receives blood from the paired internal carotid arteries and vertebral arteries.

 

(2) These four arteries anastomose (fuse) to form the circle of Willis. The circle of Willis then gives off branches that supply most of the brain. The details are as follow.

 

(a) Each internal carotid artery divides to form an anterior and middle cerebral artery.

 

(b) The two anterior cerebral arteries anastomose with each other via the anterior communicating artery.

 

(c) The two vertebral arteries anastomose to the brain cavity to form the basilar artery. The basilar then joins the two middle cerebral arteries by way of the posterior communicating arteries. This completes the circle.

 

(3) In most mammals it is possible to block both carotids and still receive sufficient blood flow into the circle of Willis via the vertebral arteries. This is not true for human beings.

 

c. Fetal circulation - The fetus derives all of its nutrition, disposes of wastes, and exchanges gases via the placental attachment with its mother. The fetal circulation is modified to reflect these facts.

 

(l) Pulmonary bypass - The lungs are non-functional and collapsed in the fetus. As a consequence blood is shunted from the right side of the circulation to the left side before blood ever gets to the lungs. This is accomplished by two structures.

 

(a) Foramen ovale - This is a hole in the wall which separates the left atrium from the right atrium. This permits blood returning to the right side of the heart to move directly to the left side.

 

(b) Ductus arteriosus - This is a shunt vessel that connects the pulmonary artery to the aorta. Blood which gets out of the right ventricle will move through this shunt to the aorta and therefore bypass the lungs.

 

(c) At birth the lungs expand, a flap folds over the foramen ovale, and the smooth muscle in the ductus arteriosus contract thereby closing it off. Tissue eventually fills the foramen ovale making it a solid partition and the ductus eventually becomes a solid ligament (ligamentum arteriosum).

 

(2) Placental connection - Blood leaves the fetus via extensions of the internal iliac arteries known as the umbilical arteries. Blood returns from the placenta via the umbilical vein. This vein divides to form two major branches.

 

(a) Ductus venosus - This branch joins with the inferior vena cava.

 

(b) A second branch joins up with the hepatic portal system of the fetus.

 

(c) After birth the umbilical vein degenerates and the ductus venosus becomes a ligament. the remainder of the umbilical arteries also become ligaments.

 

C. Hemodynamics - This is the physiology of circulation.

 

1. Blood flow - This represents the volume of blood that passes through an area per unit of time. It is essential that adequate flow be maintained to tissues at all times. The mechanisms by which flow is maintained is the subject of this section.

 

a. Blood velocity - The velocity of blood flow is inversely proportional to the total cross sectional area of the vessels through which it flows. The larger the cross sectional area, the slower the flow.

 

(1) Flow is slowest in the capillaries because these vessels collectively have the largest cross sectional area. The cross sectional area of the capillaries is some 600 to 800 times as great as that of the aorta. Flow rate in the capillaries is about 0.5 mm/sec.

 

(2) Flow is most rapid in the aorta with rates approaching 20 cm/sec.

 

(3) In the largest veins, flow rates range between 10 and 13 cm/sec.

 

b. Factors which affect flow - There are two basic factors that affect blood flow. These are pressure and resistance.

 

(1) Pressure - This is the energy which is imparted to the blood by the pumping of the heart.

 

(2) Resistance - This is the friction which blood most overcome if flow is to occur. Factors involved in resistance include the following.

 

(a) Blood viscosity - This represents the internal resistance to flow. Blood is indeed thicker than water, mainly because it has a greater viscosity.

 

(b) Vessel resistance - This is the frictional resistance offered by the walls of the blood vessels. Two major factors affect this resistance.

 

/1/ Vessel length - The longer the vessel, the greater is the resistance offered to flow.

 

/2/ Vessel diameter - The smaller the diameter of a vessel, the greater the resistance offered to flow.

 

c. Relationship among flow, pressure, and resistance

 

(1) Flow is equal to pressure divided by resistance.

 

(2) If flow is to occur through a vessel then there must be a pressure drop. This means that the pressure at the end of the vessel must be less than it was at the beginning of the vessel. The "lost" pressure represents the energy expended in overcoming the resistance to flow. The relationship can be expressed mathematically as follows.

