Human Physiology Lecture 14 - Cardiovascular System PDF

Human Physiology Lecture 14 Cardiovascular System – Blood Flow and Blood Pressure Required Reading: Section 14.3 – Physical Laws Describing Blood Flow pp. 463-464 excluding System Interactions Objectives: Arteries and Arterial Blood Pressure...

Human Physiology Lecture 14 Cardiovascular System – Blood Flow and Blood Pressure Required Reading: Section 14.3 – Physical Laws Describing Blood Flow pp. 463-464 excluding System Interactions Objectives: Arteries and Arterial Blood Pressure Overview of Arterioles and Resistance to Flow Factors Affecting Arterial Blood Pressure Veins BIOL2010 Human Physiology 1  Overview of the Vasculature Aorta Vena cavae – ID: 12.5 mm – ID: 30 mm – 2 mm thick – 1.5 mm thick wall wall Arteries Veins – ID: 2-6 mm – ID: 5 mm – 1 mm thick – 0.5 mm thick walls wall Figure 14.5 The relationships of blood vessels according to size and the direction of blood flow in the systemic circuit. BIOL2010 Human Physiology 2  Arteries: A Pressure Reservoir Elastic and fibrous tissue in walls of arteries – Confers stiffness and flexibility – Act as pressure reservoirs – ensure smooth flow of blood even when heart is in diastole – Elastin fibres act as spring Store elastic force and passively recoil Figure 14.7 The role of arterioles as a pressure reservoir. BIOL2010 Human Physiology 3 Arteries: A Pressure Reservoir To serve as pressure reservoir, arteries must have low compliance Compliance: measure of how the pressure of a vessel will change with a change in volume – Low compliance (arteries) A large increase in pressure only causes small degree of expansion of vessel wall – High compliance (veins) Large increase in pressure causes a large increase in expansion of a vessel wall BIOL2010 Human Physiology 4  Arterial Blood Pressure Arterial blood pressure = pressure in aorta Pressure in the aorta varies with cardiac cycle – Systolic blood pressure = maximum pressure Due to ejection of blood into aorta Pressure in aorta = arterial blood pressure – Diastolic blood pressure = minimum pressure Slow decline in pressure to minimum just before the next systole Due to elastic recoil – Mean arterial pressure (MAP) = average pressure in arteries that occurs during one cardiac cycle BIOL2010 Human Physiology 5  Arterial Blood Pressure Measuring blood pressure – Estimate of arterial pressure – Brachial artery – Pressure cuff and sphygmomanomete r – Korotkoff sound – Blood pressure is: Figure 14.8 The events involved in blood pressure measurement. BIOL2010 Human Physiology 6  Arterial Blood Pressure The measured blood pressure is shown as SP/DP – Example: 110/70 From BP can also determine pulse pressure (PP) and mean arterial pressure (MAP): – Pulse pressure (PP) = SP – DP Example: what is PP if BP is 110/70? Why is PP useful? – MAP = SP + (2 × DP) 3 Example: (110 + 140)/3 = 83.3 mm Hg BIOL2010 Human Physiology 7  Arterioles Arterioles: resistance vessels – Part of microcirculation – Connect arteries to capillaries or metarterioles – Contain rings of smooth muscle to regulate radius and, therefore, resistance – Best site where resistance to flow can be regulated BIOL2010 Human Physiology 8  Arterioles and Resistance to Blood Flow Arterioles provide greatest resistance to blood flow Greater than 60% of TPR (total peripheral resistance) attributed to arterioles – TPR = the combined resistances of all the blood vessels with the circuit (in the systemic system) – Collectively, capillaries have less resistance Largest pressure drop in vasculature occurs along arterioles BIOL2010 Human Physiology 9  Arterioles and Resistance to Blood Flow - Average pressure entering arterioles: 75-80 mm Hg - Average pressure leaving arterioles: 35-40 mm Hg - High arteriole resistance Figure 14.9 Pressures in the vasculature. BIOL2010 Human Physiology 10  Arterioles and Resistance to Blood Flow Resistance is regulated in arterioles by: – Contraction/relaxation of circular smooth muscle Major function of arterioles is to act as points of control for regulating blood flow Arterioles function to: – Control blood flow to individual capillary beds – Regulate mean arterial pressure (MAP) BIOL2010 Human Physiology 11  Arterioles and Resistance to Blood Flow Changes in arteriole radius – Radius depends on contraction state of smooth muscle in arteriole wall – Arteriolar tone Contraction level (radius) is independent of extrinsic influences – Vasoconstriction Increased contraction = decreased radius – Vasodilation Decreased contraction = increased radius Both intrinsic and extrinsic control mechanisms alter contraction Figure 14.10 Changes in the radius of arterioles. BIOL2010 Human Physiology 12  Intrinsic Control of Arteriole Smooth Muscle Intrinsic control happens via: – Changes in metabolic activity Active hyperemia – Changes in blood flow Reactive hyperemia – Stretch of smooth muscles in arterioles – Local chemical messengers BIOL2010 Human Physiology 13  Intrinsic Control of Arteriole Smooth Muscle Changes in metabolic