Blood pressure control
Blood pressure control
Three factors play a prominent role in the regulation of blood pressure: cardiac output (CO), blood volume, and peripheral vascular resistance. This graphic depicts how the blood pressure can be increased. CO depends on the amount of blood returning to the heart (venous return) and the heart rate. The venous return depends on the preload or the capacitance of the veins. An increase in blood volume will therefore also increase the CO. CO is also under control of the sympathetic nervous system, which regulates the heart rate. The blood volume is regulated by the renin-angiotensin-aldosterone system (RAAS), which is a neurohumoral control.
RAAS activation results in an increase in aldosterone and antidiuretic hormone (ADH), which cause Na+ retention and water conservation, respectively. See renin-angiotensin-aldosterone system for a more detailed description of this system. The peripheral vascular resistance is under control of both the sympathetic nervous system and RAAS. Atrial natriuretic peptide (ANP) is the important hormone in decreasing blood pressure (not depicted here). ANP is released from the right atrium and increases water and Na+ excretion from the kidneys by blocking aldosterone and aldosterone release. ANP also induces peripheral vasodilation.
Sudden assumption of an upright position from supine position causes an initial decrease in:
Extra info: The consequences are: venous pooling, reduced cardiac output, increased total peripheral resistance, tachycardia, and increase in stimulation of the rennin-angiotensin system.
The cardiac output cannot increase indefinitely because
A severe decrease in blood volume (due to trauma) leads to the following:
The amount of blood that is forced out of the heart depends on
Following acute haemorrhage, the following compensatory mechanism occurs:
Extra info: Reduced blood volume decreases baroreceptor stretch and consequently increases sympathetic output. This gives rise to vasoconstriction except for blood vessels in heart and brain. Angiotensin is secreted upon decreased renal blood flow (indirectly via renin). Acute blood loss induces tachycardia, hypotension, arteriolar vasoconstriction, and venoconstriction.