Blood Vessels

 

Blood vessels form the closed delivery system that begins and ends at the heart. They are dynamic structures that pulsate, constrict, and relax, and even proliferate.

1. General structure and function of vascular system.

There are three types of blood vessels: 1. arteries 2. capillaries 3. veins

Blood is forced into the large arteries leaving its ventricles. It then moves into successively smaller arteries, finally reaching their smallest branches, the arterioles, which feed into the capillary beds of body organs and tissues.

Blood draining from the capillaries is collected by venules, small veins that merge to form larger veins that ultimately empty into the great veins converging on the heart. There is about 60,000 miles of vessels within the adult body. Arteries and veins act simply as conduits for blood. Only the capillaries come into intimate contact with tissue cells and directly serve cellular needs.

2. Structure of vessel walls.

The three vessel types vary in length, diameter, and the relative thickness and tissue makeup of their walls.

The walls of all blood vessels except the capillaries are composed of three distinct layers, or tunics. The tunics surround a central blood-containing space called the lumen.

The inner most tunic, which is intimate contact with the blood, is the tunica intima. It contains the endothelium that lines the lumen of the vessel, and its flat cells fit closely together, forming a slick surface that minimizes friction.

The middle layer, the tunica media consists mostly of circularly arranged smooth muscle cells and elastic fibers. The activity of the vascular smooth muscle is regulated by vasomotor fibers of the sympathetic system. Depending on the needs of the body, the vasomotor fibers can cause either vasoconstriction or vasodilation. Since small changes in blood vessel diameter greatly influence blood flow and blood pressure, the activities of the tunica media are critical in regulating circulatory dynamics. The tunica media is usually the bulkiest layer in arteries.

The outer most layer is the tunica adventitia and is composed of loosely woven collagen fibers that protect the vessel and anchor it to surrounding structures.

3. Arterial system.

Normally arteries carry oxygenated blood and veins carry oxygen-poor blood, except for the pulmonary circulation which is reversed.

Arteries can be divided into three groups: 1. elastic arteries 2. muscular arteries 3. Arterioles

Elastic arteries are large, thick-walled vessels near the heart, such as the aorta and its major branches. They are the largest and most elastic. Their large-diameter lumen allows them to serve as low-resistance conduits. The elastic arteries contain more elastin than any other vessel type. The abundance of elastin enables these arteries to withstand and smooth out large pressure fluctuations by expanding when the heart forces blood into them, and then recoiling to propel blood onward into the circulation when the heart relaxes. The alternating expansion and recoil of elastic arteries during each cardiac cycle creates a pressure wave called a pulse. The arterial pulse rate reflects heart rate.

Muscular arteries, (distributing arteries), deliver blood to specific body organs and account for most of the named arteries. Their diameter ranges from that of a little finger to that of a pencil lead. They have the thickest tunica media of all vessels. Their tunica media contains relatively more smooth muscle and less elastic tissue. They are more active in vasoconstriction and are less distensible.

Arterioles have a lumen smaller than 0. 3 mm and are the smallest of the arterial vessels. The largest of the arterioles exhibit all three tunics, but the tunica media is chiefly smooth muscle with few elastic fibers. Blood flow into the capillary beds is determined by alterations in arteriole diameter in response to changing neural stimuli and local chemical influences. When arterioles constrict the tissues served are largely bypassed. When arterioles dilate, blood flow into the local capillaries increases dramatically.

4. Capillary structure and function.

Capillaries are the smallest blood vessels. Their exceedingly thin walls consist of just a thin tunica intima. The average capillary length is 1 mm and the average lumen diameter is 8-1 0 μm just large enough for RBC's to slip through. Given their location and the thinness of their walls, capillaries are ideally suited for their role in exchange of materials between the blood and interstitial fluid.

Structurally, capillaries are classified as: 1. continuous 2. Fenestrated

Continuous capillaries are abundant in the skin and muscles. They are continuous in the sense that their endothelial cells provide an uninterrupted lining.

Fenestrated capillaries are essentially similar to the continuous variety except that some of the endothelial cells are riddled with oval pores or fenestrations (windows). The fenestrations are usually covered by a very delicate membrane or diaphragm, so this variety has greater permeability to fluids and small solutes. They are found where active capillary absorption occurs, such as in the small intestines, and in endocrine organs which allow hormones to gain rapid entry into the blood. Fenestrated capillaries with perpetually open pores occur in the kidneys, where rapid filtration is essential. Highly modified, leaky capillaries (sinusoids) connect the arterioles and venules in the liver, bone marrow, lymphoid tissues, and some endocrine glands. Sinusoids have large, irregularly shaped lumens, and are usually fenestrated. These modifications allow large molecules to pass between the blood and surrounding tissues. Blood flows sluggishly through the sinusoid channels, allowing time for it to be processed or modified in various ways (absorbing nutrients, removing and destroying microbes).

