Blood Pressure

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Blood Pressure:

The heart generates blood flow  opposed by resistance in peripheral circulation  pressure.

Arterial blood pressure:
• arterial pressure is the force that pumps blood through the arterial part of the systemic circulation
• depends on elastic properties of arteries and the volume of blood forced into arteries
• blood pressure changes in a regular fashion with contraction of left ventricle  pulse
• systolic pressure –
• diastolic pressure –

Capillary pressure:
• pressure has dropped to 40mmHg by time reaches capillary beds
• higher pressure would rupture the fragile capillaries

Venous blood pressure:
• usually steady and changes very little during cardiac cycle

Factors aiding venous return:
1) Respiratory pump:
• pressure changes in ventral body cavity during breathing (the respiratory pump)  draws blood toward the heart
• inhale  increase in abdominal pressure  squeezes local veins  blood flows toward heart as valves prevent back flow
2) Muscular pump:
• as skeletal muscles around veins contract  pump blood toward heart as valves prevent back flow

Control of blood pressure
Blood pressure (BP) = cardiac output (CO) x peripheral resistance (PR)
Cardiac output is dependent on blood volume  blood pressure varies directly with cardiac output, peripheral resistance and blood volume
Vessel diametre is main determinant of peripheral resistance

Vasomotor centre:
• located in medulla
• transmits impulses via vasomotor fibres to innervate the smooth muscle of blood vessels – impulse are constant  arterioles are almost always in state of moderate constriction (vasomotor tone)
• has inputs from
• baroreceptors (pressure sensitive mechanoreceptors that respond to stretch – when stretched  more impulses to vasomotor centre  inhibits vasomotor centre  vasodilation  decrease in blood pressure)
• chemoreceptors (respond to changes in blood vessels of oxygen, carbon dioxide and H+)
• higher brain centres (hypothalamus and cerebrum)  modifies neural control

Chemical controls:
• adrenal medulla hormones (epinephrine  increases cardiac output and vasoconstriction)
• atrial natriuretic factor (ANF), (peptide hormone produced by atria of heart; decreases blood volume and pressure; ANF is antagonistic to aldosterone  excretion of sodium and water)
• antidiuretic hormone (ADH), (causes vasoconstriction when blood pressure falls to very low levels)
• Angiotension II (released by kidneys when renal perfusion is inadequate  vasoconstriction  rapid rise in blood pressure)
• endothelium derived factors (eg endothelin  potent vasoconstrictor release in response to low blood flow)
• Nitric oxide (NO) – previously called EDRF – endothelium derived relaxing factor (secreted by endothelial cells in response to a high blood flow and in response to signalling molecules (eg acetylcholine, bradykinin)  vasodilation)
• alcohol (inhibits ADH release and depresses vasomotor centre  vasodilation)
• inflammatory chemicals (histamine, kinins, etc)  potent vasodilators)

Renal regulation:
• regulate blood pressure by regulating blood volume

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