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Cardiovascular System: Blood Vessels
Up Course Syllabus Basic Chemical and Cellular Biology Organization of the Body and Body Tissues Integumentary System;Bones and Skeletal Tissue The Skeleton and Joints The Muscular System Cardiovascular System: Blood Cardiovascular System: The Heart Cardiovascular System: Blood Vessels Test Scores/Final Grades Practical exams



Structure of Blood Vessels

a) Three distinct layers or tunics form the walls of blood vessels:

---The tunica interna (tunica intima) is the innermost layer. It contains the endothelium which is a continuation of the endocardial lining of the heart. The endothelium contains  simple squamous epithelial cells that form a flat slick surface inside the blood vessels which minimizes friction as blood moves through the lumen (central blood containing space of blood vessel).  In vessels larger than 1 mm  in diameter a subendothelial layer of loose connective tissue (a basement membrane) supports the endothelium----

---The tunica media is the middle layer and consists circularly arranged smooth muscle cells and sheets of elastin.  The smooth muscle is innervated by vasomotor nerve fibers of the sympathetic system of the autonomic nervous system.  The smooth muscles can be used to reduce the diameter of the lumen due to the contraction of the smooth muscle fibers (vasoconstriction), or to increase the diameter of the lumen due to smooth muscle relaxation  (vasodialation). ---

---The tunica externa  (tunica adventitia) is the outermost layer and is composed largely of collagen fibers that protect and reinforce the blood vessel and anchor it to surrounding structures.  The vasa vasorum  (vessels of the vessels) is found within the tunica externa of larger vessels which nourish the more external tissues of the vessel wall.  The innermost or luminal portion of the vessel obtains its nutrients directly from the blood in the lumen.----

The Arterial System

---Consist of arteries which are  blood vessels that transport blood away from the heart---

---Arteries are divided into three groups:

1) Elastic (conducting) Arteries)

a) They are thick walled arteries near the heart (the aorta and its major branches) and are the largest in diameter ranging from 2.5 cm to 1 cm.

b) They are the most elastic of arteries and are called conducting arteries because  their large lumen allows them to serve as low-resistance pathways that conduct blood from the heart to medium-sized arteries.

c) They contain more elastin than any other type.  The elastin is present in all three tunics. The tunica media contains the greatest amount of elastin where it lies between the layers of smooth muscle cells in "holey" sheets that appears like slices of Swiss cheese. The elastin fibers allow the elastic arteries to withstand and smooth out large pressure fluctuations by expanding when the heart pumps blood into them and recoiling to propel blood onward into circulation when the heart relaxes.  The pressure smoothing effect of the elastic arteries keep the arteries that it is conducting blood to from being damaged by high pressures. 

c) Elastic arteries are inactive in vasoconstriction and are just simple elastic tubes that expand and recoil as blood flows through them from the heart.

2) Muscular (Distributing) Arteries

a) Distally the elastic arteries give way to the muscular (distributing) arteries deliver blood to the specific body organs.  Their internal diameter ranges from 1 cm to 0.3mm.

b) They have the thickest tunica media of all vessels, containing more smooth muscle and less elastin than elastic arteries. They are more active in vasoconstriction and do not expand as much as elastic vessels.

c) There is an elastic lamina on each face of the tunica media in muscular arteries.

3) Arterioles

a) Muscular arteries divide to form the smallest of  arteries called arterioles, having a lumen diameter from 0.3mm to 10 micrometers.

b) Larger arterioles have all three tunics, and the smaller arterioles which lead into the capillary beds have only a single layer of smooth muscle surrounding and endothelial lining.