Capillaries do not function independently; they tend to form interweaving networks called capillary beds. In most body regions, a capillary bed consists of two types of vessels:

1. vascular shunt (metarteriole-thoroughfare channel) - short vessel that directly connects the arteriole and venule at opposite ends of the bed.

2. true capillary

The terminal arteriole feeding the bed leads into a metarteriole which is directly continuous with the thoroughfare channel. The channel, in turn , joins the postcapillary venule that drains the bed. Typically, a cuff of smooth muscle (precapillary sphincter), surrounds the root of each true capillary at the metarteriole and acts as a valve to regulate the flow of blood into the capillary. Blood flowing through a terminal arteriole may take one of two routes:

1. through the true capillaries

2. through the shunt

Blood flowing through true capillaries takes part in exchanges with tissue cells, whereas blood flowing through shunts bypasses the tissue cells. The relative amount of blood flowing into the true capillaries is regulated by vasomotor nerve fibers and local chemical conditions. The capillary beds may be flooded with blood or almost completely bypassed.

5. Venous system

Blood is carried from the capillary beds toward the heart by veins. The vessels increase in diameter, and their walls gradually thicken as they progress from venules to the larger and larger veins. Venules range are about 8-100μm in diameter, and are formed when capillaries unite. The smallest venules (postcapillary venules) consist entirely of endothelium around which a few fibroblasts congregate. The venules are extremely porous, and inflammatory fluid and WBC's move easily from the blood stream through their walls. Venules join to form veins, which usually have three distinct tunics, but their walls are always thinner and their lumens larger than those of corresponding arteries. Veins are usually collapsed, and their lumens appear slit-like in routine tissue preparations. The tunica media tends to be thin and there is little smooth muscle or elastin.

In the vena cava, the tunica adventitia is further thickened by longitudinal bands of smooth muscle.

With their large lumens and thin walls, veins can accommodate a fairly large blood volume. Up to 65% of the body's total blood supply is found in the veins at any time. Even so, veins are normally only partially filled with blood.

Blood pressure within the veins is low and some special adaptations that help return blood to the heart must be made. The large diameter lumens are one structural adaptation. Another is the presence of valves that prevent blood from flowing backward. Valves are most abundant in the veins of the limbs. They are absent in veins of the ventral body cavity.

6. Structure and function of vascular anastomoses

Where vascular channels unite or interconnect, they form anastomoses. Most organs receive blood from more than one arterial branch, and nearby arteries supplying the same territory often merge, forming arterial anastomoses, which permits free communication between vessels involved and provide alternate pathways for blood to reach a given body region. Arterial anastomoses are abundant in the abdominal organs and around joints, where active movement may hinder blood flow through one channel.

Veins interconnect much more freely than arteries, and as a result, occlusion of venous channels rarely blocks blood flow or leads to tissue death.

7. Physiology of circulation

The heart is the pump, the arteries are conduits, the arterioles are resistance vessels, the capillaries are exchange sites, and the veins are blood reservoirs.

Blood flow is the actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period of time. For the entire vascular system, blood flow is equivalent to cardiac output. Blood flow through individual body organs may vary widely and is intimately related to their immediate needs.

 

Blood pressure is the force per unit area exerted on the wall of a blood vessel by its contained blood. Differences in pressure within the vascular system provide the immediate driving force that keeps blood moving through the system. The term blood pressure means systemic arterial blood pressure in the largest arteries near the heart, and it is expressed in terms of millimeters of mercury.

Peripheral resistance is opposition to flow and is a measure of the amount of friction the blood encounters as it passes through the vessels. Most friction is encountered in the peripheral circulation.

There are three important sources of resistance:

1. blood viscosity

2. vessel length

3. vessel diameter

Blood viscosity is the internal resistance to flow and is related to the thickness or stickiness of a fluid. The greater the viscosity, the less easily molecules slide past one another. The longer the vessel, the greater the resistance. Since viscosity and vessel length are normally unchanging, these factors can be considered constant.

Blood vessel diameter changes often and is a very important factor in altering peripheral resistance. Fluid close to the walls of a tube or channel is slowed by friction as it passes along the wall, whereas fluid in the center of the channel flows more freely and faster. The smaller the tube, the greater the friction because relatively more fluid contacts the tube walls. Because arterioles are small diameter vessels and can enlarge or constrict in response to neural and chemical controls, they are the major determinants of peripheral resistance.

