Regulation of Blood Pressure PDF

Regulation of Blood Pressure 1 Blood Vessels • The middle layer of the venous wall is poorly developed compared to arteries and their lumens have a greater diameter • Capillary walls composed of endothelium which allow exchange of substances between the blood and tissues. 2 Direction of blo...

Regulation of Blood Pressure 1 Blood Vessels • The middle layer of the venous wall is poorly developed compared to arteries and their lumens have a greater diameter • Capillary walls composed of endothelium which allow exchange of substances between the blood and tissues. 2 Direction of blood flow • Heart à aorta à arteries à arterioles à capillaries • Capillaries à venules à venes à vena cava à heart • The heart is the pump, the arteries are pressure reservoirs, the arterioles are resistance vessels that control distribution, the capillaries are exchange sites, and the veins are blood reservoirs. 3 Blood pressure Blood pressure is the force that blood exerts against the inner walls of blood vessels. Human Physiology: An Integrated Approach 6e Pearson Blood pressure rises and falls in a regular fashion, it is pulsatile. The pressure is highest in arteries and lowest in veins 4 Arterial Blood Pressure The maximum pressure during ventricular contraction is called the systolic pressure (100-120 mmHg). During diastole the ventricles relax, the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction is the diastolic pressure (70-90 mmHg). Pulse pressure = the difference between the systolic and diastolic pressures. It is felt as a throbbing pulsation in an artery (a pulse) during systole. 5 Mean arterial pressure Mean arterial pressure (MAP) is closer to diastolic pressure than to systolic pressure because diastole lasts twice as long as systole. Mean arterial pressure = Diastolic pressure + 1/3 (pulse pressure) or Mean arterial pressure = (2Diastolic pressure + Systolic pressure)/3 Example: If the systolic pressure is 124 mmHg and the diastolic pressure is 82, calculate the mean arterial pressure 6 a wave - atrial contraction c wave- ventricles begin to contract v wave slow flow of blood into the atria 7 Indirect blood pressure measurement The blood pressure cuff is inflated until the cuff pressure exceeds systolic pressure, blood flow into the arm is stopped and a brachial pulse cannot be felt or heard. The cuff pressure is gradually reduced. The pressure read when the first soft tapping sounds are heard is systolic pressure. When the artery is no longer constricted sounds disappear. The pressure at which the sounds first disappear is the diastolic pressure. Blood flow stops Small amound of blood Turbulent blood flow Sound first heard Korotkoff sounds Pressure gauge sphygmomanometer Normal blood flow Korotkoff sounds are created by pulsatile blood flow through the compressed artery. Pressure (mmHg) Systolic pressure Sounds disappear Cuff pressure Diastolic pressure Time 8 Explain how blood returns to the heart 10 Venous blood return Venous pressure is normally too low to promote adequate venous return. • The muscular “pump.” As the skeletal muscles surrounding the deep veins contract and relax, they direct blood toward the heart, and once blood passes each successive valve, it cannot flow back. • The respiratory “pump”. As we inhale, the pressure in the chest decreases, allowing thoracic veins to expand and speeding blood entry into the right atrium. • layer of smooth muscle around the veins that constricts under sympathetic control, increasing venous return. 11 The sympathetic nervous system innervate smooth muscle in blood vessel walls • Contraction of smooth muscle à reduction in the diameter of the vessel (vasoconstriction). • If vasomotor impulses are inhibited, the muscle fibers relax, and the diameter of the vessel increases (vasodilation). • Changes in the diameters of arteries and arterioles greatly influence blood flow and blood pressure. • Changes in the diameter of veins affect the amount of blood returning to the heart. 