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20. 6. 2012.

Vascular system

The vascular system contains a series of vessels that transport blood from the heart to the tissues and back: the arteries, arterioles, capillaries, venules and veins.
Arteries are large muscular, elastic, conduit vessels for transporting blood away from the heart to the arterioles. The aorta is the major artery transporting blood from the left ventricle to all regions of the body, and it branches into smaller arteries that become progressively smaller, finally branching into arterioles. The arterioles are the site of greatest control of the circulation by the sympathetic nervous system, so arterioles are sometimes called resistance vessels.
From the arterioles, blood enters the capillaries, the narrowest vessels, with walls only one cell thick. Virtually all exchange between the blood and the tissues occurs at the capillaries. Blood leaves the capillaries to begin the return trip to the heart in venules, and the venules form larger vessels – the veins. The vena cava is the great vein transporting blood back to the right atrium from all regions to the body above(superior vena cava) and below(inferior vena cava) the heart.

Blood pressure

Blood pressure is the pressure exerted by the blood on the vessel walls, and the term usually refers to arterial blood pressure. It is expressed by two numbers: the systolic blood pressure(SBP) and the diastolic blood pressure(DBP). The higher number is the SBP; it represents the highest pressure in the artery that occurs during ventricular systole. Ventricular contraction pushes the blood through the arteries with tremendous force, and that force exerts high pressure on the arterial walls. The lower number is the DBP and represents the lowest pressure in the artery, corresponding to ventricular diastole when the ventricle is filling.
Mean arterial pressure(MAP) represents thee average pressure exerted by the blood as it travels through the arteries. Since diastole takes about twice as long as systole in a normal cardiac cycle, mean arterial pressure can be estimated from the DBP and SBP as follows:

MAP = 2/3 DBP + 1/3 SBP


MAP = DBP + [0.333 x (SBP – DBP)].

(SBP – DBP) is also called pulse pressure.
To illustrate, with a normal resting blood pressure of 120 mmHg over 80 mmHg, the MAP = 80 + [0.333 + (120-80)] = 93 mmHg.

General hemodynamics

Recall that the cardiovascular system is a continuous closed-loop system. Blood flows in this closed-loop system because of the pressure gradient that exists between the arterial and venous sides of the circulation. To understand regulation of blood flow to the tissues it is necessary to understand the intricate relationship between pressure, flow and resistance.
In order for blood to flow in a vessel there must be a pressure difference from one end of the vessel to the other end. Blood will flow from the region of the vessel with high pressure to the region of the vessel with low pressure. Alternatively, if there is no pressure difference across the vessel, there is no driving force and therefore no blood flow. In the circulatory system, the mean arterial pressure in the aorta is approximately 100 mmHg at rest, and the pressure in the right atrium is very close to 0 mmHg. Therefore, the pressure difference across the entire circulatory system is 100 mmHg – 0 mmHg = 100 mmHg.
The reason for the pressure differential from the arterial to the venous circulation is that the blood vessels themselves provide resistance or impedance to blood flow. The resistance that the vessel provides is largely dictated by the properties of the blood vessels and the blood itself. These properties include the length and radius of the blood vessel and the viscosity of thickness of the blood flowing through the vessel. Resistance to flow can be calculated as

resistance = [ἠL/r4]

where ἠ is the viscosity of the blood, L is the length of the vessel, and r is the radius of the vessel, which is raised to the fourth power.
Blood flow is proportional to the pressure difference across the system and is inversely proportional to resistance. This relationship can be illustrated by the following equation:

blood flow = ▲pressure/ resistance

Notice that blood flow can increase by either an increase in the pressure difference(▲pressure), a decrease in resistance, or a combination of the two. Altering resistance to control blood flow is much more advantageous because very small changes in blood vessel radius equate to large changes in resistance. This is due to the fourth-power mathematical relationship between vascular resistance and vessel radius.
Changes in vascular resistance are largely due to changes in blood vessel radius or diameter, as the viscosity of blood and the length of the vessels do not change significantly under normal conditions. Therefore, regulation of blood flow to organs is accomplished by small changes in blood vessel radius through vasoconstriction and vasodilatation. This allows the cardiovascular system to divert blood flow to the areas where it is needed most.
As mentioned already, most resistance to the blood flow occurs in the arterioles. Figure below shows the blood pressure changes across the entire vasculary system. The arterioles are responsible for -70% to 80% of the drop in mean arterial pressure across the entire cardiovascular system. This is important because small changes in arteriole radius can greatly affect the regulation of mean arteriole pressure and the local control of blood flow. At the capillary level, changes due to systole and diastole are no longer evident and the flow is smooth(laminar) rather than turbulent.

Distribution of blood

Distribution of blood to the various body tissues varies tremendously depending on the immediate needs of a specific tissue compared with that of other areas of the body. At rest under normal conditions, the most metabolically active tissues receive the greatest blood supply. The liver and kidneys combine to receive almost half the blood being circulated, and resting skeletal muscles receive only about 15% to 20%.
During exercise, blood is redirected to the areas where it is needed most. During heavy endurance exercise, muscles receive up to 80% or more of the available blood. This redistribution, along with increases in cardiac output, allows up to 25 times more blood flow to active muscles.

Similarly, after one eats a big meal, the digestive system receives more of the available cardiac output than when the digestive system is empty. Along the same lines, during increasing environmental heat stress, skin blood flow increases to a greater extent as the body attempts to maintain normal temperature. The cardiovascular system responds accordingly to redistribute blood, whether it is to the exercising muscle to match metabolism, for digestion, or to facilitate thermoregulation. These changes in the distribution of the cardiac output are controlled by the sympathetic nervous system, primarily by increasing or decreasing arteriolar diameter. These vessels have a strong muscular wall that can significantly after vessel diameter, are highly innervated by sympathetic nerves, and have the capacity to respond to local control mechanisms.