8. Systemic blood pressure

Any fluid driven by a pump through a circuit of closed channels operates under pressure, and the nearer the fluid is to the pump, the greater the pressure it is under. Blood flows through the blood vessels along a pressure gradient, always moving from higher to lower pressure areas. The pumping action of the heart generates blood flow. Pressure results when flow is opposed by resistance.

Blood pressure in the elastic arteries close to the heart reflects two factors: 1. how much those arteries can be stretched 2. the volume of blood forced into them at any time

Blood pressure changes in a regular fashion in the elastic arteries near the heart, and blood flow within them is pulsatile. As the left ventricle contracts and expels blood into the aorta, it stretches the elastic walls of the aorta causing aortic pressure to reach its peak. This pressure peak is called the systolic arterial blood pressure and averages about 120 mm Hg. During diastole which occurs during ventricular relaxation, closure of the semilunar valve prevents blood from flowing back into the heart, and the walls of the aorta recoil, maintaining continuous pressure on the reducing blood volume. During this time, aortic pressure drops to its lowest level (diastolic pressure).

Systemic blood pressure is highest in the aorta and declines throughout the length of the pathway to finally reach 0mm Hg in the right atrium. The steepest change in pressure occurs in the arterioles, which offer the greatest resistance to blood flow. So long as a pressure gradient exits, blood flow continues until it completes the circuit back to the heart.

Venous blood pressure is steady and changes very little during the cardiac cycle.

The main factors influencing blood pressure are:

1. cardiac output

2. peripheral resistance

3. blood volume

blood pressure = cardiac output X peripheral resistance

Cardiac output (ml/min) = stroke volume (ml/beat) X heart rate (beats/min)

Cardiac output is about 5.5L/min

 

Blood vessel diameter changes are the most important influence on pressure and blood flow patterns. Very small changes in diameter can produce substantial changes in resistance and blood pressure, because resistance varies inversely with the fourth power of vessel radius (1/2 the diameter).

This means if the radius of a vessel is doubled, the resistance is then 1/16 as much (r4 = 2x2x2x2 = 16 and

1 /r4 = 1/ 16)

When arterioles constrict blood backs up in the arteries and blood pressure increases. Blood pressure varies directly with the amount of blood in the vascular system. Blood volume varies with age and gender and is usually maintained at about 5 liters in adults.

9. Control of blood pressure

Maintaining a steady flow of blood from the head to the toes is vital for proper organ function. Blood pressure is regulated by neural, chemical and renal controls.

Neural controls of blood vessels are directed primarily at:

1. altering blood distribution

2. maintaining adequate systemic blood pressure

Under conditions of low blood volume, all vessels except those supplying the heart and brain are constricted.

The nervous system controls blood pressure and distribution by altering the diameter of arterioles.

The sympathetic nervous system enervates the smooth muscle layer of blood vessels (arterioles) via N.E. a potent vasoconstrictor.

Baroreceptors are mechanoreceptors that detect changes in arterial pressure. They are located in the carotid sinuses and the aortic arch plus in nearly every large artery of the neck and thorax. When pressure rises and stretches these receptors, they send signals to the vasomotor center which results in vasodilation and decline in pressure.

Signals also reach the cardiac inhibitory center in the medulla leading to a reduction in heart rate and force. Decreased pressure leads to the opposite responses.

When the oxygen content or pH of the blood drops sharply, chemoreceptors in the aortic arch and large arteries of the neck transmit impulses to the vasomotor center, and reflex vasoconstriction occurs. The rise in blood pressure helps speed blood return to the heart and then to the lungs. The most prominent of these receptors is the carotid and aortic bodies.

During periods of stress, the adrenal medulla releases norepinephrine and epinephrine into the blood which enhances the sympathetic response.

ADH produced by the hypothalamus stimulates the kidneys to conserve water. ADH is released in greater amounts when blood pressure falls to dangerously low levels and helps to restore arterial pressure by causing intense vasoconstriction.

The kidneys act directly and indirectly to regulate arterial pressure and provide the major long-term mechanism of blood pressure control: 1. direct - control water retention 2. indirect - release renin which results in the formation of angiotensin, a powerful vasoconstrictor.

 

 

10. Variations from normal blood pressure

Hypotension: Low blood pressure; generally considered to be a systolic pressure below 100mm Hg. Hypotension simply reflects individual variations and is no cause for concern. Often associated with long life and old age free of illness. Elderly people are prone to orthostatic hypotension - temporary low blood pressure and dizziness when they rise suddenly from a reclining or sitting position. Blood pools in the lower extremities. Chronic hypotension may hint at poor nutrition because the poorly nourished are often anemic and have inadequate levels of plasma proteins.

