PBL: BLOOD ON THE ROAD How does the body maintain blood pressure and why is this important? Victoria Venning The maintenance of blood pressure (BP) is dependent upon intrinsic, reflex, hormonal, renal, and microvascular control systems. The regulation of BP is dependent upon the interaction from each control system. Intrinsic Regulation: -Central determinants of BP are stroke volume and heart rate, both of which are influenced by cardiac output. -Frank Starling mechanism. -Amount of resistance offered by the left ventricle during contraction (afterload). A higher afterload will decrease stroke volume and therefore decrease BP. -Changes in peripheral vascular resistance are detected in the smooth muscle cells of arteries. Inadequate oxygen or build up of metabolites will promote vasodilation. F = ∆P/R Where F = Cardiac Output (flow rate) ∆P = Mean arterial pressure R = total peripheral resistance OR Mean arterial pressure = CO x total peripheral resistance Reflex Regulation/SNS of BP: Involves the sympathetic nervous system (SNS) and provides a quick response to a drop in BP (ie. haemorrhage). The baroreflex system is composed of: -Pressure sensitive baroreceptors located in the aorta and carotid artery sense tiny changes in BP. - When ↓BP, sensory nerve fibers signals receptors to and from the cardiovascular control center located in the medulla of the brain stem. -The smooth muscle of the arteriolar walls are richly innervated by sympathetic nerve fibers. -Nerve fibers act on smooth muscle cells to alter cardiac contractility, heart rate, and peripheral vascular resistance. -Preganglionic neurons release acetylcholine -> noradrenaline -> activates adrenergic receptors on peripheral target tissue -> SNS effects. For example, ↑ arteriolar vasoconstriction -> affects total peripheral resistance thus affecting BP as F = ∆P/R. Also, certain levels of sympathetic control contribute to vascular tone (ongoing, partial state of constriction). Neural and hormonal. Renal (Long term control) Kidneys maintain BP due to its effects on blood volume. Kidneys, like the aorta and carotids contain pressure sensitive receptors in the arteries. BP ↓ kidneys release renin -> produces angiotensin II (potent blood vessel constrictor) -> BP ↑. Angiotensin II also results in sodium retention -> blood volume ↑ -> further contributing to BP ↑. Vasopressin is involved in maintaining water balance via the kidneys (during urine formation) and therefore can retain blood volume. Vasopressin and angiotensin II are both potent vasoconstrictors. When BP rises too far, the kidneys excrete more sodium and water, ↓ BP into normal range. If BP is not regulated may result in hypertension. Hypertension is defined as either a systolic BP >140mmHg or diastolic BP >90mmHg. Hypertension affects the whole cardiovascular system and is associated with a higher incidence of myocardial infarction, stroke, congestive heart failure and chronic kidney disease.