Fundamentals of Physiology and Anatomy

Fundamentals of Physiology and Anatomy Control of Blood Pressure Pressure Changes throughout the Vasculature • • • • Pressure in LV = between 0-120 Pressure in large arteries = 90-120 Pressure then dramatically falls in smaller arteries and arterioles Pressure continues to fall in capillaries, venules and great veins. Pressure in pulmonary = 1/5 of pressure in systemic Because resistance is much lower in pulmonary circuit. • • RV pressure = between 0-20 Decreasing pressure throughout the circulation. • Segment pressure IS NOT mean BP o It influences it Why does Pressure not fluctuate between systole and diastole in arteries as much as inside the heart? • During systole: o Heart forces blood into aorta o Aorta expands o Increased radius reduces resistance = reduces pressure o This ensures pressure in systolic circuit doesn’t exceed 120mmHg • During diastole o Elastic recoil keeps diastolic pressure at 80mmHg o Maintains flow o Prevents vessel collapse • If aorta/large arteries were rigid, o Systolic pressure would be very high o Diastolic = near 0 o NOT GOOD! Fundamentals of Physiology and Anatomy Defining MABP MABP is defined through Darcy’s Law P = QR P = pressure Q = flow R = resistance Mean Arterial Blood Pressure = CO x TPR TPR = total resistance of all systemic arteries and arterioles ▪ ▪ Cardiac output – the main determinant of pulse pressure (systolic-diastolic BP) Total peripheral resistance – main determinant of diastolic BP (afterload) • • MABP is average pressure in systemic circulation (L side of heart) Lower pressures in pulmonary circulation (R side) o Because pulmonary TPR is much lower HOWEVER, flow (cardiac output) is the same on both sides. • Put another way: • Contraction of heart = driving force behind blood flow o CARDIAC OUTPUT • So control of cardiac output o Regulates total blood flow throughout the body o Hence regulates MABP • However, TPR (and therefore MABP) are regulated by vascular system Function of Arterioles • • The resistance vessels of CVS Two important functions: o Main site of control of TPR ▪ Therefore major determinant of MABP o Controls relative blood flow through different vascular beds ▪ At any given MABP Fundamentals of Physiology and Anatomy How do Arterioles alter TPR and MABP? • • • Changes in vessel diameter (of arterioles) -> biggest changes in pressure Branching increases parallel resistances. o So, total arteriolar resistance drops BUT o R is inversely proportional to r4 o Small decreases in vessel radius -> relatively large increases in vessel resistance Arterioles can actively alter radius via constriction/dilation of smooth muscle layer. This alters resistance. Relation between Resistance and Pressure Large arteries are wide = low resistance Arterioles are very narrow = high resistance At the capillaries, resistance has dropped because of branching. At venules, resistance starts to increase because branches join together. Vasoconstriction At rest, vessel is partially constricted. Additional smooth muscle contraction: • Reduces radius • Increases resistance to flow • Increases pressure upstream Vasoconstrictor stimuli = NA, angiotensin II, endothelin Vasodilation At rest, partially constricted. Smooth muscle relaxation: • Increases radius • Reduces resistance • Reduces pressure upstream Vasodilator stimuli = adrenaline, Ach, histamine Fundamentals of Physiology and Anatomy Mechanism of Contraction/Relaxation 1. 2. 3. 4. 5. 6. 7. Activation of phospholipase C Triggers release of stored Ca2+ Ca2+ enters cell (via non-selective cation channels) and increase intracellular conc. Causes depolarisation Opens VGCC Depolarisation spreads through gap junctions to other muscle cells Smooth muscle contraction occurs 1. 2. 3. 4. 5. 6. K+ channels open cAMP and cGMP are produced Steps 1 and 2 cause hyperpolarisation Closes VGCC Reduces [Ca2+] Smooth muscles relax Blood Flow Velocity and Total Cross-Sectional Area Are inversely proportional Arteries -> arterioles -> capillaries • As no. of branches increase -> total cross-sectional area increases • But the branches get narrower -> decreases blood flow velocity Capillaries -> venules -> veins • Total area decreases • Velocity increases REMEMBER – FLOW IS ALWAYS THE SAME; ONLY VELOCITY CHANGES. Importance of Blood Flow Velocity Low capillary flow velocity = maximises time for exchange between blood and interstitial fluid Exchange of Fluid in Capillaries Occurs between plasma and interstitium. Interstitium = space between cells