 

Flow = P1 - P2

Resistance

 

(3) If flow time is set at one minute and resistance is equal to the resistance offered by the systemic circulation (peripheral resistance), then flow becomes equal to the cardiac output and the formula may be rewritten as follows.

 

Cardiac Output = P1 - P2

Peripheral resistance

(4) P1 represents the pressure in the aorta, the beginning of the systemic circuit, and P2 represents the pressure in the right atrium, the end of the systemic circuit. For all practical purposes the pressure in the right atrium is 0 and therefore pressure becomes equal to the mean (average) arterial pressure.

 

(5) As there are two different blood pressures (diastolic and systolic) an average must be obtained. This average is the mean arterial pressure. The mean arterial pressure (MAP) can be approximated by the following formula.

 

MAP = systolic pressure + 2(diastolic pressure)/3

 

(6) The final relationship which has been developed is as follows.

 

Cardiac output = mean arterial pressure

peripheral resistance

 

This can be rearranged to yield the following equation.

 

Mean arterial pressure = cardiac output X peripheral resistance

2. Blood pressure - Whenever this term is used without modifiers it is really referring to arterial pressure. As the previous discussion has shown, blood pressure is absolutely essential for blood flow.

 

a. Expressing blood pressure - There are two different pressures measured in the aorta. Systolic pressure is the higher pressure while diastolic pressure is the lower value. The two pressures correspond to systole and diastole in the heart. They are usually written with the systolic value over the diastolic value. An example would be 120/80. The mean arterial pressure is the average value and must be calculated.

 

b. Pressure units - Pressure is usually expressed as the force that a column of liquid of a given height will exert. The liquid used is usually mercury (Hg) because of its great density. Physiological pressures are expressed as millimeters of mercury (mm/Hg). For example, a blood pressure of 90 would exert the same force as a column of mercury 90 millimeters high.

 

c. Pulse pressure - This is the difference between systolic and diastolic pressure. It is responsible for what we feel as our pulse. A large difference between the two values yields a hard or firm pulse while a small difference results in a weak pulse.

 

d. Factors which affect blood pressure - There are six major factors that have an affect on blood pressure.

 

(1) Blood volume - All other things being equal, the greater the volume of blood in the circulatory system, the greater will be the blood pressure. Loss of volume usually results in a loss of pressure although certain physiological mechanisms may counteract the pressure drop.

 

(2) Viscosity of blood - Viscosity is largely due to the formed elements. The higher the hematocrit, the greater the viscosity. Elevated viscosity forces the ventricle to develop more pressure in order to move the blood.

 

(3) Elasticity - The elasticity of the blood vessels buffers the blood pressure preventing both systolic and diastolic extremes. During systole the arteries "give" and consequently the pressure inside does not go as high as it would in a rigid pipe. During diastole the arteries recoil maintaining pressure on the blood and preventing extremely low diastolic pressures.

 

(4) Gravity - Blood moving upward requires more pressure than blood moving downward because of the pull of gravity. As a result of this blood pressure in the head is always less than in the feet (when standing). In a horizontal position pressure if fairly uniform through out the arterial tree.

 

(5) Cardiac output - The more blood that is put into the arterial tree per minute, the greater will be the pressure (all other things being equal).

 

(6) Peripheral resistance - This is the total resistance offered to flow by the vascular bed. It can be varied by regulating the diameter of the arterioles. When the arterioles are constricted the blood is held back in the arterial side and pressure increases. When the arterioles dilate peripheral resistance decreases and more blood can leave the arterial tree thereby dropping pressure.

 

e. Of the six factors described, only two can be varied on a short terms basis. These are cardiac output and peripheral resistance. Therefore, the equation

 

MAP = cardiac output X peripheral resistance

 

is valid for most physiological situations.

 

f. Central regulation of blood pressure - Nervous control - As the body's demand for blood flow changes, the blood pressure must change as well. Insufficient pressure results in poor flow and tissue death, while highly elevated pressures result in cardiovascular damage. Consequently precise homeostatic controls exist to maintain proper pressure and flow. These controls are for cardiac output and peripheral resistance. The control of cardiac output have already been discussed and the control of peripheral resistance will be discussed below.