Hypertension: Transient elevations occur as normal adaptations during fever, physical exercise, and emotional upset. Persistent hypertension is common in obese people because of the total length of their blood vessels is relatively greater than that in thinner people. Chronic hypertension is a common and dangerous disease that warns of 'increased peripheral resistance. Although hypertension is usually asymptomatic for the first 10-20 years, it slowly strains the heart and damages the arteries (silent killer). Prolonged hypertension is the major cause of heart failure, vascular disease, renal failure and strokes.

Because the heart has to work harder, the myocardium enlarges, the heart weakens and its walls become flabby. The vessels also develop small tears 'in the endothelium and accelerate the progress of atherosclerosis. As the vessels become increasingly blocked, blood flow to the tissues becomes inadequate, and vascular complications of vessels 'in the brain, retinas heart and kidneys begin to appear.

Hypertension is defined physiologically as a condition of sustained elevated arterial pressure of 140/90 or higher. As a general rule, elevated diastolic pressures are more significant medically, because they always indicate progressive occlusion and/or hardening of the arterial tree.

About 90% of hypertensive people have "primary or essential" hypertension which cannot be assigned to any specific organic cause. Factors such as diet, obesity, heredity, race, and stress are believed to be involved. Clinical signs usually appear after the age of 40. Dietary factors include high sodium, saturated fat, and cholesterol and deficiencies in certain metal ions (potassium, calcium, magnesium). Factors that most accurately predict risk are the seventy of blood pressure elevation, blood cholesterol levels, cigarette smoking, presence of diabetes mellitus, and stress levels.

Primary hypertension cannot be cured. Most can be controlled by diet, losing weight, and antihypertensive drugs.

Secondary hypertension accounts for 10% of 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. Treatment is directed at the underlying disorder.

 

11. Physiology of blood flow.

The velocity of blood flow changes as blood travels through the systemic circulation. It is fastest in the aorta and other large arteries and slowest in the capillaries. Velocity is inversely related to the cross-sectional area of the vessel to be filled. Blood flows fastest where the total cross-sectional area is least. As the arterial system continues to branch the total cross-sectional area of the vascular bed increases, and velocity of blood flow declines proportionally.

12. Autoregulation of localized blood flow

Automatic adjustment of blood flow to each tissue in proportion to its requirements at any point in time. Regulated by local conditions and is largely independent of systemic factors. Since mean arterial pressure is identical throughout the body, changes in blood flow through individual organs are controlled intrinsically by modifying the diameter of local arterioles feeding the capillaries. Thus organs regulate their own blood flow by varying resistance of their arterioles. In most tissues, declining levels of nutrients, particularly oxygen, are the strongest stimuli for autoregulation.

 

In the brain, a localized increase in carbon dioxide (< pH) is an even more potent trigger.

Local physical factors are also important autoregulatory stimuli. Vascular smooth muscle responds directly to passive stretch and causes vasoconstriction. Reduced stretch promotes vasodilation. Such responses to changing volume entering an arteriole are called myogenic responses. Generally both chemical and physical factors determine the final autoregulatory response of a tissue.

The net result of autoregulation is immediate vasodilation of the arterioles serving the capillary beds of needy tissues, and blood flow to the area is temporarily increased. This is accompanied by relaxation of precapillary sphincters allowing blood to surge through the true capillaries.

Autoregulation in the brain, heart, and kidneys is extraordinarily efficient. Adequate perfusion is maintained even in the face of fluctuating mean arterial pressure.

If the nutrient requirements of a tissue are more than the short-term autoregulatory mechanism can easily supply, a long-term autoregulatory mechanism may evolve over a period of weeks or months to enhance blood flow. The number of vessels in the region increases, and the existing vessels enlarge. This is common in the heart when a coronary vessel is partially occluded; it occurs throughout the body in people who live 'in high-altitude areas.

13. Blood flow in special areas

Each organ has special requirements and functions that are revealed in its pattern of autoregulation.

Skeletal muscles: extremely changeable and varies with the degree of muscle activity. Resting skeletal muscles receive about 1 liter of blood per minute, and only about 25% of their capillaries are open. During this time, myogenic and general neural mechanisms predominate. When muscles become active, blood flow increases in direct proportion to their greater metabolic activity (active or exercise hyperemia). During vigorous exertion, blood flow can increase tenfold or more, and virtually all capillaries are open. Autoregulation occurs almost entirely in response to the decreased oxygen concentration. Systemic adjustments also occur to ensure that more blood reaches the muscles. Vasoconstriction of vessels in the digestive viscera and skin diverts blood away from these areas temporarily.

