REGULATION OF BLOOD FLOW.

1. REGULATION OF BLOOD FLOW.

2. Common characteristic of blood flow • The direct effect of heart contraction is creation of certain level of blood pressure, which is allows blood circulation. • Blood flow is continuous, although heart pumps the blood by separate portions. It caused by functioning of all components of cardio-vascular system: heart, arteries, arterioles, capillaries, venuls and veins. Besides that continuous blood flow is caused by extracardial factors as skeletal muscle contraction and pressure gradient between abdominal and thoracic cavities. Cardiac output depends on high, mass and area of human body surface. Cardiac output is regulated by contractive activity of cardiac muscle; valve function of full value; blood volume, vascular tonus, blood flow in capillaries; value of blood returning to the heart. In general distribution of cardiac output between different organs corresponds to its functional activity. Part of common blood supply, which every organ gets, depends on necessity in O2 and substrates of energy exchange. Average time of blood circulation measures 20-23 s.

3. Powers, which causes blood flow • Blood flows in vessels from high pressure to low. Heart pumping causes initial pressure. The highest pressure is large arteries ascending from the heart. Pressure in aorta at the end of systole is 110-125 mm Hg, at the end of diastole - 70-80 mm Hg. In pulmonary trunk during systole the blood pressure is 20-25 mm Hg, in diastole - 10-15 mm Hg. In large arteries blood flow velocity is 0.1-0.2 m/s. In large veins returning blood flow to the heart is caused by lowest blood pressure - 0 mm Hg.

4. Functional importance of blood circulation system. • Both pulmonary and systemic circulation, compose entire system of blood circulation and function in correlation. The right ventricle is responsible for blood pumping into pulmonary circulation. Here blood is oxygenated and CO2 is taken out. The left ventricle pumps blood into systemic circulation. Blood flow in this part of vascular system provides performing of all other blood functions as regulatory, protective, excretory and others. Both right and left parts of heart pump equal portions of blood into corresponding vessels and function in interconnection to each other. The minute blood volume in pulmonary and systemic circulation is the same.

5. Circulating blood volume • Blood flowing in vessels is similar to stream of fluid in the pipe, but has a lot of specificities. Fluid stream in the pipe is described by formula: • Q= (P1-P2)/R, where • Q - fluid volume, • P1 - pressure in the beginning of the pipe, • P2 - pressure in the end of the pipe, • R - peripheral resistance of the pipe. • So fluid volume, which flows through the pipe is directly proportional to pressure difference from the end to beginning of pipe; and inversely proportional to peripheral resistance of pipe. As vessels have elastic walls, the blood flow in it, is differ from the same in pipe. Vessel cross-section may change due to neural and endocrine influences according to necessity. • Blood volume flowing through every part of vascular system per time unit is the same. It means that through aorta or cross-section of all arteries, capillaries or veins flows equal volume of blood. This volume per minute is called minute blood volume and measures in adults in rest 4.0 - 6.5 l/min.

6. Peripheral resistance of vessels • Peripheral resistance in vessels according to Poiseuille's formula depends on length of vessels (l), viscosity of blood (η) and cross-section of vessel (r): • R= 8lŋ/πr. • In accordance to this formula the highest peripheral resistance might be in the smallest vessels. In reality the highest resistance is observed in arterioles. Average blood flow resistance in adults is equal to 900-2500 din·s/sm5

7. Paradoxes of blood flow. • In capillaries blood flow resistance is a bit lower because of such mechanism. In capillaries blood cells move one after another, dividing only by plasma, which decreases friction between blood cells and capillary wall. On other side, capillaries are shorter, than arterioles, which caused lower blood flow resistance too. • Viscosity of blood is also important for resistance of vessels. It depends on quantity of blood cells, protein rate in plasma, especially globulins and fibrinogen. Considerable increase of blood viscosity may cause lower blood returning to the heart and than disorders of blood circulation. • In large arteries centralization of blood flow is observed. Blood cells moves in the central part of blood stream, and plasma is peripheral. Instead increase of blood viscosity in arterioles is caused by higher friction between cells and vessels wall.

8. Linear velocity of blood flow • Blood flow also is characterized by linear velocity of blood circulation: • V=Q/πr2, where • V - linear velocity, • Q - blood volume, • r - radius of vessel. • So it is clear the wider cross-section of vessel the slower linear velocity of blood stream. In large arteries linear velocity is highest (0.1-0.2 m/s). In arterioles it measures 0.002 - 0.003 m/s, in capillaries - near 0.0003 m/s. In veins cross-section decreases and linear velocity increases to 0.001 - 0.05 m/s in large veins and to 0.1 - 0.15 in vena cava.

9. Blood pressure • Transversal pressure - is difference between pressure inside the vessel and squeeze of it from the tissues. When increasing the tissue pressure to vessel wall, it closes. Hydrostatic pressure is corresponding to weight of all blood in vessel when it has vertical position. For vessels of head and neck this pressure decreases towards the heart. For vessels of limbs it has outward direction. That is why hydrodynamic pressure in vessels over heart is decreased due to hydrostatical pressure. Below heart hydrodynamic pressure is increased, because it is summarized with hydrodynamic pressure.

