Homeostasis in mammals

Homeostasis involves internal/external stimuli, receptors (detect changes), a coordination system (nervous/endocrine), effectors (muscles/glands that bring about a response), and negative feedback (reverses the initial change).

The kidney consists of the fibrous capsule, cortex (outer layer), medulla (inner layer), renal pelvis (collects urine), ureter (carries urine to bladder), and branches of the renal artery and renal vein (supply and drain blood).

The nephron includes the glomerulus (capillary network), Bowman's capsule (surrounds glomerulus), proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. These structures are associated with afferent and efferent arterioles and peritubular capillaries.

Ultrafiltration occurs in the Bowman's capsule where high hydrostatic pressure forces water, ions, glucose, and amino acids out of the glomerulus capillaries and into the Bowman's capsule, forming glomerular filtrate. Blood cells and large proteins remain in the blood.

Selective reabsorption occurs in the proximal convoluted tubule where useful substances like glucose, amino acids, ions, and water are reabsorbed from the glomerular filtrate back into the blood via active transport and osmosis.

The Bowman's capsule has a specialized structure with podocytes and fenestrations that allows for efficient ultrafiltration. Podocytes have foot-like processes that create filtration slits, preventing large molecules from entering the filtrate.

ADH (antidiuretic hormone), released by the posterior pituitary gland, increases the permeability of the collecting ducts to water. This allows more water to be reabsorbed back into the blood, resulting in a more concentrated urine and maintaining blood water potential.

Aquaporins are channel proteins in the collecting duct membranes that facilitate water reabsorption. ADH stimulates the insertion of more aquaporins into the collecting duct membrane, increasing water permeability and reabsorption.

Glucagon binds to a specific receptor protein on the liver cell membrane. This binding causes a conformational change in the receptor, initiating the signalling cascade.

cAMP acts as a second messenger. It activates protein kinase A (PKA), which then initiates an enzyme cascade by phosphorylating other enzymes.

Signal amplification occurs through the enzyme cascade. Each activated enzyme phosphorylates and activates many more downstream enzymes, leading to a large overall effect.

Insulin stimulates the insertion of GLUT4 glucose transporter proteins into the muscle cell membrane. This increases the permeability of the membrane to glucose, allowing more glucose to enter the cell.

Insulin increases the rate of glycogenesis in liver cells. Glucose is converted to glycogen and stored, reducing blood glucose concentration.

Glucagon stimulates glycogenolysis in liver cells, breaking down glycogen into glucose. It also promotes gluconeogenesis, synthesizing glucose from non-carbohydrate sources, both increasing blood glucose concentration.

When blood glucose is high, insulin is released, lowering blood glucose. When blood glucose is low, glucagon is released, raising blood glucose. These opposing actions maintain glucose within a narrow range.