12 Peripheral resistance Changes in arteriole diameters regulate peripheral resistance. Blood vessels with smaller diameters offer a greater resistance to blood flow, factors that cause arteriole vasoconstriction increase peripheral resistance, which raises blood pressure. Peripheral resistance is under tonic sympathetic control. Increased sympathetic activity à vasoconstriction Increased sympathetic activity constricts veins à increase venous return 13 Factors that influence blood pressure Arterial pressure is a balance between blood flow into the arteries and blood flow out of the arteries. Blood flow into the aorta is equal to the cardiac output of the left ventricle. out, blo w lo f s d e e c in ex When flow Blood flow out of the arteries is influenced primarily by peripheral resistance reases c in e r u s s e ries à pr e t r a e h t in od collects When flow out exceeds flow in à mean arterial p re ssure falls Mean arterial pressure = cardiac output × total peripheral resistance 14 Blood flow through a blood vessel or a series of blood vessels is determined by two factors: the pressure difference between the two ends of the vessel (the inlet and the outlet) the resistance of the vessel to blood flow. Q=ΔP/R (Ohm’s Law ΔV = I × R ) 15 Human Physiology: An Integrated Approach 6e Pearson 17 18 Human Physiology: An Integrated Approach 6e Pearson Factors that influence blood pressure Human Physiology: An Integrated Approach 6e Pearson Arteries are low-volume vessels that contain only about 11% of total blood volume. Veins are high-volume vessels (volume reservoir) that hold about 60% of the circulating blood volume at any one time. 19 How the arterial blood pressure is regulated? 1. Sensory input (sensors) 2. Sensory neuron 3. Integration center (Cardiovascular control center located in brain stem) 4. Response – regulation by sympathetic and parasympathetic nerve fibers 21 Regulation of blood pressure Pressure changes are sensed by baroreceptors and chemoreceptors (1) Baroreceptors: pressure-sensitive mechanoreceptors that respond to changes in arterial pressure and stretch (2) Chemoreceptors: receptors that respond to changes in blood levels of oxygen, carbon dioxide, and H+ Baroreceptors are neurons located in the walls of the aorta and carotid arteries that sense changes in blood pressure Guyton & Hall: Textbook of Medical Physiology 12th Edition Chemoreceptors 22 Human Physiology: An Integrated Approach 6e Pearson 23 When the arterial pressure increases, nerve impulses transmitted from the baroreceptors to the cardiac center increases. This center relays parasympathetic impulses to the SA node, and the heart rate decreases. As a result of this cardioinhibitor reflex, cardiac output falls, and blood pressure decreases toward the normal level. 24 Decrease in arterial blood pressure initiates the cardioaccelerator reflex, which sends sympathetic impulses to the SA node and contractile cells. The heart beats faster and stronger increasing cardiac output and arterial pressure. 25 26 Orthostatic Hypotension {orthos, upright + statikos, to stand} The decrease in blood pressure upon standing • When you are lying flat, gravitational forces are distributed evenly up and down the length of your body, and blood is distributed evenly throughout the circulation. • When you stand up, gravity causes blood to pool in the lower extremities. à decrease in venous return à decrease in cardiac output à decrease in blood pressure upon standing = orthostatic hypotension • Orthostatic hypotension triggers the baroreceptor reflex 27 Factors that influence blood pressure Human Physiology: An Integrated Approach 6e Pearson 28 Regulation of Arterial Pressure Rapid control of arterial pressure – baroreceptor & chemoreceptor mediated • Reflex regulation of blood pressure beginning within seconds e.g. These reflexes may increase the pressure to two times normal within 5 to 10 seconds. Long-term control of arterial pressure by kidneys & several hormones • Long-term control of arterial pressure is regulated by homeostasis of body fluid volume & electrolyte • Regulation of fluid and electrolyte excretion by kidneys (Antidiuretic hormone, natriuretic peptides, renin, angiotensin, aldosterone) 29 Antidiuretic hormone (ADH, Vasopressin) Human Physiology: An Integrated Approach 6e Pearson 30 Renin-Angiotensin-Aldosteron System Next slide 31 Renin-Angiotensin-Aldosteron System Human Physiology: An Integrated Approach 6e Pearson 32 Hypotension • If blood pressure falls too low, the driving force for blood flow is unable to overcome opposition by gravity. à blood flow and oxygen supply to the brain are impaired à the person may become dizzy or faint. 33 Hypertension (elevated blood pressure) Sustained elevation of the systemic arterial pressure. A systolic pressure that is consistently over 140 mm Hg at rest, or a diastolic pressure that is chronically over 90 mm Hg, is considered a sign of hypertension It can be secondary, caused by another problem (e.g. kidney disease, high sodium intake, obesity and psychological stress). High blood pressure with unknown cause - essential hypertension Uncontrolled hypertension à left ventricle works harder to pump sufficient blood, the mycardium thickens, enlarging heart. *left axis deviation 34