---the smallest of blood vessels containing a thin wall consisting only of the tunica interna (a single endothelial cell can form the entire circumference of the capillary wall)---

---Along the outer surface of the capillaries are spider-shaped pericytes, smooth muscle-like cells that stabilize the capillary wall.----

---Average lumen diameter is 8-10 micrometers only large enough for red blood cells to pass through in single file----

--Sites for the exchange of gases, nutrients, hormones... between the blood and interstitial fluid---

----Their are three types of capillaries:

1) Continuous capillaries

a) abundant in the skin and muscles and the most common

b) They are continuous in that their endothelial cells provide an uninterrupted lining.  The adjacent endothelial cells are joined by incomplete tight junctions in which gaps called intercellular clefts are found.  It is through the intercellular clefts that the that fluids and small solutes are allowed to pass out of the capillary. The endotheilial cytoplasm also has small vesicles that can transport fluids across the capillary wall.  The tight junctions are complete within the capillaries carrying the blood to the brain and constitute the structural basis of the blood brain barrier.

2) Fenestrated capillaries

a) Similar to continuous capillaries except that some of their endothelial cells have oval pores or fenestrations.  The fenestrations present allow these capillaries a greater permeability to fluids and small solutes.

b) They are found where active capillary absorption or filtrate formation occurs such as in the small intestines where fenestrated capillaries absorb digested nutrients, in endocrine glands that allow hormones rapid entry into the blood stream, and in the kidneys (with perpetually open pores) where rapid filtration of blood plasma is essential

3) Sinusoids

a) Sinusoidal capillaries (sinusoids) are highly modified "leaky" capillaries found in organs such as the liver, bone marrow,  lymphoid tissue, and some endocrine organs.

b) Their lumens are large and irregularly shaped and are fenestrated.

c) The endothelial lining has fewer tight junctions  and larger intercellular clefts which allows larger molecules (such as proteins) to pass between the blood and the surrounding tissue. 

d) In the liver the endothelium is discontinuous and large macrophages called Kupffer cells form part of the lining. In the spleen phagocytes  located outside the sinusoids stick cytoplasmic extensions through the intercellular clefts into the sinusoidal lumen  to get their "prey".

e) Blood flows very slow and "sluggishly" through the irregularly shaped lumens to allow time for the blood to be modified or processed.  The liver for example receives venous blood from the digestive organs and must have time to absorb the nutrients and remove any bacteria.

Capillary Beds

a) Capillary beds are regions where the arterial circulation (blood being carried away from the heart) is transformed into venous circulation (blood being carried back to the heart). Arteries divide into arterioles, arterioles into capillaries  which converge together to form venules and veins after exchange of materials have occurred.

b) A capillary bed consists of two types of vessels;

1) The vascular shunt (metarteriole) which is a short vessel that connects the arteriole and venule at opposite ends of the bed.  It is directly continuous with the thoroughfare channel (intermediate between a capillary and a venule) which joins the postcapillary venule.

2) True capillaries which are the actual exchange  vessels. They number 10 to 100 per capillary bed depending  on the organ or tissues served, and branch off the metarteriole (proximal end of the shunt) and return to the throughfare channel (the distal end of the shunt, but occasionally they spring from the terminal arteriole and empty directly into the venule.  Cuffs of smooth muscle fibers called precapillary sphincters surround the root of each true capillary at the metarteriole and acts as a valve to regulate the flow of blood.   Blood through a terminal arteriole may take one of two routes: through the true capillaries or through the shunt. When the precapillary sphincters are relaxes (open), blood flows through the true capillaries and takes part in exchanges with tissue cells.  When the sphincters are contracted (closed), blood flows through the shunts and bypasses the tissue cells.  Blood flow can be altered via the precapillary sphincters depending upon activities of the body: an increase in blood flow to muscles during exercise, or an increase in blood flow to digestive organs following a meal


--Blood is carried toward the heart from the thoroughfare channel into small postcapillary venules, into larger venules and eventullay into veins---


a) The smallest venules are the postcapillary venules and consist entirely of endothelium around which are a few pericytes. All venules are very porous and fluid and white blood cells move easily from the bloodstream through their walls. The larger venules have one or two layers of smooth muscle cells and thin externa.


a) Veins are formed by venules forming together. Veins usually have three tunics, but their walls are thinner and their lumen larger than arteries.