 

(1) Regulation of peripheral resistance - Regulation of resistance is largely a regulation of the arterioles. These vessels contain smooth muscle which receives innervation from the sympathetic division of the ANS. When there are high levels of sympathetic activity the arterioles in most tissues constrict and resistance increases. When the sympathetic activity is low the arterioles dilate and resistance decreases. An exception to this rule is that the arterioles in the skeletal muscles and brain which have beta receptors instead of alpha receptors. When these receive sympathetic impulses they dilate due to a release of nitric oxide.. Regulation of sympathetic activity comes from the vasomotor center.

 

(a) Vasomotor center - This is a sympathetic reflex center located in the medulla which controls the amount of sympathetic activity to the arterioles.

 

/1/ Increased vasomotor activity results in increased sympathetic outflow and increased vasoconstriction to most tissues. This results in increased resistance and increased blood pressure.

 

/2/ Decreased vasomotor activity results in decreased sympathetic outflow and decreased vasoconstriction (vasodilation). This results in a decrease in resistance and decreased blood pressure.

 

The vasomotor center is turn is controlled by special pressure receptors known as the baroreceptors.

 

(b) Baroreceptors - These pressure receptors are located in the walls of the common carotid arteries (carotid sinuses) and the aorta (aortic sinus). These receptors are actually stretch receptors which respond to the stretch of the artery walls which in turn varies with blood pressure.

 

/1/ As pressure increases the walls of the arteries stretch and the activity of the baroreceptors increases. Increasing vollies from the baroreceptors cause a decrease in vasomotor activity which results in decreased peripheral resistance and a decrease in blood pressure.

 

/2/ Falling pressure reduces the stretch on the arteries and the activity of the baroreceptors decreases. Decreasing baroreceptor activity results in increasing vasomotor activity, increased vasoconstriction, and increased blood pressure.

 

/3/ The baroreceptors also send signals to the cardiac control centers. An increase in baroreceptor activity stimulates the cardiac inhibitory center and inhibits the cardiac accelerator center. The result is a decrease in cardiac output and a decrease in blood pressure. A drop in baroreceptor activity has the exact opposite effect. The connection of the baroreceptors to both the vasomotor center and the cardiac control centers insures that both will be synchronized during blood pressure regulation.

 

(2) Summary of nervous regulation - The blood pressure receptors are the baroreceptors, the integrating centers are the vasomotor center and cardiac control centers, and the effectors are the arterioles and heart. Summary examples of their interaction are as follows.

 

(a) Increasing blood pressure results in increased baroreceptor activity. The increased impulses from the baroreceptors

 

/1/ stimulate the cardiac inhibitor center (decrease cardiac output).

 

/2/ inhibit the cardiac accelerator center (decrease cardiac output).

 

/3/ inhibit the vasomotor center (decrease peripheral resistance).

 

The collective effect is reduced cardiac output and reduced peripheral resistance which of course result in reduced blood pressure.

 

(b) Decreasing blood pressure results in decreased baroreceptor activity. The decreased impulses from the baroreceptors

 

/1/ inhibits the cardiac inhibitor center (increases cardiac output).

 

/2/ stimulates the cardiac accelerator center (increase cardiac output).

 

/3/ stimulates the vasomotor center (increase peripheral resistance).

 

The collective effect is increased cardiac output and increased peripheral resistance which of course results in increased blood pressure.

 

g. Central regulation of blood pressure by hormones - angiotensin system - In addition to the nervous control of blood pressure there is a chemical method that also regulates blood pressure. This is known as the angiotensin system and is initiated by the kidney.

 

(1) Because of its role in filtering blood, the kidney is extremely sensitive to reductions in blood flow. If blood flow drops below normal the kidney responds by releasing a hormone known as renin.

 

 

(2) Renin acts on a blood protein produced by the liver known as angiotensinogen, converting it into an active form known as angiotensin.

 

(3) Angiotensin increases blood pressure by two different methods.

 

(a) It causes vasoconstriction of the arterioles thereby increasing peripheral resistance.

 

(b) It increases the level of the hormone aldosterone which in turn increases the total volume of fluid in the vascular bed by affecting salt and water reabsorption in the kidney.

 

(4) Blood pressure may be brought down through this system as follows.

 

(a) An enzyme, angiotensinase, inactivates angiotensin.

