Short-term adjustments of blood pressure are made by altering cardiac output (heart rate and contractility) and peripheral resistance at the arterioles. Blood volume increases blood pressure by pushing harder on the vessel walls. Blood volume is primarily regulated by the kidneys, which can preserve body fluids by making the urine more concentrated. Blood viscosity also increases BP, because it is harder to pump viscous blood. A high hematocrit increases viscosity.
The kidneys regulate water reabsorption (conserve water) to maintain constant blood volume. GFR remains relatively constant, except with extreme fluid loss, when it slows dramatically. Dilute urine is produced in the absence of regulatory hormones that act on the distal tubules. Obligatory water reabsorption always occurs in PCT and ascending limb of the loop of Henle. Regulated (facultative) water reabsorption depends on ADH (vasopressin) and aldosterone acting on the distal tubules.
Hypovolemia is decreased blood volume, which results in decreased blood pressure and renal blood flow. Reduced blood volume increases plasma osmolarity causing ADH release from the posterior pituitary. ADH (anti-diuretic hormone) inserts aquaporins into distal nephrons to allow water reabsorption. Low blood volume also activates the renin-angiotensin system.
Atrial natriuretic peptide (ANP) is produced by the atria when they are stretched due to increased blood volume. It is a diuretic that increases urine volume by reducing water reabsorption, thus reducing blood volume.
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