10. Role of changing body position • In horizontal position of the body hydrostatical pressure is equal in every part of the body and hydrodynamic pressure doesn't depend on it. In vertical position transversal pressure in vessels of limbs creates tension of vessels walls (Laplas low): • Pt=F/r, where • Pt – transversal pressure, • F - vessel tension, • r - radius of vessel. • So it is shown the smaller radius of vessel, the lower tension in vessels walls. Due to this capillaries with thinnest wall don't crush because of its smallest diameter. Existence of precapillary sphincters permits proper direction of blood pressure so that capillaries may close (plasmatic capillaries).

11. Local regulatory mechanisms • Collagen fibers of vessels walls form net, which prevent its tension or decrease tone. Smooth muscle cells combine with elastic and collagen fibers in vessels walls. Contracting and stretching these fibers smooth muscle cells produce active tension of vessel wall - tonus of vessels. • There are some mechanisms in regulation of vessel tonus by smooth muscles. When rapid increasing of blood pressure, smooth muscles contract and decrease tension by decreasing vessel diameter. In slow rising of pressure tension decrease by dilation of smooth muscles and increase vessel diameter. These mechanisms occur more often in veins than in arteries. Veins have equal elasticity in systemic and pulmonary circulation, but arteries are more extensible in pulmonary circulation. Arteries in pulmonary circulation contain a lot of elastic and smooth muscle fibers.

12. Functional types of vessels • - Elastic (damping) vessels. Large arteries belong to this group. The main function of these vessels is to turn ejection of blood into continuous blood flow. It is possible due to elastic properties of its wall; • - Resistive vessels are arterioles, precapillary sphincters and venuls. These vessels may regulate the blood flow in capillaries by changing their tonus; • - Exchange vessels are capillaries. Their walls due to the special structure permit exchange of materials between blood and tissues; • -Capacitive vessels are veins. To sure one-way direction of blood flow veins have valves if lying below the heart. Veins contain 75-80 % of circulating blood. Veins of skin and abdominal cavity may function as depot of blood.

20. Local regulation of blood flow • Role of metabolic factors: Greater rate of metabolism or less blood flow causes decreasing O2 supply and other nutrients. Therefore rate of formation vasodilator substances (CO2, lactic acid, adenosine, histamine, K+ and H+) rises. When decreasing both blood flow and oxygen supply smooth muscle in precapillary sphincter dilate, and blood flow increases. In is a vasodilator substance as nitric oxide released from endothelial cells released from endothelial cells. It causes secondary dilation of large arteries when micro vascular blood flow increases. Cardiac muscle utilizes fatty acids for energy. Cardiac muscle utilizes glucose through glycolisis that results in formation of lactic acid.

21. Basal tone of vessels. • When arterial pressure suddenly increases local blood flow tends to increase. It leads to sudden stretch of arterioles cause smooth muscles in their wall to contract. Than local blood flow decreases to normal level. Vessel walls are capable to prolonged tonic contraction without tiredness even at rest. Such a condition is supported by spontaneous myogenic activity of smooth muscles and efferent impulsation from autonomic nerve centers, which control arterial pressure. Partial state of contraction in blood vessels caused by continual slow firing of vasoconstrictor area is called vasculomotor tone. Due to regulatory nerve and humoral influences this basal ton changes according to functional needs of curtain organ.

22. Neuro-humoral regulation of systemic circulation • a) Afferent link. Nerve receptors, which are capable react to changing blood pressure, lays in heart cameras, aorta arc, bifurcation of large vessels as carotid sinus and other parts of vascular system. Irritation of these mechanical receptors produce nerve impulses, which pass to higher nerve centers for processing sensory information from visceral organs.

23. b) Central link. Vasoconstrictor area of vasculomotor center is located bilaterally in dorsolateral portion of reticular substance in upper medulla oblongata and lower pons. Its neurons secrete norepinephrine, excite vasoconstrictor nerves and increase blood pressure. It transmits also excitatory signals through sympathetic fibers to heart to increase its rate and contractility. • Vasodilator area is located bilaterally in ventromedial of reticular substance in upper medulla oblongata and lower pons. Its neurons inhibit dorsolateral portion and decrease blood pressure. It transmits also inhibitory signals through parasympathetic vagal fibers to heart to decrease its rate and contractility. • Posterolateral portions of hypothalamus cause excitation of vasomotor center. Anterior part of hypothalamus can cause mild inhibition of one. Motor cortex excites vasomotor center. Anterior temporal lobe, orbital areas of frontal cortex, cingulated gyrus, amygdale, septum and hippocampus can also control vasomotor center.

24. c) Efferent link. Stimulation of sympathetic vasoconstrictor fibers through alfa-adrenoreceptor causes constriction of blood vessels. Stimulation of sympathetic vasodilator fibers through beta-adrenoreceptors as in skeletal muscles causes dilation of vessels. • Parasympathetic nervous system has minor role and gives peripheral innervations for vessels of tong, salivatory glands and sexual organs.