b) They have very little smooth muscle and elastin in the tunica media even in the largest of veins. The tunica externa is the largest layer (several times thicker than the tunica media )and consists of thick bundles of collagen fibers and elastic networks. Longitudinal bands of muscle are found in the tunica externa of the larger veins such as the vena cavae.

c) Veins can accomodate a large volume of blood due to their large lumens and thin walls. About 65% of the bodies total  blood supply at any one time is found in the veins, which is why veins are called capacitance vessels or blood reservoirs. The veins still only partially fill with blood.

d) Blood pressure is lower in our veins, so they are not in as much danger of bursting as are arteries even with their thin walls.  Veins have special adaptations that enable the blood to return to circulation at the same rate that it entered from the arteries.  One adaptation is the increased size of the lumen which offers less resistance to blood flow, and another are valves. Venous valves are formed from folds of the tunica interna and they resemble the semilunar valves of the heart. Venous valves are most abundant in the veins of the limbs where the upward flow of blood is opposed by gravity.  They are absent in the veins of the ventral body cavity.

e) Highly specialized flattened veins called venous sinuses such as the coronary sinus of the heart and the dural sinuses of the brains have thin walls composed of only endothelium.  They are supported by the tissues that surround them rather than by additional tunics. The dural sinuses for example which receive crebrospinal fluid and blood draining from the brain are reinforced by the dura mater that covers the brain surface.

Vascular Anastomoses

a) Anastomoses can occur in both the arterial and the venous system. Anastomoses are blood vessels formed from two or more blood vessels joining together.  An organ usually receives blood from more than one artery and when the arteries merge then it forms an arterial anastomoses.  Arterial anastomoses provide alternate pathways called collateral channels for blood to reach a given body region.  If one vessel is cut or blocked by a clot, the collateral channel will provide the organ with an adequate amount of blood.  Arterial anastomoses occur around joints where active movement may hinder blood flow through one channel. They are also common in abdominal organs, the heart, and the brain. Arteries that do not anastomose, or which have a poorly developed collateral circulation supply the retina, kidneys, and spleen. If their blood supply is interrupted cells supplied by such vessels die.

b) Examples of arteriovenous anastomoses are vascular shunts (metarteriole --thoroughfare channel that connects the arteriole with the venule at opposite ends of the capillary bed). Venous anastomoses are more common and much more abundant than arterial anatomoses, and occlusion of a vein rarely blocks blood flow or leads to tissue death (CABG grafts use veins from our legs without any change in circulation).

Physiology of Circulation

Blood Flow, Blood Pressure, and Resistance

a) Blood flow is the actual volume of blood flowing through a vessel, an organ, or the entire circulation at a given time.  Cardiac output is equal to the blood flow through the entire circulation at a given time and is relatively constant under resting conditions, but can change as our physical activity may change.  Blood flow through individual organs may vary at any given time.  We will be studying differences in blood flow through various organs.

b) Blood pressure (BP) is the force per unit area exerted on the wall of a blood vessel by the blood.  It is measured in mm of Hg (mercury). For example blood pressure equal to  120 mm Hg is equal to the pressure of a column of mercury 120 mm high.  When we talk about blood pressure we are speaking about the pressure of our blood in the largest arteries near the heart only, or systemic arterial pressure.  If one were to measure the blood pressure in the vessels as the blood flows from the heart from large arteries, to smaller arteries, arterioles and in to capillaries, we find that the pressure gradually decreases.  The reason our blood moves is because fluids move from a region of high to lower pressure. It's the gradual decrease in pressure that causes our blood to move.

c) Resistance is the opposition to the flow of blood due to the friction between the blood and the walls of the blood vessel. Resistance to blood flow can depend upon viscosity (how thick or sticky the blood is), length of blood vessels (the longer the blood vessel the greater the resistance), diameter of blood vessels (smaller the diameter the greater the resistance).

d) Blood flow is directly proportional to blood pressure (as blood pressure increases then blood flow increases), and is inversely proportional to peripheral resistance (as resistance increases then blood flow decreases). As blood is carried further from the heart it encounters the greatest resistance so we use the term peripheral resistance when we speak of blood flow.