 

(b) A second hormone, Atrial natriuretic factor (ANF), is released by the atrial myocardial cells in response to stretch brought about by high blood pressure. This hormone reduces aldosterone secretion and reduces total blood volume by increasing the loss of salt and water from the body. This results in a drop in pressure. ANF will be discussed in greater detail with fluid and electrolyte balance.

h. Other chemical controls

 

(1) Adrenal medulla hormones - This gland releases both epinephrine and norepinephrine. The combined effects of these hormones is to increase cardiac output and increase vasoconstriction. This results in an increase in blood pressure. These hormones become significant during periods of stress.

 

 

(2) Antidiuretic hormone (ADH) - This hormone plays its major role in regulating fluid balance in the body, but as fluid balance also affects blood pressure, ADH plays a role. Although of little significance normally, it becomes important when blood pressure drops to dangerously low levels by promoting intense vasoconstriction as well as fluid retention.

 

3. Local control of blood flow - autoregulation - It is often necessary to increase blood flow to selected tissues without changing the central blood pressure. This can be accomplished by local dilation of arterioles supplying the tissues. This local dilation is brought about by the following mechanisms.

 

a. Decreased oxygen levels - This results in dilation and relaxation of precapillary sphicters.

 

b. Accumulations of metabolic end products - Increased levels of carbon dioxide and other metabolites cause local dilation of arterioles.

 

c.  Release of nitric oxide (endothelium-derived relaxation factor, EDRF) by the endothelial cells causes relaxation. It now appears that NO is the most important factor in causing arteriole dilation. Numerous chemical factors as well as increasing blood pressure cause the release of NO. Sympathetic nervous activity apparently is more important in vasoconstriction.

 

d. Increased potassium, decreased pH, mediators of inflammation, and elevated local temperature will also promote vasodilation.

 

c. Myogenic effects - Arteriole smooth muscle contracts more forcefully when it is stretched and relaxes when stretching is less. The amount of stretch is dependent upon the amount of blood flowing. If blood flow decreases, the muscle tends to relax, vasodilation occurs, and blood flow again increases.

 

Autoregulatory ability is particularly well developed in skeletal muscle, cardiac muscle, and gastrointestinal tract.

 

4. Interaction of central blood pressure mechanisms and autoregulation - During periods of extreme exercise the autoregulatory mechanisms result in massive dilations of the arterioles in the skeletal muscles. This results in decreasing peripheral resistance and a drop in pressure. Simultaneously, oxygen demand of the muscles will result in an increase in cardiac output that tends to neutralize the drop in pressure. The blood pressure therefore remains fairly constant until maximum cardiac output is reached. At that point, if local dilation continues, pressure will begin to drop. This can continue until adequate pressure can no longer be maintained and a loss of consciousness will occur.

 

5. Venous pressure

 

a. Venous pressure is much less uniform than is arterial pressure.

 

b. Blood leaving the capillaries has very little pressure due to it being expended in forcing the blood through the capillaries.

 

c. Due to gravity on the column of blood, venous pressure in the ankles of a standing person may be as high as 80 to 100 mm/Hg. In a sitting person the pressure would be only half of that value.

 

d. In the subclavian vein, venous pressure may be only 8 to 10 mm/Hg.

 

e. Venous return to the heart is brought about largely by two mechanisms.

 

(1) Contraction of skeletal muscle - The contracting muscles "milk" the veins forcing blood upward from the legs and lower parts of the body. At strategic points veins possess valves which prevent blood from flowing backward if the muscles relax.

 

(2) Siphon action - The circulatory system is closed and continuous. Therefore when blood is pumped from the heart it tends to pull blood in from the venous side of the circulation. Reduced atmospheric pressure in the thoracic cavity during breathing tend to pull blood into the great veins.

 

f. A person standing for extended periods without moving will have blood pool in the legs. Eventually so much blood may pool that venous return will be reduced enough that cardiac output will fall dramatically leading to a drop in blood pressure. The response of the body to a loss of blood pressure in the brain is to faint and therefore bring the body into a horizontal position where reduced blood pressure will suffice.

 

g. Persons who stand relatively still for long periods of time may have their surface veins in the legs distend, a painful condition known as varicose veins. If the distended veins occur in the rectum then the condition is known as hemorrhoids.