Interstitial fluid volume = plasma volume x4 Increase blood volume = increase blood pressure Capillary hydrostatic pressure • Induces filtration bulk flow of fluid through capillary pores Plasma protein osmotic pressure • Causes capillary to absorb fluid from interstitium Balance between filtration and absorption = no net movement of fluid A shift in balance = alter plasma volume and MABP Fundamentals of Physiology and Anatomy Short-term Control of MABP Via the Baroreceptor Reflex Short-term means responses to changes occurring sec-hrs in duration. Longer-term changes are dealt with through changes in blood volume. Maintaining MABP Requires : • Sensors o E.g. pressure transducers • Integrating centre • Effectors o E.g. pump or resistance controls Baroreceptor Reflex Cardiac Output Control Sensors = baroreceptors (found in carotid sinus and aortic arch) Integrating centre = CV centre in medulla Effector = SA node and cardiac muscle Afferent pathway • Glossopharyngeal nerve • Vagus nerve Efferent pathway • Vagus nerve • Sympathetic and cardiac nerves TPR Control Effectors = arterioles and veins Fundamentals of Physiology and Anatomy Baroreceptors • • • Are nerve endings Found in anterior layer of carotid sinus Respond to stretch of carotid sinus o Stretch is caused by pressure change Arterioles • • • Have many sympathetic nerve endings innervating smooth muscle layer Particularly strong innervation in skin, kidneys, gut and spleen. o These are the most important vascular beds. Sympathetic nerves release NA -> acts primarily on α-adrenergic receptors -> vasoconstriction Medulla • • Compares baroreceptor output to set-point Responds to changes by altering outputs to heart and vasculature. Changes in Baroreceptor Output Increased MABP • Medulla decreases sympathetic output • And increases parasympathetic output • HR and contractility are reduced o This lowers SV = lowers CO • Reduced vasoconstriction of arterioles reduces TPR • Lowers MABP back to normal Decreased MABP • Medulla increases sympathetic output • And decreases parasympathetic output • HR and contractility are increased o This increases SV = increases CO • Increased vasoconstriction of arterioles = increases TPR • Raises MABP back to normal NEGATIVE FEEDBACK! Veins • • Also effectors in baroreceptor reflex Innervated by sympathetic NS 1. 2. 3. 4. NA constricts veins Increases venous pressure Helps push blood back up to heart, against gravity Partial restoration of venous return Increase venous pattern = increase CO Fundamentals of Physiology and Anatomy Response to Postural Change 1. 2. 3. 4. 5. 6. 7. 8. 9. Moving from supine to standing position Gravity pulls blood to the legs Reduced venous return Reduces stroke volume Reduced MABP (NOT GOOD BECAUSE LACK OF BLOOD SUPPLY TO BRAIN – FAINTING) Less baroreceptor firing Increase sympathetic, decrease parasympathetic outputs Increase TPR, HR and venous return MABP returns to normal • The increase in HR cannot fully compensate for reduced SV o Hence CO is still reduced The large increase in TPR brings MABP up to/slightly above normal Initial drop in MABP is normally transient Without baroreceptor reflex, o Orthostatic hypotension occurs (dizziness, fainting) • • • Role of Baroreceptors in Maintaining MABP during Exercise During exercise: • Metabolic demand increases several-fold • Blood flow must also increase o Achieved through local vasodilation (muscles) • The heat generated triggers thermoregulatory reflex o Results in further vasodilation (in skin) • Muscle and skin are large vascular beds o Their vasodilation greatly lowers TPR • Vasoconstriction in other parts of body – especially kidney and spleen. o Maintains TPR • Since MABP = CO x TPR o Low TPR will dangerously lower MABP unless its corrected Cardiac Output and Venous Return Circulation is a closed circuit. Cardiac output = venous return 1. 2. 3. 4. 5. 6. 7. Increased sympathetic output Venous constriction Increased venous pressure Increased venous return Starling mechanism Increased stroke volume Increased CO Fundamentals of Physiology and Anatomy Regional Changes in Blood Flow during Exercise • • Total CO at rest = 5L/min Moderate exercise doubles total cardiac output o Doesn’t change much in brain o Doubles in heart o 6-fold increase in skeletal muscle o Also increases in skin o Everywhere else, it reduces. ▪ Kidneys ▪ Gut Long-term Control of BP Measuring MABP Fundamentals of Physiology and Anatomy Practical 3 – Diving Bradycardia HR decreases – efficient way of preserving oxygen Questions