AY24 CVS Physio 3_Circulatory system and BP.pdf

HSC1007 ANATOMY AND PHYSIOLOGY 1 CARDIOVASCULAR PHYSIOLOGY CIRCULATORY SYSTEM AND BLOOD PRESSURE ANDY LEE (PHD) ASSISTANT PROFESSOR FACULTY HALL #04-17 OFFICE PHONE: 6592 2524 [email protected] LEARNING OUTCOMES At the end of the lesson, you should be able to:  Describe the main feat...

HSC1007 ANATOMY AND PHYSIOLOGY 1 CARDIOVASCULAR PHYSIOLOGY CIRCULATORY SYSTEM AND BLOOD PRESSURE ANDY LEE (PHD) ASSISTANT PROFESSOR FACULTY HALL #04-17 OFFICE PHONE: 6592 2524 [email protected] LEARNING OUTCOMES At the end of the lesson, you should be able to:  Describe the main features of the different blood vessels  Explain how arteries function as a pressure reservoir  Explain how the sounds of Korotkoff are used to measure blood pressure  Distinguish between pulse pressure, arterial blood pressure and mean arterial blood pressure  Describe the mechanisms by which blood flow to different organs/tissues can be regulated  Describe the factors regulating venous return  Describe the factors that affect mean arterial blood pressure  Describe both short and long term control of blood pressure INSIDE THE HUMAN BODY: BLOOD CIRCULATION ORGANIZATION OF CVS Fig 10-4, Sherwood FEATURES OF BLOOD VESSELS Table 10-1, Sherwood Endothelium Elastin fibers Smooth muscle Collagen fibers RECONDITIONING BLOOD  Blood is transported to all parts of the body through a system of vessels  Brings fresh supplies to the vicinity of all cells while removing their wastes  Reconditioning organs receive more blood than what they need to perform homeostatic adjustments to blood  Example:  Digestive tract (20% of CO; to pick up nutrient supplies)  Kidneys (20% of CO, to eliminate waste, adjust water and electrolytes) PRESSURE VS FLOW RATE  Flow rate  Volume of blood passing through per unit time  Proportional to pressure gradient and inversely proportional to resistance  Blood pressure  Force exerted by the blood against a vessel  Pressure gradient  Difference in pressure between the beginning and end of a vessel  Resistance Fig 10-2a, Sherwood  Friction between the blood and vascular wall ARTERIES  Arteries serve as rapid- transit passageways to the organs and as a pressure reservoir  Heart contracts to pump blood into arteries and relaxes to refill with blood from veins BLOOD PRESSURE  Force exerted by blood against a vessel wall  Depends on volume of blood within the vessel; and  Distensibility of the vessel walls  Systolic pressure  Maximum pressure when blood is ejected into the arteries  Diastolic pressure  Minimum pressure when blood is draining into the rest of the vessel during diastole  Indicated as systolic/diastolic (120/80 mmHg) BLOOD PRESSURE Fig 10-7, Sherwood Korotkoff sounds BLOOD PRESSURE https://www.youtube.com/watch?v=bHXvhOQ0hYc PULSE PRESSURE  Pulse pressure = systolic – diastolic  Pulse wave can be FELT over major arteries  Wave results from difference in systolic and diastolic pressure Fig 10-7, Sherwood  Strong pulse wave is just strong difference between systolic and diastolic pressure  Pulsating in nature (pulsatile)  Elastic properties of arteries help convert pulsatile flow of blood from heart into more continuous flow in the capillaries MEAN ARTERIAL PRESSURE (MAP)  Main driving force for blood flow  Average pressure driving blood forward  Is the pressure that is monitored and regulated by body’s blood pressure reflexes (homeostasis)  MAP = Diastolic pressure + 1/3 pulse pressure; or MAP = 2/3 diastolic + 1/3 systolic  Example: if car drives 80km/h for 40min and 120km/h for 20min. Average speed is 93km/h and not 100km/h. BLOOD PRESSURE In arteries, little resistance, little pressure lost, so pressure is around the same throughout arterial tree. Pressure drops from