 

6. Distribution of blood - At any given moment, the distribution of blood in the body is as follows.

 

a. Venous side - 64%

b. Systemic arteries - 15%

c. Heart - 7%

d. Systemic capillaries - 5%

e. Pulmonary veins - 9%

 

Because of their large capacity the veins serve as blood reservoirs. Even a small amount of venous contraction can shift large volumes of blood into other parts of the circulation.

 

7. Exchange of materials between the blood and tissues - The capillaries are the sites of exchange. Hydrostatic blood pressure will force fluid and small molecules across the capillary wall into the tissue spaces, a process called filtration. The osmotic pressure of the blood will then draw much of this fluid back into the circulation. The mechanisms by which this occurs is as follows.

 

a. Capillaries are permeable to water and small molecules.

 

b. There are two forces that move fluid into and out of the capillary bed. One is hydrostatic pressure (blood hydrostatic pressure, BHP) and the other is osmotic (oncotic) pressure, OP. BHP pushes fluid out of the capillaries while OP pulls it back into the capillaries. The difference between these two pressures is the net filtration pressure (NFP).

 

c. On the arterial side of the capillary bed, BHP exceeds OP and there exists a positive NFP which pushes fluid out of the capillary bed into the tissue spaces.

 

d. OP does not decrease in the capillary because protein is too large to filter across the capillary wall. Eventually a point is reached when the osmotic pressure exceeds the hydrostatic pressure. At this point the NFP becomes negative and fluid begins to move back into the capillary bed.

 

e. By the time the blood reaches the vennules the osmotic pressure gradient has returned most of the fluid initially forced out by the hydrostatic pressure.

 

g. The relationship discussed can be expressed as follows.

 

NFP = BHP - OP

 

Consider the following example.

 

 

 

At the arterial end: NFP = (BHP 35) - (OP 26) = + 9

 

At the venous end: NFP = (BHP 18) - (OP 26) = - 8

 

Note that overall there is a net filtration pressure of + 1 which means that not all (but most) of the fluid forced out by BHP is returned to the capillary by OP. The remainder is returned by the lymphatic system.

 

C. Lymphatic system - This is an adjunct to the circulation.

1. Functions

 

a. Tissue fluid is drained by the lymphatic system and returned to the circulation.

 

b. The tissue fluid is filtered and purified by the lymphatic system.

 

c. The lymphatic system is the major source of lymphocyte production.

 

d. The lymphoid tissues play major roles in the immune response system.

 

2. Anatomy - The lymphatic system has the following components.

 

a. Lymph - This is the fluid of the system. It is essentially the same as tissue fluid except that it contains lymphocytes.

 

b. Lymph vessels - These represent a series of vessels that are similar to veins. Lymph capillaries are also included here.

 

c. Lymphoid organs - These are organs that receive lymphatic drainage. Most play a role either in filtration and purification of lymph, or in the immune response. They include the lymph nodes, Peyer's patches in the intestinal wall, the spleen, thymus, and tonsils.

 

3. Organization

 

a. The lymphatic system begins as small blind (one end closed) capillaries which are dispersed among the circulatory capillaries. The capillaries pick up tissue fluid.

 

b. Capillaries drain into larger vessels. Along these vessels are located the lymph nodes. These are connective tissue capsules which surround aggregations of lymphoid tissue. Large numbers of macrophages and lymphocytes are found here. Filtration and immunity are major functions of these nodes.

 

c. The lymphatic vessels eventually drain into the thoracic duct and the right lymphatic duct which return the fluid to the subclavian veins.

 

D. Pathologies

 

1. Hypertension (high blood pressure) - Arterial pressures of 150/90 are considered hypertensive. Hypertension can lead to heart attack, heart failure, stroke, and kidney damage. Obviously such high arterial pressures force the heart to work harder and lead to enlargement of the heart muscle but not an increase in cardiac output.

 

a. Primary (Essential) hypertension - This is hypertension due to unknown causes. It constitutes about 90% of the cases.

 

b. Secondary hypertension - This is due to circulatory stress in the kidneys, tumors on the adrenal cortex leading to hyper aldosterone secretion, or tumors on the adrenal medulla leading to hyper epinephrine/norepinephrine secretion.

