THE BLOOD VESSELS & BLOOD PRESSURE Lecture – 9 Dr. Zahoor Ali Shaikh 1

BLOOD VESSELS Arteries – pressure vessels [reservoir] Arterioles – resistance vessels Capillaries – exchange vessels Vein – capacitance vessel [blood reservoir] 60 – 70% of blood is in the veins 2

PATTERN & PHYSICS OF BLOOD FLOW Blood is transported to all parts of body through blood vessels. Blood vessel bring Oxygen and nutrition. They remove waste product. We will study general principles regarding blood flow, Physics of blood flow, Role of blood vessels, then blood pressure regulation. 3

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DISTRIBUTION OF CARDIAC OUTPUT Digestive organ, kidney, skin receive blood in excess of their own needs, therefore, can withstand better main blood flow is reduced. Brain suffers irreparable damage when blood supply is not there for more than 4mins. If oxygen is not supplied to brain, permanent damage occurs. 5

BLOOD FLOW Blood Flow through a vessel depends on - Pressure Gradient - Vascular Resistance What is Flow Rate of Blood? It is volume of blood passing through a vessel per unit time. Flow rate is directly proportional to the pressure gradient and inversely proportional to vascular resistance. 6

BLOOD FLOW Pressure Gradient - Flow Resistance - Flow F = ΔP / R F = Flow rate of blood through vessel ΔP = Pressure Gradient R = Resistance of blood vessel 7

PRESSURE GRADIENT Pressure Gradient is difference in pressure between the beginning and end of vessel. Blood flows from area of high pressure to an area of low pressure, down the pressure gradient. When heart contracts, it gives pressure to the blood, which is main driving force for flow through a vessel. Due to resistance in the vessel, the pressure drops as blood flows. 8

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RESISTANCE What is Resistance? It is measure of hindrance or opposition to blood flow through a vessel, caused by friction between the blood in the vessel wall. If resistance to flow increases, it is difficult for blood to pass through a vessel, therefore, flow rate decreases. When resistance increases, the pressure gradient must increase to maintain the same flow rate. 10

APPLIED If vessels offer more resistance to flow e.g. increased peripheral resistance, which occurs in high blood pressure then heart must work harder to maintain adequate circulation. 11

RESISTANCE Resistance to blood flow depends on three factors: 1. Viscosity of blood 2. Vessel length 3. Radius of the vessel – this is most important. 12

RESISTANCE 1. Viscosity of blood We write η for Viscosity. Viscosity refers to the friction, which is developed between the molecules of fluid as they slide over each other during flow of fluid. Greater the viscosity, Greater the resistance to flow. Blood viscosity is determined by number of circulating RBC, blood viscosity is increased in Polycythemia and decreased in Anaemia. 13

RESISTANCE 2. Vessel length Greater the length of a vessel, more will be the resistance. How length of a vessel affects the resistance? When blood flows through a vessel, blood rubs against the vessel wall, greater the vessel surface area, greater will be the resistance to the flow. 14

RESISTANCE 3. Radius of the vessel It is the most important factor to determine the resistance to flow. Fluid passes more readily through a large vessel. Slight change in radius of a vessel brings great change to flow because Resistance is inversely proportional to the fourth power of the Radius [multiplying the radius by itself four times]. R α 1/r 4 15

RESISTANCE 3. Radius of the vessel Therefore, doubling the radius, reduces the resistance to 1/16 its original value. [r 4 = 2×2×2×2 = 16 or R α 1/16 ] and there is increased flow through a vessel 16 fold. On the other hand, when we decrease the radius to the half, blood flow will be decreased 16 times. 16

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APPLIED Clinically radius of arteriole can be regulated and is the most important factor in controlling resistance to blood flow throughout the vascular system. 18

POISEUILLE’S LAW The factors that affect the flow rate through a vessel are integrated in Poiseuille’s Law. Flow rate = πΔPr 4 8ηL ΔP = Pressure Gradient [ Pressure Gradient = Flow] r = Radius [ Radius = Flow] η = Viscosity [ Viscosity = Flow] L = Length [ Length = Flow] 19

VASCULAR TREE Systemic Circulation Arteries give small arteries which branch into many arterioles. The volume of blood flowing through an organ can be adjusted by the caliber [internal diameter] of the arteriole. Arteriole branch into capillaries – where ex- change occurs with the cells. Capillary exchange is the entire purpose of circulatory system. 20

