Roger Evans; Week 10 MED1022; Physiology
MAP is determined by CO and TPR. TPR is sum resistance of all the blood vessels. Mainly determined by arterioles. Changes in TPR are brought about by changes in state of constriction or dilation of arterioles. Change in pressure = flow x resistance. For whole circuit, change in pressure is mean aortic pressure - mean right atrial pressure; mean right atrial pressure = 0mmHg so change in pressure = mean aortic pressure, and since pressure in all large arteries is essentially the same m aortic p = mean arterial pressure. Therefore mean arterial pressure = CO x TPR. If MAP were to fall it could be returned to normal by increasing CO, TPR or a combination of the two. CO main controllers are heart rate and stroke volume. TPR major controller is resistance of arterioles controlled by both reflex and local mechanisms and contraction/relaxation of smooth muscle.
Total blood volume determines mean arterial pressure, if it falls MAP falls and vice versa. The two factors determining the pressure in an elastic container are volume of fluid and distensibility of walls. Volume affects pressure by affective venous pressure and therefore SV and CO. Short term control of MAP is baroreceptor reflex and hormonal control, long term is by regulation of blood volume. Detectors are arterial baroreceptors, control center is medullary CV centre, effectors are the heart, arterioles and veins. Baroreceptors are in aortic arch and carotid sinus, detect pressure, rate of AP firing is proportional to arterial pressure (rises- increases). Stretch receptors are in atria and large veins, regulate blood volume and body water as well as long term MAP control. Chemoreceptors are in aorta and carotid sinus, detect changes in O2, CO2 and pH. Influences both resp and CV function.
Medullary centre can be divided to cardiac and vasomotor centres. Integrates and issues commands. Baroreceptor reflex is quick, works for orthostatic hypertension. PHT increases risk of falls in elderly, when hypovolemic and when in a state of accentuated peripheral vasodilation. If MAP remains low, baroreceptors reset. Most long term control of BP occurs with kidneys through regulation of blood volume.
ADH constricts arterioles, stimulates kidneys to preseve water which leads to increased blood volume so increased CO. Aldosterone also increases water retention, increases blood volume and CO. ANP decreases.