7- Introduction and functional anatomy of vascular physiology
The cardiovascular system consists of theheart, blood vessels, Blood, andthe lymphatic system. Lymphatic system consists of lymphatic vessels, lymph nodes, and lymph.
History:
The English physician William Harvey (1578 – 1657) demonstrated that the heart pumps blood through a closed system of vessels.
Functions of CVS:
1- The blood and lymph carry absorbed products of digestion to the liver. Then the blood transport of these substances which are essential for cellular metabolism.
2- The blood carries oxygen from the lungs to all the body cells.
3- Excrete metabolic waste products such as urea, excess water and ions in the urine. Carbon dioxide produced by cell is carried by blood to the lungs for elimination in the exhaled air.
4- It contributes in both hormonal and temperature regulation.
5- It protects against blood loss through clotting mechanism and foreign microbes by immune function.
Vascular system:
The blood vessels are a closed system, which carry blood from the heart to the tissue, and back to the heart.
The wall of blood vessels is made of:-
A- Inner layer includes endothelial and sub endothelial connective tissue (Intima).
B- Middle layer includes smooth muscles (Media).
C- Outer layer includes connective tissue (Adventitia). See figure (34).
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Figure (34): Structure of normal muscle artery(Ganong's review of medical physiology 2010).
Types of blood vessels:
1-Arteries: The arteries are thick-walled structures with extensive development of elastic tissue. They are stretched during systole and recoil during diastole, such property prevents an excessive rise in blood pressure (during systole) and excessive fall (during diastole). They carry blood from the heart mostly oxygenated (except pulmonary artery), under the highest pressure to tissues. The volume of blood in the arteries is called the stressed volume. The muscles of arteries are innervated by noradrenergic nerve fibers, in some instances by cholinergic fibers. The elasticity of arteries is decreased by aging process due to atherosclerosis.
2-Arterioles: They are the smallest branches of the arteries. Their walls have less elastic tissue but much more innervated by noradrenergic nerve fibers and some time by cholinergic nerve fibers, which constrict or dilate the vessels. They are the site of highest resistance to blood flow. They can alter blood flow to tissue because of their ability of changing their diameter.
3-Capillaries: The arterioles divide into smaller muscle-walled vessels called metarterioles and these in turn feed into capillaries. The openings of the capillaries are surrounded by minute smooth muscle (precapillary sphincters). The capillaries are thin-walled structures which are about 1 μm thick, lined with a single layer of endothelial cells. They are the site where nutrients, gases, water and solutes exchange between the blood and the tissues. They are about 5m in diameter at the arterial end and 9 m at venous end. The diameter of capillaries is just sufficient to permit red blood cells in single file; the total area of all capillary walls in the body are more than 6300 m² in the adult. See figure 35.
Figure (35): Microcirculation (Fox, 2006).
4- Venules and Veins: The venules collect blood from the capillaries and coalesce into larger veins. The walls of them are only slightly thicker than those of the capillaries. The veins have thin wall in comparism to the arteries, easily distended. The veins have a very large capacitance (capacity to hold blood). They contain the largest percentage of blood in the cardiovascular system (65-75% of circulating blood volume), figure37. The volume of blood in the veins is called the unstressed volume. The smooth muscle in the walls of the veins is innervated by sympathetic nerve fibers. The blood in veins is under lower pressure. The veins contain valves prevent retrograde flow. These valves function was first demonstrated by William Harvey in 17th century. No valves are present in the very small veins or the veins from the brain and viscera. The veins carry blood from tissue to right atrium and collect blood from lung to the left atrium.
Arteriovenous anastomosis: In the finger tips, palms of hand and ear lobes, there are short channels that connect arterioles to venules (arterio-venous anastomosis or shunt). These permit the passage of large molecules such as WBCs. See figure36.
Figure (36): The blood percentage in CVS (Fox 2006).
Neural innervations of the blood vessels:
Sympathetic nerves innervate the most blood vessels by release norepinephrines which bind to specific receptors on the smooth muscles called alpha receptors. Stimulation of the alpha receptors causes the smooth muscles to contract. Blood vessels supplying skeletal muscles have a different type of receptors called β2 receptors, when stimulated by norepinephrine cause the vessels to relax. Sympathetic vasodilator response plays a significant role in the exercise, serving to prime the skeletal muscles with oxygen and nutrient. Also skeletal muscles blood vessels do not appear to be innervated by parasympathetic neurons.
Lymphatic system
Formation of lymph: As fluid pressure increase in the interstitial space, this will open the valve at the terminal lymphatic capillary, so protein and fluid move to lymphatic vessel. Normally filtration of fluid out the capillaries is slightly greater than absorption of fluid into the capillaries. The excess filtered fluid and protein are return to circulation via lymph.
Function of lymph:
1- It transports interstitial fluid to the blood.
2- It transports of absorbed fat from small intestine.
3- Lymphocytes help to provide immunological defenses against infectious disease.
Lymph comes back to the circulation by following mechanisms:
1- Intrinsic rhythmic contraction at rate 10 – 15 per minutes.
2- Contractile filaments and valves in wall of lymphatic capillaries.
3- Contraction of surrounding skeletal muscles.
4- Movement of organs of the body.
5- Pulsation of arteries adjacent to the lymphatic vessels.
6- Interstitial fluid pressure.
7- One way flap valves which permit interstitial fluid to enter lymph vessels but not
leave it. Figure (37).
Figure (37): structure of lymphatic capillary (Guyton & Hall 2006).
Structures of lymphatic vessel:
A-Lymphatic capillaries:
Their diameter 10 – 50 µm, very thin wall, highly permeable to plasma proteins. The junction of Endothelial cells run very obliquely and may function like flap valves allowing fluid to enter but closing to prevent back flow.
B- Collecting or afferent lymphatic trunk.
C- Lymph nodes.
D- Efferent lymphatic trunk.
E- Major lymphatic duct.
All the lymph of body flow to the thoracic duct and right lymphatic duct which empty into venous system. Normal lymph flow is 2 – 3 L / day. The rate of lymph flow varies in different organs, highest in the GIT and the liver. Figure 38.
Figure (38): lymphatic pathway(Fox 2006) .