activity – Vascular smooth muscle is sensitive to conditions of ECF and respond to changes in concentrations of carbon dioxide, potassium ions and hydrogen ions (among others) – The changes in concentrations are due to changes in metabolic activity – General rule of thumb: Changes associated with increased metabolic activity generally cause vasodilation, whereas changes associated with decreased metabolic activity induce vasoconstriction BIOL2010 Human Physiology 14  Intrinsic Control of Arteriole Smooth Muscle Active Hyperemia Reactive Hyperemia Results from Results from changes changes in O2 and in O2 and CO2 in CO2 in response to response to change change in in blood flow metabolic activity Note: changes in blood flow are direct effect of changes in oxygen and carbon dioxide on arterioles themselves, therefore is intrinsic BIOL2010 Human Physiology 15  Intrinsic Control of Arteriole Smooth Muscle Stretch of smooth muscles in arterioles – Some tissues have stretch-sensitive fibres which stretch when BP in arterioles increases = myogenic response – Increase in perfusion pressure  arteriolar smooth muscles contract  vasoconstriction  increased resistance decreased flow Local chemical messengers – Many secreted by endothelial cells themselves Nitric oxide and prostacyclin – vasodilation Endolethin-I - vasoconstriction BIOL2010 Human Physiology 16 Extrinsic Control of Arteriole Smooth Muscle BIOL2010 Human Physiology 17  Capillaries and Venules Microcirculation – Simplest way to regulate exchange of material across capillary walls  regulate blood flow via Arterioles Metarterioles Precapillary sphincters Figure 14.17 Microcirculation. BIOL2010 Human Physiology 18  Capillaries and Venules Metarterioles – Intermediate structurally between arterioles and capillaries – Have rings of smooth muscle – Act as shunts from arterioles to venules Precapillary sphincters – Regulate flow through capillary bed – When sphincters contract, capillaries contract and flow through capillary bed decreases Figure 14.17 Microcirculation. BIOL2010 Human Physiology 19  Capillaries and Venules Exchange Across Capillary Walls Transport mechanisms: Simple diffusion Mediated transport Transcytosis Figure 14.18 Exchange of materials across the wall of a continuous capillary. BIOL2010 Human Physiology 20  Capillaries and Venules Bulk Flow Across Capillary Walls – Because capillary walls are permeable to water and small solutes, fluid can move from: Blood to interstitial fluid i.e. filtration Interstitial fluid to blood i.e. absorption – Purpose of bulk flow is to maintain balance between interstitial fluid and plasma – Shift of fluid from plasma to interstitial fluid = edema BIOL2010 Human Physiology 21  Capillaries and Venules Bulk Flow Across Capillary Walls – Starling Forces – forces that drive movement of fluid in/out of cell Capillary hydrostatic pressure (PCAP) Interstitial fluid hydrostatic pressure (PIF) Capillary osmotic pressure (πCAP) Interstitial fluid osmotic pressure (πIF) BIOL2010 Human Physiology 22  Capillaries and Venules Bulk Flow Across Capillary Walls Net filtration pressure (NFP) = filtration pressure – absorption pressure – NFP = (PCAP + πIF) – (πCAP + PIF) – Arteriole end: (38 + 0) – (25 + 1) = 12 mm Hg  filtration – Venous end: (16 + 0) – (25 + 1) = -10 mm Hg  absorption – Net across capillary: filtration (at arteriole end) - absorption at venule end BIOL2010 Human Physiology 23  Veins Veins are a volume reservoir Veins have high compliance – Expand with little change in pressure – Function as blood reservoir compared to arteries – 60% total blood volume in systemic veins at rest Figure 14.21 Distribution of blood volume in the various portions of the cardiovascular system. BIOL2010 Human Physiology 24  Factors That Influence Venous Pressure and Venous Return Skeletal muscle pump – One-way valves in peripheral veins; absent from central veins – Contraction/relaxation of skeletal muscle Figure 14.22 The skeletal muscle pump. BIOL2010 Human Physiology 25  Factors That Influence Venous Pressure and Venous Return Respiratory pump – Inspiration Diaphragm moves down - decreases pressure in thoracic cavity; increases pressure in abdominal cavity Pressure on veins in abdominal cavity creates gradient favoring blood movement to thoracic cavity – Blood moves from abdominal veins to central veins in thoracic cavity  blood flows towards heart – Exhalation Reverses the pressure and creates pressure gradient that would favour backward movement of blood  does not happen Why? BIOL2010 Human Physiology 26  Factors That Influence Venous Pressure and Venous Return Blood volume – Increased blood volume → increased venous pressure – Decreased blood volume → decreased venous pressure Venomotor tone – Smooth muscle tension in the veins – Increase in venomotor tone has two effects: Constriction of veins raises venous pressure  blood moves towards heart Increased wall tension reduces compliance  raises venous pressure  increased stroke volume BIOL2010 Human Physiology 27