Blood flow = difference in blood pressure/peripheral resistance

***When arterioles serving a particular organ dilate the resistance decreases and causes the blood flow to increase while the systemic pressure is unchanged or even  falling.

Arterial Blood Pressure

a) Remember when we speak of systemic blood pressure we are referring to the larger elastic arteries near the heart such as the aorta.  Pressure these arteries depend upon two factors: how much they are able to stretch, and the volume of blood flowing into them.  The less they stretch the greater the pressure like pinching a garden hose and the more the faucet is turned on in a garden hose the greater the pressure.  If a constant supply of blood were entering the aorta from the left ventricle without any interruption, then the arterial pressure would always be the same.  After the ventricle contracts there is a pause and the arterial pressure rises and falls because  of the elasticity of the vessels and the fact that the ventricle is repolarizing and the amount of blood entering the aorta decreases at that point.  Table 20.5 illustrates this showing an EKG, the high point of blood pressure is 120 (systolic pressure) when the ventricles contract and the aorta expands and the low point is 80 (diastolic pressure) when the ventricle is repolarizing and the atria relaxes.  This occurs in a regular fashion and is said to be pulsatile (pulsating or the presence of a pulse) 

b) The difference between the systolic and diastolic pressure is called pulse pressure.  For example in a person of normal blood pressure the pulse pressure would be equal to 40 mm Hg:

pulse pressure = 120 - 80 = 40

c) The mean (average) arterial pressure (MAP) is equal to the diastolic pressure plus 1/3 of the pulse pressure. So in a person of normal blood pressure MAP would be equal to 93 mm Hg:

MAP = 80   +   40/3 = 93

***The mean is ordinarily calculated by adding the numbers and dividing by the number of numbers in the list.  If one did it this way the MAP would be equal to 100 mm Hg (120 + 80/2 = 100).  We use the other formula because the diastolic pressure is being exerted for a longer period of time than the systolic pressure.

***MAP and pulse pressure decrease with an increasing distance from the heart until pulse pressure is phased out and there is an even steady flow of blood

Capillary Blood Pressure

a) From figure 20.5, by the time the blood reaches the capillaries, blood pressure has dropped to approximately 40 mm Hg and to 20 mm Hg or less by the end of the capillary beds.   The low pressure in our capillaries is desirable because capillaries are very fragile and higher pressures would rupture them, and it would increase the flow of solute containing fluids to the tissues too quickly.

Venous Blood Pressure

a) Venous pressure is steady and changes little during the cardiac cycle (20 mm Hg average pressure from the venules to the vena cava as opposed to an average pressure of 60 mm Hg from the aorta to the ends of the arterioles).  The difference in pressure can be observed when rapid spurts of blood are ejected from a severed artery as compared with the slow even flow of blood from a vein.  By the time the blood reaches our veins the blood has lost a lot of its energy due to the cumulative effects of peripheral resistance as heat.

b) Venous return of blood to the heart is aided in addition to large lumens and valves, by the respiratory and skeletal muscle pumps. The respiratory pump involves changes in pressure as we inhale air into our lungs. As we inhale abdominal pressure increases, squeezing the local veins, and since venous valves prevent the backflow of blood, the blood is forced upward toward  the heart.  At the same time, the pressure within the chest decreases, allowing thoracic veins to expand and speeding blood entry into the right atrium of the heart.  The muscular pump occurs when the skeletal muscles surrounding the deep veins contract and relax and "milk" the blood toward the heart, again preventing from flowing back because of the valves.  Skeletal muscle inactivity can cause people to have swollen ankles because the blood pools in their feet and legs such as people who stand on their feet a lot (hairdressers), and in people who are bedridden.