Brain: total blood flow to the brain averages about 750 ml/min and is maintained at relatively constant levels. Cerebral blood flow is regulated by one of the most precise autoregulatory systems in the body and is tailored to local neuronal need. Brain tissue is exceptionally sensitive to declining pH and increased carbon dioxide. Oxygen deficits are much less potent stimulus for autoregulation. Greatly excessive carbon dioxide levels abolish autoregulatory mechanisms and severely depress brain activity. The brain also has myogenic mechanisms that protect it from possible damaging changes in blood pressure. Fainting occurs when mean arterial pressure falls below 60 mm Hg and cerebral edema is the usual result of pressures over 160mm Hg.

Lungs: blood flow through the lungs is unusual in many ways. The pathway is short, and the arteries and arterioles are similar to veins and venules in structure (thin walls, large lumens). There is little resistance to blood flow, and less pressure is needed. Arterial pressure in the pulmonary circulation is much lower (about 25/10 versus 120/80).

The autoregulatory mechanism is exactly opposite that seen in most tissues (low oxygen causes vasoconstriction). When the air sacs of the lungs are flooded with oxygen-rich air, the pulmonary capillaries become flushed with blood and ready to receive the oxygen. If the air sacs are blocked, the oxygen content in those areas will be low, and blood will largely bypass those areas.

Heart: movement of blood through the smaller vessels of the heart is influenced by aortic pressure and the pumping activity of the ventricles. When the ventricles contract, the coronary vessels are compressed, and blood flow stops. As the heart relaxes, the high aortic pressure forces blood through the circulation. Under resting conditions, blood flow through the heart is about 250ml/min. During strenuous exercise, the coronary vessels dilated in response to a local accumulation of ADP and carbon dioxide, and blood flow can increase 3 to 4 times. This increased blood flow is important because cardiac cells use as much as 65% of the oxygen carried to them in blood under resting conditions (most other tissue use 25%). Increased blood flow is the only way to make sufficient oxygen available.

14. Blood flow through capillaries.

Capillaries: blood flow is slow and intermittent, rather than steady (vasomotion), and reflects the "on-off' opening and closing of the precapillary sphincters.

Fluid movements: fluid (water + solutes) is forced out of the capillaries through the clefts at the arterial end of the bed, but most is returned at the venous end.

Hydrostatic pressure: force exerted by fluid pressing against a wall. In capillaries, hydrostatic pressure is the same as capillary blood pressure -- pressure of blood against the capillary wall. Capillary hydrostatic pressure is also called filtration pressure because it forces fluids through the capillary walls.

In theory, blood pressure -- which forces fluid out of the capillaries -is opposed by hydrostatic pressure of the interstitial fluid (back pressure).

Osmotic pressure: created by the presence in a fluid of large nondiffusible molecules, such as plasma proteins. Such substances draw water toward them. Osmotic pressure does not vary form one end of the capillary bed to the other.

At any point along a capillary, fluids will leave the capillary if net hydrostatic pressure is greater than net osmotic pressure, and fluids will enter if net osmotic pressure exceeds net hydrostatic pressure.

Hydrostatic forces dominate at the arterial end while osmotic forces dominated at the venous end. Thus, net fluid flows out of the circulation at the arterial ends of the capillary beds and into the blood stream at the venous end.

More fluid enters the tissue spaces than is returned to the blood. This fluid and any leaked proteins are picked up by the lymphatic vessels.

15. Forms of circulatory shock

Circulatory shock: Condition in which blood vessels are inadequately filled and blood cannot circulate normally. If this condition persists, cell death and organ damage occurs.

Hypovolemic shock is the most common form of shock. It results from large-scale blood loss such as might follow acute hemorrhage, severe vomiting or diarrhea, or extensive bums. If blood volume drops rapidly, heart rate increases, creating a weak, "thready" pulse. Intense vasoconstriction alsooccurs moving blood away from blood reservoirs into the major circulatory channels. Blood pressure is stable at first, but eventually drops if blood volume loss continues. The key to management is fluid replacement as quickly as possible.

Vascular shock - blood volume is normal and constant. Poor circulation results from extreme vasodilation that leads to an abnormal expansion of the vascular bed. The huge drop in peripheral resistance is revealed by rapidly falling blood pressure. The most common causes are loss of vasomotor tone due to failure of autonomic nervous system regulation and septicemia.

Cardiogenic shock, or pump failure occurs when the heart is so inefficient that it cannot sustain adequate circulation. Its usually cause is myocardial damage from numerous myocardial infarcts.