Blood Pressure

Blood pressure A constant flow of blood is necessary to transport oxygen to the cells of the body. The arteries maintain an average blood pressure of around 90 mmHg. This helps push the blood from the arteries into the capillaries. In the capillaries, oxygen transfers from the blood to the cells.

Systole and Diastole The cardiac cycle consists of a period of relaxation called diastole, during which the heart fills with blood, followed by a period of contraction called systole. The arteries fluctuate between a state of systole and diastole. In systole, the pressure in the arteries increases as the heart pumps blood into the arterial system. As the pressure increases, the elastic walls of the arteries stretch. This can be felt as a pulse in certain arteries. In diastole, the recoil of the elastic arteries forces blood out of the arterial system into the capillaries. The pressure in the arteries falls as blood leaves the system. Minimum diastolic pressure is typically 70-80 mmHg. Maximum systolic pressure is typically 110-120 mmHg.

BP Formula Blood pressure depends on cardiac output (CO) and systemic vascular resistance (SVR) BP = CO x SVR* Cardiac output depends on heart rate and stroke volume CO = HR x SV* *SVR = total resistance of arterioles to flow of blood *SV = the amount of blood pumped by the heart each cycle

Control of Blood Pressure The body responds quickly to falls in arterial pressure This immediate response is to increase cardiac output (CO) and systemic vascular resistance (SVR) Sympathetic activity causes vasoconstriction. This increases SVR. Sympathetic activity causes an increase in both heart rate and stroke volume. These both increase cardiac output CO = HR x SV Both SVR and CO have now increased so BP will increase BP = CO x SVR

Baroreceptors How does the body know that there has been a fall in blood pressure? Baroreceptors on the aorta and carotid artery respond to falls in BP. They send signals to the cardiovascular centre in the brain stem medulla. The medulla sends signals along the sympathetic nerves to the arterioles and heart, increasing SVR and cardiac output.

A fall in BP triggers vasoconstriction

A fall in BP triggers an increase in cardiac output

Antihypertensive Drugs 90% of cases of hypertension are of unknown origin. Hypertension is defined as having a systolic pressure of over 140 mm Hg and diastolic pressure over 90 mm Hg. Mild hypertension, between 140 – 159 mm Hg may initially be treated by changes in lifestyle. More aggressive treatment is warranted as the problem gets worse.

Antihypertensive Drugs Prolonged high blood pressure can damage organs, especially the brain, kidneys and cardiovascular system and may result in haemorrhagic stroke, renal failure and myocardial infarction (heart attack). Atherosclerosis, a disease where fatty deposits accumulate and damage blood vessels is accelerated in people with high blood pressure, especially if they are also diabetic.

Antihypertensive Drugs All antihypertensive drugs act on the familiar formula… BP = SVR x CO (HR v SV) They act by Reducing SVR .. or by... Reducing cardiac output …by… Reducing heart rate …or by… Reducing stroke volume

Renin, angiotensin, aldosterone system BP Renin Angiotensinogen Angiotensin I Angiotensin Converting Enzyme Angiotensin II

Renin, angiotensin, aldosterone system Angiotensin II ADH- vasopressin Aldosterone Vasoconstriction Thirst Blood Pressure Sodium retention

Angiotensin-II receptor blockers Beta blockers Beta-blockers are beta-adrenoceptor antagonists. They bind to and block β1 receptors on the heart so reducing its responsiveness to sympathetic activity Angiotensin-II receptor blockers These drugs are used to treat essential hypertension and some are also used for heart failure. Angiotensin-II is a potent vasoconstrictor that causes an increase in SVR. Angiotensin-II receptor blockers bind to and block the AT1 receptor and therefore reduce the vasoconstrictive effect of angiotensin-II.

Calcium-channel blockers (CCBs) Calcium-channel blockers (CCBs) exert their effects by different mechanisms. Calcium channel blockers act on the heart’s conduction tissue to reduce heart rate, the myocardium to reduce stroke volume and the smooth muscles of the arterioles to reduce systemic vascular resistance.