Blood Pressure The maintenance of blood pressure is dependent upon intrinsic (stroke volume, heart rate and cardiac output) , reflex (baroreceptors and chemoreceptors), hormonal, renal, and micro vascular control systems
Blood pressure Blood pressure changes by: • Blood flow (blood volume) • Resistance (blood vessels area) • Both 1 and 2
Blood Volume Blood volume is determined by the amount of water and sodium : • Ingested • Excreted by the kidneys, gastrointestinal tract, lungs and skin
How Blood Volume Affects Blood Pressure An increase in blood volume • Increases central venous pressure • Increase atrial and ventricular pressure • Increases stroke volume and cardiac output
What alters blood volume? • Removing H2O from the body decreases the volume of blood; i.e., failing to retain H2O by failing to retrieve it from urine when that is concentrated in the kidney causes blood volume to decrease along with blood pressure. VP/ADH stimulates the kidney distal tubules to recapture the H2O in urine passing through the tubules. • Increasing the osmotic or ionic content of blood will increase blood volume by drawing H2O from body tissues to dilute dissolved materials, e.g., glucose, or ions, e.g., Na+. Sodium ion recovery from urine is stimulated by the Renin-Angiotensin-Aldosterone system which is stimulated by low blood pressure.
How are changes in blood pressure detected? Brain, heart, & kidney respond to changes in blood pressure via pressure receptors, baroreceptors, & to changes in ionic composition of blood via osmoreceptors. Baroreceptors exist in large blood vessels, heart, & the kidney glomerulus (maculodensa). Osmoreceptors occur in hypothalamic & glomerulus (juxtaglomerular) cells.
Hormonal Regulation of the Cardiovascular System Several hormones affect blood pressure (BP) and flow by acting on the heart, by altering blood vessel diameter, or adjusting the total blood volume.
Examples of Hormones that affect BP These include (but not all): 1) Renin-angiotensin-aldosterone (RAA) system - angiotensin II raises BP by causing vasoconstriction, elevating CO, and stimulating aldosterone and ADH secretion to raise total blood volume (review aldosterone and ADH). It also stimulates thirst. 2) Epinephrine and norepinephrine - from adrenal medulla, act to increase CO and selective vasoconstriction/vasodilation to adjust blood flow in flight/fight. 3) Antidiuretic hormone - How would this affect total blood volume and therefore BP? It also has vasomotor abilities. 4) Atrial natriuretic peptide (ANP) - lowers BP by causing vasodilation and promoting loss of salt and water in urine to decrease total blood volume. 5) Erythropoietin stimulates RBC production if BP declines or blood becomes hypoxic. This will elevate blood volume (by addition of cells) and improve its oxygen-carrying capacity.