arteries to veins – increasingly non- pulsatile Fig 10-8, Sherwood CAPILLARIES  Ideally suited to serve as site of exchange  Very thin walled, extensive branching and proximity of almost every cell to a capillary  RBC in a single file  Blood velocity in capillaries is very slow as compared to arteries  Total flow rate (~5L/min) is the same throughout circulatory tree (due to increased surface area) CAPILLARIES Fig 10-17, Sherwood CAPILLARIES Fig 10-15, Sherwood PRE-CAPILLARY SPHINCTERS  Tissues that are metabolic active have more capillaries  Many capillaries are not open under resting conditions  Capillaries have no smooth muscles  Thus, pre-capillary sphincters serve to control blood flow  Sensitive to local metabolic activity changes ↑ metabolic activity → sphincter relax → more open capillaries → increased blood flow to active tissues Fig 10-18, Sherwood REGULATION OF BLOOD FLOW AND DISTRIBUTION  Local control of arteriolar (arterioles)  Local vasoregulation of radius is important in determining arterioles the distribution of cardiac output  Vasoconstriction  The fraction of the total CO delivered  - to each organ varies depending on  Vasodilation demands for blood  -  Differences in flow to organs are  Vascular tone determined by differences in  state of partial constriction of vascularization and differences in arteriolar smooth muscle resistance offered by arterioles  Establishes a baseline of arteriolar supplying each organ resistance VASOREGULATION OF ARTERIOLES Fig 10-9, Sherwood EXTRINSIC CONTROL OF ARTERIOLAR RADIUS  Extrinsic control of arteriolar radius is important in regulating blood pressure  Influence of total peripheral resistance (TPR) on mean arterial pressure  Sympathetic fibers supply arteriolar smooth muscle everywhere (except in brain)  Increased sympathetic activity – generalized vasoconstriction  Decreased sympathetic activity – generalized vasodilation EXTRINSIC CONTROL OF ARTERIOLAR RADIUS  Local controls overriding sympathetic vasoconstriction  Riding bicycle (exercise) → increased sympathetic activity → generalized vasoconstriction → local metabolic activity in leg muscle induces vasodilation (override) → more blood to leg muscle → lesser blood to arm muscles and viscera etc  No parasympathetic innervation to arterioles (except in the penis and clitoris) EXTRINSIC CONTROL OF ARTERIOLAR RADIUS  Cardiovascular control center  Control the sympathetic output  Adrenal hormones  Norepinephrine produces generalized vasoconstriction (Epinephrine too, but weaker)  Epinephrine reinforces local vasodilatory mechanisms in tissues (mostly in skeletal muscles and heart)  Vasopression and angiotensin II (potent vasoconstrictors)  Vasopressin maintains water balance  Angiotensin II regulates salt balance DURING EXERCISE  Cardiac output ↑  Vasodilation in skeletal muscle and heart  More blood diverted to these organs  Body heat ↑  Vasodilation of the skin also (flushing of skin) DURING EXERCISE Sherwood, Chapter 10, Closer look at Exercise Physiology BLOOD PRESSURE  Affected by cardiac output and total peripheral resistance Blood Pressure  Needs to be closely regulated  High enough (adequate pressure) to maintain blood flow and tissue Cardiac TPR perfusion (fluid exchange) Output  Not too high to overwork heart and cause vascular damage and Arteriolar HR radius rupture of small vessels Blood SV viscosity VENOUS RETURN  Blood returning to the heart  Veins  Low resistance, less elastic recoil, less smooth muscle  Slow transit time through it (act as a blood reservoir)  Storage as moving not as quickly to the heart to be pumped out  Venous capacity (amount of blood veins can hold)  Depends on distensibility of vessel walls and other external pressures such as skeletal muscles squeezing it  