 

Approximately 20% of the population can expect to have hypertension. The percentage is much higher in black Americans. Hypertension can usually be controlled.

 

2. Atherosclerosis This is a narrowing of the arteries due to the build up of deposits known as plaque. Overgrowth of smooth muscle, deposition of lipids and connective tissue, and sometimes calcification can occur. These deposits can greatly restrict blood flow and generate thromboses which completely block the vessels. Atherosclerosis is the major cause of coronary occlusions and strokes.

 

a. Causes of atherosclerosis - the cholesterol connection - Cholesterol is an essential lipid that is an integral component of cell membranes. It is also the starting point for vitamin D and the steroid hormones. Cholesterol, like all lipids, is insoluble in the watery plasma. Therefore it must be combined with a protein to make it soluble. Such a combination is termed a lipoprotein. Lipoproteins are usually measured in milligrams per deciliter (l00 ml) of blood. There are three types of cholesterol bearing lipoproteins.

 

(1) Low density lipoprotein (LDL)- This is the most common form and functions to transport cholesterol to the cells. There it interacts with receptors which take it into the cell via receptor mediated endocytosis. If an individual has a genetic predisposition that leads to too few receptors then LDL levels in the blood will be very high and the LDL will result in the build up of plaque in the arteries. Individuals with normal numbers of receptors but who have diets very high in fat will also end up with elevated LDL and the same atherosclerotic plaques.

 

(2) Very low density lipoprotein (VLDL) - These transport principally triglycerides and only small amounts of cholesterol. However, after depositing some of their triglycerides in fat cells, these are converted into LDL. This is a major means by which a high fat diet can contribute to plaque formation.

 

(3) High density lipoprotein (HDL) - This is the "good" cholesterol. These remove cholesterol to the liver where it can be eliminated and therefore prevents the accumulation of cholesterol in the blood. The higher the percentage of HDL, the lower is the risk of atherosclerosis.

 

A lipid profile of the blood measures total cholesterol, HDL, and triglycerides (VLDL). LDL is then calculated by subtracting the HDL and triglycerides/5. For adults the desirable levels would be a total cholesterol of 200 or less, LDL under 130 and HDL over 40. The risk of atherosclerosis may be predicted by the ratio of total cholesterol to HDL. For example, a person with a total cholesterol of 150 and a HDL of 50 would have a ratio of 3 and be considered low risk. Ratios above 4 are considered undesirable and the higher the ratio, the greater the risk.

Cholesterol levels can be controlled by diet, exercise, and drugs.

 

3. Shock - This refers to reduced cardiac output. Usually there is not sufficient blood to perfuse all of the tissues and therefore the body closes down all of the circulatory pathways except to the most essential areas (CNS and heart). If the condition is not corrected promptly then total cardiovascular collapse will occur and death will ensue. Causes of shock include:

 

a. heart failure.

b. hemorrhage.

c. reduced venous return, perhaps due to massive vasodilation as occurs during anaphylaxis.

 

Persons in shock will have a pale ashy appearance and the skin will be cool to the touch. The pulse will be rapid and weak. Treatment is by fluid infusion and the use of vasoconstrictive agents.

 

4. Edema - This is an accumulation of excess tissue fluid. It results in swelling and may impair transfer of materials to the tissue. It can result from

 

a. increased capillary pressure.

b. increased capillary permeability.

c. blocked lymphatics.

d. decreased osmotic pressure in the plasma due to reduced plasma proteins. This can result from liver or kidney failure.

 

E. Effects of exercise on the cardiovascular system - A regular program of aerobic (isotonic) exercise has beneficial effects on the circulation. There seem to be two major effects.

 

1. Reduction in peripheral resistance - This leads to decreased blood pressure.

 

2. Increased cardiac output - This occurs as a result of an increase in stroke volume. Exercise causes the volume (capacity) of the heart to increase, and as a result the heart will pump more blood per beat. This means there is an increase in the efficiency of cardiac activity. This is why trained athletes have lowering resting heart rates than do non-athletes.

 

Exercise has beneficial effects in terms of minimizing cardiovascular disease. Overall cholesterol levels are reduced and HDL levels are increase. Evidence indicates that regular moderate exercise can reduce the incidence of heart attacks by half.