Question

how the body regulates blood pressure when blood pressure is low. Please include relevant organs, hormones, and actions.

Answer 1

Normal blood pressure in humans is 90-120 mm of Hg systolic and 60-90 mm Hg of diastolic blood pressure. Vascular Baroreceptors detect changes in blood pressure. These baroreceptors are present in sinuses within the aorta and carotid arteries. The walls of venae cavae and right atrium also express low pressure baroreceptors. When blood pressure decreases, these baroreceptors stretch slowly. This decreases the action potential generated, causing decreased rate of firing. The medulla oblongata has a cardiovascular system, which receives this input from baroreceptors. A reflex is triggered to maintain homeostasis. Peripheral vessels are sympathetically stimulated and there is vasoconstriction along with increase in cardiac output, causing the blood pressure to decline. The cardiac center in medulla oblongata will stimulate cardiac output by increasing contraction as well as heart rate.

Vasoconstriction results from increased concentration of calcium ions and phosphorylation of myosin in vascular smooth muscle cells. Increased intracellular calcium due to IP3 mediated release form the sarcoplasmic reticulum activates myosin light chain kinase, which phosphorylates myosin light chain to initiate cross bridging. ATP, epinephrine and angiotensin II are the endogenous vasoconstrictors. Vasoconstriction is involved in short term control of blood pressure. Smooth muscles in arterioles are innervated by sympathetic motor neurons. Cardiovascular system also receives inputs form chemoreceptors that monitor levels of oxygen and carbon dioxide.

Epinephrine and norepinephrine secreted by the adrenal medulla will increase blood pressure by increase heart contraction. They also cause vasoconstriction of arteries and veins.

Long term control is monitored by the renin angiotensin II system. When blood pressure lowers, juxtamedullary cells in the kidneys will secrete renin directly in circulation. Renin will then cause conversion of angiotensinogen to angiotensin I in liver. This angiotensin I is converted to angiotensin II in the lungs by actions of angiotensin converting enzyme ACE. Angiotensin II is a vasoconstrictive peptide that induces vasoconstriction in arterioles. It also stimulates the adrenal cortex to secrete aldosterone. Aldosterone stimulates the reabsorption of water and salt from the renal tubules in kidneys. Due to reabsorption of water, the fluid in blood increases, thereby increasing the blood pressure. Angiotensin II also induces anti diuretic hormone secretion in pituitary. ADH stimulates water absorption in collecting duct. Angiotensin II also induces the reabsorption of sodium and chloride by renal tubules as well as excretion of potassium. All these effects will increase the volume of blood, and perfusion of juxtamedullary apparatus.