Intrinsic control of blood flow

Intrinsic control of blood distribution refers to the ability of the local tissues to vasodilate or vasoconstrict the arterioles that serve them and alter regional blood flow depending on the immediate needs of those tissues. With exercise and the increased metabolic demand of the exercising skeletal muscles, the arterioles undergo locally mediated vasodilation, opening up to allow more blood to enter that highly active tissue.
There are essentially three types of intrinsic control of blood flow. The strongest stimulus for the release of local vasodilating chemicals is metabolicm in particular an increased oxygen demand. As the tissue’s oxygen use increases, available oxygen is diminished. Local arterioles vasodilate to allow more blood, and thus more oxygen, to perfuse that area. Other chemical changes that can stimulate increased blood flow are decreases in other nutrients and increases in by-products(carbon dioxide, K+, H+, lactic acid) or inflammatory chemicals. Second, several vasodilating substances can be produced in the endothelium(inner lining) of arterioles and initiate vasodilation in the vascular smooth muscle of the arterioles. These substances include nitric oxide(NO), prostaglandins, and endothelium-derived hyperpolarizing factor(EDHF). These endothelium-derived vasodilators are important to the regulation of blood flow at rest and during exercise in humans, although the precise mechanisms and interplay between these vasodilators are still being studied. Finally, pressure changes within the vessels themselves can also cause vasodilation and vasoconstriction. This is reffered as the myogenic response. The vascular smooth muscle contracts in response to an increase in pressure across the vessel wall and relaxes in response to a decrease in pressure across the vessel wall. Additionally, acetylcholine and adenosine also have been proposed as potential vasodilators for the increase in muscle blood flow during exercise. Increased blood flow can either bring in needed substances such as oxygen, or clear out metabolic waste such as carbon dioxide, or usually both. Figure below illustrates the three types of intrinsic control of vascular tone.

Extrinsic neural control

This concept of intrinsic local control explains redistribution of blood within an organ or tissue mass; however, the cardiovascular system must divert blood flow to where it is needed, beginning at a site upstream of the local environment. Redistribution at the system or body level is controlled by neural mechanisms. This is known as extrinsic neural control of blood flow, because the control comes from outside the specific area(extrinsic) instead of from locally inside the tissues(intrinsic).
Blood flow to all body parts is regulated largely by sympathetic nervous system. The circular layers of smooth muscle within the artery and arteriole walls are supplied by sympathetic nerves. In most vessels, an increase in sympathetic nerve activity causes these muscle cells to contract, constricting blood vessels and thereby decreasing blood flow.
Under normal conditions, the sympathetic nerves transmit impulses continuously to the blood vessels, keeping the vessels moderately constricted to maintain adequate blood pressure. This state of tonic vasoconstriction is referred to as vasomotor tone. When sympathetic stimulation increases, further constriction of the blood vessels in a specific area decreases blood flow into that area and allows more blood to be distributed elsewhere. But if sympathetic stimulation decreases below the level needed to maintain tone, constriction of vessels in the area is lessened, so the vessels passively vasodilate, increasing blood flow into that area. Therefore, sympathetic stimulation will cause vasoconstriction in most vessels, but blood flow is altered by either increasing or decreasing the amount of vasoconstriction relative to normal vasomotor tone.

Distribution of venous blood

While flow to tissues is controlled by changes on the arterial side of the system, most of the blood volume normally resides in the venous side of the system. At rest, the blood volume is distributed among the vascular shown in the figure below. The venous system has a great capacity to hold blood volume. There is a little vascular smooth muscle in the veins, and they are very elastic and “balloon-like”. Thus, the venous system provides a large reservoir of blood available to be rapidly distributed back to the heart(venous return) and to the arterial circulation. This is accomplished through sympathetic stimulation of the venules and veins, which causes the vessels to constrict.

Integrative control of blood pressure

Blood pressure is normally maintained by reflexes from the autonomic nervous system. Specialized pressure sensors located in the aortic arch and the carotid arteries, called baroreceptors, are sensitive to changes in arterial pressure. They send information about the current blood pressure to the cardiovascular control centers in the brain where autonomic reflexes are initiated to respond to changes in blood pressure. For example, when blood pressure is elevated, the baroreceptors are stimulated by an increase in stretch. They relay this information to the cardiovascular control center in the brain. In response to the increased pressure there is a reflex increase in vagal tone, to decrease heart rate, and a decrease in sympathetic activity, which serves to normalize blood pressure. In response to a decrease in blood pressure, less stretch is sensed by the baroreceptors, and the response is to increase heart rate by vagal withdrawal and to increase sympathetic nervous activity, thus correcting the low-pressure signal.
There are also other specialized receptors, called chemoreceptors and mechanoreceptors, that send information about the chemical environment in the muscle and the length and tension of the muscle to the cardiovascular control centers. These receptors can also modify the blood pressure response and are especially important during exercise.

Return of blood to the heart

Because we spend so much time in an upright position, the cardiovascular system requires mechanical assistance to overcome the force of gravity when blood returns from the lower parts of the body to the heart. Three basic mechanisms assist in this process:
  • Valves in the veins
  • The muscle pump
  • The respiratory pump.

The veins contain valves that allow blood to flow in only one direction, thus preventing backflow and pooling of blood in the lower body. These venous valves also complement the action of the skeletal muscle pump, mechanical compression of the veins from rhythmic skeletal muscle contraction. This pushes blood volume in the veins back toward the heart. Finally, the changes in pressure in the abdominal and thoracic cavities during breathing assist blood return to the heart.

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