Monitoring Circulatory Efficiency

1) The Pulse

---usually taken in the wrist where the radial artery surfaces; Other pulse points are illustrated in figure 20.11 are also called pressure points which are commonly compressed to stop bleeding in certain areas of the body. The average pulse (resting heart rate) of a healthy individual is around 66 beats per minute -lying down-, 70 beats per minute-sitting up-, 80 beats per minute-standing up-, 140-180 beats per minute during vigorous exercise or emotional upset.

2) Blood Pressure 

---Blood pressure is measured using  the blood pressure cuff or shygmomanometer wrapped around the brachial artery. The cuff is tightened until the cuff pressure exceeds systolic pressure, and blood flow into the lower arm is stopped. As the cuff is gradually reduced, the examiner ausculates (listens) for sounds in the brachial artery using a stethoscope. The first soft tapping sounds heard as the pressure is reduced is the blood starting to spurt through the artery and is recorded as the systolic pressure. As the pressure is further reduced, these sounds called the sounds of Korotokoff, become louder and more distinct, but when the artery is no longer constricted and blood flows freely, the sounds can  no longer be heard.  The pressure at which the sounds disappear is recorded as the diastolic pressure.--

---Resting systolic pressure in normal adults varies between 110 and 140 mm Hg, and diastolic pressure between 75 and 80 mm Hg.  Blood pressure varies with age, sex, weight, race, and socioeconomic status.  Blood pressure varies with mood, physical activity, and posture.--

---Hypotension or low blood pressure is a systolic blood pressure below 100 mm Hg. Low blood pressure is often associated with long life and and old age free of illness.---

--- Elevations in blood pressure  can naturally occur during fever, exercise, and emotional upset.  Persistent high blood pressure or hypertension is common in obese people because the total length of their blood vessels is greater than in thinner individuals.  About 90% of hypertensive people have primary,or essential, hypertension, in which no underlying cause has been identified. The following factors are believed to be involved: 

1) Obesity

2) Age (around the age of 40)

3) Diet (Dietary factors that contribute to hypertension include high sodium, saturated fat, and cholesterol intake and deficiencies  in certain metal ions (K+, Ca2+, and Mg2+)

4) Race (more blacks than whites)

5) Heredity (Children of hypertensive families are twice as likely to develop hypertension as are children of normotensive parents)

6) Stress

7) Smoking ( Nicotine enhances the sympathetic nervous system's vasoconstrictor effects)

***Secondary hypertension (10 % of the cases) is due to identifiable disorders, such as excessive renin secretion by the kidneys, arteriosclerosis, and endocrine disorders such as hyperthyroidism and Cushing's disease. 


Blood Flow Through Body Tissues

---When the body is at rest, the brain receives about 13% ,of the total blood flow, the heart 4%, kidneys 20%, and the abdominal organs 24%, and skeletal muscle which makes up about half of the body mass receives about 20%.  During exercise, however, nearly  all of the increased cardiac  output flushes into the skeletal muscles and blood flow to the kidneys and digestive organs is actually reduced (figure 20.12).  Note that the total blood flow to the brain remains at about 750 ml/min during rest and exercise. Neurons are totally intolerant of ischemia (decreased blood flow).  Certain parts of the brain may receive more of that blood flow at certain times as in running.  The neurons in that part of the brain controlling the movement of your legs would receive more blood than other areas of the brain. 

Major Blood Vessels of the Body

1) Table 20.4 and Table 20.9 illustrate the major arteries and veins of the body respectively.

2)The notable  blood vessels above also include the following: 

---The Circle of Willis: An arterial anastomosis that encircles the pituitary gland and optic chiasma and unites the brain's anterior and posterior blood supplies.  It also acts to equalize blood pressure in the two brain areas and provides alternate routes for blood to reach the brain tissue if a carotid or vertebral artery becomes occluded (figure 20.20)----

---The brachial and radial arteries (pressure point for measuring pulse and blood pressure---

---The Great Saphenous Vein is the longest most superficial vein in the body. This is the vein which is used in CABG (coronary artery bypass grafts)