During exercise  Stored blood needed – increase in venous return REGULATION OF VENOUS RETURN  Sympathetic innervation  Veins have low tone, but high sympathetic innervation: vasoconstriction, ↑ venous pressure  Note: Vasoconstriction in veins: ↑ flow (from ↓ capacity, squeezing out more blood already present); while arteriole vasoconstriction ↓ flow (from ↑ resistance)  Skeletal muscle activity  Many large veins lie between skeletal muscles in arms/legs  When muscle contracts, veins compressed (skeletal muscle pump)  ↓ venous capacity, ↑ venous pressure (extra way that blood returns to heart during exercise) REGULATION OF VENOUS RETURN  Gravity  Lying down, equal  Standing up- vessels below heart subject to gravity (pressure caused by weight of blood from heart to vessel)  Distensible veins yield under hydrostatic pressure- ↑ capacity  In leg, post-capillary blood pools in extended veins, ↓ VR, ↓CO Fig 10-26, Sherwood EFFECT OF GRAVITY  Lie down- stand up compensations:  Triggers sympathetic venous vasoconstriction, driving some of the pooled blood forward  Skeletal muscle pump “interrupts” column of blood  Sympathetic venous vasoconstriction cannot completely compensate without skeletal pump  Scenario 1. Person stands still for long time, blood flow to brain reduced (↓ in circulating volume, despite reflexes for maintaining MAP), leads to fainting- horizontal positioning 2. Postural hypotension EFFECT OF GRAVITY  Valves stop blood from going backwards:  One-way valves spaced 2-4 cm away  Also help counteract gravity (minimize back flow)  Varicose veins  Incompetent venous valves (can no longer support the column of blood above it and collapse)  Aggravated by frequent, prolonged standing OTHER FACTORS  Respiratory activity  Pressure within chest cavity is 5 mmHg less than atmospheric pressure  External pressure gradient between the lower veins (atmospheric pressure) and chest veins (less than atmospheric pressure)  Cardiac suction  Ventricular contraction, AV valves closed but drawn downward, enlarging atrial cavity so atrial pressure transiently drops lower than 0mmHg – sucking more blood into atria  Ventricular relaxation, AV valves open, rapid expansion of ventricles creates a suction effect sucking blood from atrium and veins  FACTORS FACILITATING VENOUS RETURN Fig 10-25, Sherwood SUMMARY? Fig 10-29, Sherwood SUMMARY OF NERVOUS INPUT ON BP Fig 10-32, Sherwood SHORT TERM REGULATION OF BP (BARORECEPTOR REFLEX)  Any change in MAP will trigger an automatic baroreceptor reflex in attempt to restore BP to norm  BP ↓ - detected by baroreceptor – Cardiovascular center - ↑ sympathetic and ↓ parasympathetic – ↑ HR, ↑ SV, arteriole vasoconstriction (↑ total peripheral resistance) and venous vasoconstriction (↑ CO) - ↑ BP Fig 10-30, Sherwood LONG TERM REGULATION OF BLOOD PRESSURE  Regulating blood volume  ↓ blood volume results in ↓ arterial BP  Major compensation is reabsorption of fluid by kidney (salt and water balance)  Renin-Angiotensin system of the kidney HYPERTENSION  Hypertension is a national public-health problem, but its causes are largely unknown  Types of hypertension: primary and secondary (secondary to other known problem/disease)  Baroreceptor adaptation during hypertension: adapt to operate at a higher level  Causes? (Too much dietary salt? Disturbance of renin-angiotensin system leading to increase blood volume?)  Complications of hypertension: left ventricular hypertrophy, stroke, heart attack, kidney failure, and progressive vision loss HYPERTENSION MOH Clinical Practice Guidelines on Hypertension, 2017 American College of Cardiology, American Heart Association, … Guidelines, 2017 REFERENCES  Chapter 10  Sherwood, L. (2016) Human Physiology: From Cells to Systems. 9th edition. Cengage