25. Mechanical receptors reflexes. • These are spray-type nerve endings, which are stimulated by stretch. In increasing blood pressure, from the wall of carotid sinus impulses pass through Hering's nerve to glossopharyngeal nerve to solitary tract in medulla. Secondary signals inhibit vasoconstrictor center and excite vagal center. It results in peripheral vasodilatation and decreasing heartbeat. When arterial pressure decreases whole processes lead to exciting dorsolateral portion of vasomotor center and increasing blood pressure and heartbeat. Similar reflex mechanism starts from receptors of aortic arc. • Bainbridge reflex is observed when arterial pressure increases due to increasing blood volume and blood return. Atria and SA node are stretched and send nerve signals to vasomotor center. Increasing heart rate and heart contractility prevent damming up of blood in pulmonary circulation.

26. Reflexes from proprio-, termo- and interoreceptors • Contraction of skeletal muscle during exercise compress blood vessels, translocate blood from peripheral vessels into heart, increase cardiac output and increase arterial pressure. Stimulation of termoreceptors cause spreading impulses from somatic sensory neurons to autonomic nerve centers and so leads to changing tissue blood supply. Irritation of visceroreceptors results in stimulation of vagal nuclei, which cause decreasing blood pressure and heartbeat.

27. Haemodinamic in special body conditions • Changing body position. • Change body position from vertical to horizontal and vice versa is followed by redistribution of blood. Under the influence of gravity veins in lower half of the body are dilated and may contain additional near 0,5 l of blood. After this impulsations from baroreceptors is activated and resistive vessels are contracted, mainly in skin and muscles. At the same time rate of heartbeat increases, which permit make up for cardiac output. In insufficient reflex regulation orthostatic unconsciousness may occur.

28. Regulation of blood flow in physical exercises • In physical exercises impulses from pyramidal neurons of motor zone in cerebral cortex passes both to skeletal muscles and vasomotor center. Than through sympathetic influences heart activity and vasoconstriction are promoted. Adrenal glands also produce adrenalin and release it to the blood flow. Proprioreceptor activation spread impulses through interneurons to sympathetic nerve centers. So, contraction of skeletal muscle during exercise compress blood vessels, translocate blood from peripheral vessels into heart, increase cardiac output and increase arterial pressure.

29. Changing blood volume after bleeding. • In changing blood volume volumic receptors in vena cava or atria are activated. These impulses spread to both medulla oblongata and osmolarity regulating neurons in hypothalamus. In consequence decreasing blood volume heart activity rises through sympathetic activation and vasopressin in released from hypophisis.

30. Cerebral circulation • Intensity of blood flow is 750 mL/min; 54 mL/100 g/min. • Except for a small contribution from the anterior spinal artery to the medulla, the entire arterial inflow to the brain in humans is via 4 arteries: 2 internal carotids and 2 vertebrals. The vertebral arteries unite to form the basilar artery; and the circle of Willis, formed by the carotids and the basilar artery, is the origin of the 6 large vessels supplying the cerebral cortex. Venous drainage from the brain by way of the deep veins and dural sinuses empties principally into the internal jugular veins in humans, although a small amount of venous blood drains through the ophthalmic and pterygoid venous plexuses, through emissary veins to the scalp, and down the system of paravertebral vein in the spinal canal.

31. Regulatory influences • The sympathetic fibers on the pial arteries and arterioles come from the cervical ganglia of the sympathetic ganglion chain. The parasympathetic fibers pass to the cerebral vessels from the facial nerve via the greater superficial petrosal nerve. Cerebral has autonomic circulation. Adenosine, histamine, serotonine, prostaglandines caused vasodilatation.

32. Coronary circulation • 2 coronary arteries that supply the myocardium arise from the sinuses behind the cusps of the aortic wave at the root of the aorta. The right coronary artery has a greater flow in 50 % of individuals, the left has a greater flow in 20 %, and the flow is equal in 30 %. There are 2 venous drainage systems: a superficial system, ending in the coronary sinus and anterior cardiac veins, that drains the left ventricle; and a deep system that drains the rest of the heart.

33. Compressive influences role • The heart is a muscle that compresses its blood vessels when it contracts. The pressure inside the left ventricle is slightly greater than in the aorta during systole. Consequently, flow occurs in the arteries supplying the subendocardial portion of the left ventricle only during diastole, although the force is sufficiently dissipated in the more superficial portions of the left ventricular myocardium to permit some flow in this region throughout the cardiac cycle.

34. Metabolic and chemical factors significance • Factors that cause coronary vasodilatation: O2, CO2, H+, K+, lactic acid, prostaglandines, adenine nucleotides, adenosine. Asphyxia, hypoxia, intracoronary injections of cyanide all increase coronary blood flow 200-300 % in denervated as well as intact hearts.

35. Neural regulation • The coronary arterioles contain alpha-adrenergic receptors, which mediate vasoconstriction, and beta-adrenergic receptors, which mediate vasodilatation. Activity in the noradrenergic nerves cause coronary vasodilatation.

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