SYSTEMIC CIRCULATION Capillaries rejoin to form Venules, which merge to form small veins, that leave the organ. Small veins unite to form large veins, that empty into the heart. The arterioles, capillaries and Venules are collectively referred as the MICROCIRCULATION, because they are visible only through microscope. 21

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IMPORTANT Human Physiology – Lauralee Sherwood {Page 347}: If all of the vessels in the body are strung [connected], end to end, they would circle the circumference of the Earth twice! 23

SYSTEMIC CIRCULATION We will discuss the role of different vessels in Systemic Circulation. Arteries Functions - Serve as rapid transit passage way for blood from heart to organs. - They have large radius, therefore, offer little resistance to blood flow. - They act as Pressure Reservoir and provide driving force for blood when heart is relaxing. 24

SYSTEMIC CIRCULATION Arteries Functions [cont] Arterial wall has two type of connective tissue – Collagen Fibers which provide tensile strength against high driving pressure of blood ejected from the heart. – Elastin Fibers which give elasticity to arterial wall, so that they behave like a balloon. Because of elastic recoil, pressure is maintained in the arteries, therefore, blood flows to the organs when heart is relaxing. 25

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ARTERIAL BLOOD PRESSURE What is Blood Pressure? It is the pressure or force exerted by the blood against the vessel wall. Systolic Pressure - It is maximum pressure exerted in the arteries during systole of ventricle. Average 120mm Hg. Diastolic Pressure – Minimum pressure within the arteries during the diastole of ventricle. Average 80mm Hg. 28

ARTERIOLES Artery branches into numerous arterioles within the organ. Arterioles are the major resistance vessels. WHY? Because their radius is small. As arteriolar resistance is high, it causes marked drop in mean pressure as blood flows through arteriole. Mean Arterial Blood Pressure [ABP] of 93mm Hg in arteries falls to mean ABP of 37mm Hg as blood leaves the arteriole and enters the capillaries. 29

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ARTERIOLES Arteriolar Resistance converts the pulsatile systolic to diastolic pressure swings in the arteries into the non-fluctuating pressure present in the capillaries. Radius of the arteriole can be adjusted to achieve 2 functions: i). Adjust blood flow to the organs ii). To help to regulate blood pressure 31

ARTERIOLES Arteriolar Resistance - it is affected by - vasoconstriction [means narrowing of vessel] and vasodilation [increase in circumference and radius of vessel]. Arteriole have less elastic tissue, but thick layer of smooth muscle, which is innervated by sympathetic nerve fibers. VASCULAR TONE Arteriolar smooth muscle has vascular tone, that is partial state of constriction due to cytosolic concentration of Ca

ARTERIOLES Many factors can increase or decrease the tone and can cause vasoconstriction and vasodilation. Causes of VASOCONSTRICTION - Sympathetic Stimulation - Epinephrine and Nor Epinephrine - Angiotensin II - Vasopressin - Endothelin [from endothelial cells] - O 2 - CO 2 - Cold 33

ARTERIOLES CAUSES OF VASODILATION - Decreased sympathetic stimulation - Nitric oxide [released by endothelial cells and other tissue, it is local mediator] - Histamine - Adenosine - Prostaglandin - Decrease Oxygen - Increased CO 2 - Increase H + ion - Increase K + - Heat 34

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ACTIVE HYPEREMIA When local factors cause vasodilation, there is increased blood flow. This increased blood flow in response to increased tissue activity is called ACTIVE HYPEREMIA. Auto Regulation It is through local arteriolar mechanism which keeps blood flow constant despite changes in mean arterial blood pressure. E.g. brain – auto regulation is best kidney – auto regulation is good skeletal muscle – auto regulation is poor 36

SYMPATHETIC CONTROL OF ARTERIOLE Sympathetic Control of Arterioles is important in regulating blood pressure. Sympathetic ANS supplies arteriolar smooth muscle all over body except brain. Increase sympathetic stimulation causes arteriolar vasoconstriction. Decrease sympathetic stimulation causes arteriolar vasodilation. There is no parasympathetic innervations to arterioles. 37

WHAT YOU SHOULD KNOW FROM THIS LECTURE Pattern & Physics of Blood Flow Flow, Pressure, Resistance Relationship Types of Blood Vessels & their Functions E.g. Arteries, Arterioles, Capillaries, Veins Formula For Flow Rate Factors Affecting Resistance – Viscosity, Vessel Length, Radius of the Vessel Vascular Tree – Functions of Arteries, Arterioles Factors Causing Vasoconstriction and Vasodilation Active Hyperemia Auto Regulation Sympathetic Control of Arteriole 38

THANK YOU 39