14.1 : 血压

Blood pressure or BP is the pressure exerted by blood moving against the artery walls.

The average BP of an adult is 120 over 80 millimeters of mercury.

The numerator indicates systolic pressure, the force exerted on artery walls during ventricular contraction. The denominator signifies diastolic pressure, the pressure in arteries when the heart relaxes, and the ventricles refill with blood.

Several interconnected physiological processes regulate BP.

First is hormonal control. When BP decreases, the adrenal medullae release hormones such as adrenaline and noradrenaline, increasing cardiac output and contractility.

If BP rises, the atria secrete atrial natriuretic peptide hormone, which inhibits the renin-angiotensin system, causing vasodilation and lowering BP.

In neural control, the sympathetic nervous system promotes vasoconstriction and lowers BP when it is high. Conversely, the parasympathetic nervous system promotes vasodilation and decreases BP when it is high.

In renal control, the renin-angiotensin-aldosterone system activates when the kidneys detect low blood pressure. Renin converts angiotensinogen to angiotensin-I and angiotensin-II, which promotes vasoconstriction and aldosterone secretion, raising BP.