Organism level systems Study Guide

Homeostasis is the maintenance of a stable internal environment in the body, which is crucial for the optimal functioning of enzymes and cells. Two key aspects of homeostasis are thermoregulation (control of body temperature) and osmoregulation (control of water and salt balance). When faced with temperature challenges, the body employs various mechanisms to maintain a core temperature of approximately 37°C. If body temperature rises, the hypothalamus (the thermoregulatory centre in the brain) triggers vasodilation, increasing blood flow to the skin to dissipate heat, and stimulates sweat glands to release sweat which cools the body as it evaporates. Hairs lie flat to reduce insulation. If body temperature falls, the hypothalamus triggers vasoconstriction, reducing blood flow to the skin, and causes muscles to shiver, generating heat through contractions. Hairs stand erect to trap an insulating layer of air. For osmotic challenges, the body regulates water and salt levels, primarily through the kidneys. If water levels in the blood are too low (e.g., due to dehydration), the hypothalamus detects this high blood solute concentration (high osmolarity). This triggers the release of Antidiuretic Hormone (ADH) from the pituitary gland. ADH increases the permeability of the collecting ducts in the kidneys to water, leading to more water reabsorption into the blood and the production of a small volume of concentrated urine. Conversely, if water levels are too high, less ADH is released, leading to less water reabsorption and the production of a large volume of dilute urine. Kidneys also regulate salt balance, adjusting how much is reabsorbed or excreted to maintain blood volume and pressure. These coordinated responses ensure that internal conditions remain within narrow limits, preventing damage to cells and allowing metabolic processes to occur efficiently.