D3.3 Homeostasis

• Homeostasis maintains a stable internal environment within narrow limits

• Variables include body temperature, blood pH, glucose, gases, ions, and osmotic concentration

• Negative feedback detects deviation from a set point and reverses it

• Receptors, coordinators, effectors, and feedback loops restore normal conditions

• Beta cells secrete insulin when blood glucose rises

• Alpha cells secrete glucagon when blood glucose falls, affecting liver and muscle stores

• Type 1 diabetes results from autoimmune destruction of pancreatic beta cells

• Type 2 diabetes involves insulin-receptor/response failure and is linked to lifestyle risk factors

• Thermoregulation uses negative feedback to maintain core temperature near 37 °C

• Thermoreceptors signal the hypothalamus, which coordinates skin, muscles, liver, and hormones

• Cooling uses vasodilation, sweating, and hairs lying flat

• Warming uses vasoconstriction, reduced sweating, shivering, metabolic heat, and brown fat

• Kidneys regulate blood composition by excretion and osmoregulation

• Nephrons remove urea and adjust water and ion concentrations in urine

• Glomerular blood pressure drives ultrafiltration into Bowman's capsule

• The proximal convoluted tubule selectively reabsorbs glucose, amino acids, ions, and water

• Ascending limb pumps Na⁺/Cl⁻ and is impermeable to water

• Descending limb loses water, creating a medulla gradient for water conservation

• Hypothalamus osmoreceptors control posterior pituitary ADH release

• ADH inserts aquaporins in collecting ducts, increasing water reabsorption and concentrated urine

• Vasoconstriction and vasodilation redistribute blood according to activity

• Exercise and epinephrine increase skeletal muscle flow and reduce gut or renal flow