Anatomy and Physiology

Blood pressure can be adjusted by producing changes in the following variables:

• Cardiac output can be altered by changing stroke volume or heart rate.

• Resistance to blood flow in the blood vessels is most often altered by changing the diameter of the vessels (vasodilation or vasoconstriction). Changes in blood viscosity (its ability to flow) or in the length of the blood vessels (which increases with weight gain) can also alter resistance to blood flow.

The following mechanisms help regulate blood pressure:

• The cardiovascular center provides a rapid, neural mechanism for the regulation of blood pressure by managing cardiac output or by adjusting blood vessel diameter. Located in the medulla oblongata of the brain stem, it consists of three distinct regions:

  • The cardiac center stimulates cardiac output by increasing heart rate and contractility. These nerve impulses are transmitted over sympathetic cardiac nerves.

  • The cardiac center inhibits cardiac output by decreasing heart rate. These nerve impulses are transmitted over parasympathetic vagus nerves.

  • The vasomotor center regulates blood vessel diameter. Nerve impulses transmitted over sympathetic motor neurons called vasomotor nerves innervate smooth muscles in arterioles throughout the body to maintain vasomotor tone, a steady state of vasoconstriction appropriate to the region.

• The cardiovascular center receives information about the state of the body through the following sources:

  • Baroreceptors are sensory neurons that monitor arterial blood pressure. Major baroreceptors are located in the carotid sinus (an enlarged area of the carotid artery just above its separation from the aorta), the aortic arch, and the right atrium.

  • Chemoreceptors are sensory neurons that monitor levels of CO ₂ and O ₂. These neurons alert the cardiovascular center when levels of O ₂ drop or levels of CO ₂ rise (which result in a drop in pH). Chemoreceptors are found in carotid bodies and aortic bodies located near the carotid sinus and aortic arch.

  • Higher brain regions, such as the cerebral cortex, hypothalamus, and limbic system, signal the cardiovascular center when conditions (stress, fight‐or‐flight response, hot or cold temperature) require adjustments to the blood pressure.

• The kidneys provide a hormonal mechanism for the regulation of blood pressure by managing blood volume.

  • The renin‐angiotensin‐aldosterone system of the kidneys regulates blood volume. In response to rising blood pressure, the juxtaglomerular cells in the kidneys secrete renin into the blood. Renin converts the plasma protein angiotensinogen to angiotensin I, which in turn is converted to angiotensin II by enzymes from the lungs. Angiotensin II activates two mechanisms that raise blood pressure:

    • Angiotensin II constricts blood vessels throughout the body (raising blood pressure by increasing resistance to blood flow). Constricted blood vessels reduce the amount of blood delivered to the kidneys, which decreases the kidneys' potential to excrete water (raising blood pressure by increasing blood volume).

    • Angiotensin II stimulates the adrenal cortex to secrete aldosterone, a hormone that reduces urine output by increasing retention of H ₂O and Na ⁺ by the kidneys (raising blood pressure by increasing blood volume).

  Various substances influence blood pressure. Some important examples follow:

  • Epinephrine and norepinephrine, hormones secreted by the adrenal medulla, raise blood pressure by increasing heart rate and the contractility of the heart muscles and by causing vasoconstriction of arteries and veins. These hormones are secreted as part of the fight‐or‐flight response.

  • Antidiuretic hormone (ADH), a hormone produced by the hypothalamus and released by the posterior pituitary, raises blood pressure by stimulating the kidneys to retain H ₂O (raising blood pressure by increasing blood volume).

  • Atrial natriuretic peptide (ANP), a hormone secreted by the atria of the heart, lowers blood pressure by causing vasodilation and by stimulating the kidneys to excrete more water and Na ⁺(lowering blood pressure by reducing blood volume).

  • Nitric oxide (NO), secreted by endothelial cells, causes vasodilation.

  • Nicotine in tobacco raises blood pressure by stimulating sympathetic neurons to increase vasoconstriction and by stimulating the adrenal medulla to increase secretion of epinephrine and norepinephrine.

  • Alcohol lowers blood pressure by inhibiting the vasomotor center (causing vasodilation) and by inhibiting the release of ADH (increasing H ₂O output, which decreases blood volume).