Excretory System and Homeostasis

• Homeostasis maintains a stable internal environment despite external changes.
• The homeostatic process regulates water balance, salt concentration, pH, and temperature.
• Proper cell function, enzyme activity, and blood pressure is maintained by homeostasis.
• The endocrine, nervous, and excretory systems are key systems involved in homeostasis.

Excretory System Overview

• The excretory system removes metabolic wastes, especially nitrogenous waste.
• This system also maintains fluid and electrolyte balance.
• Major organs involved include the kidneys, ureters, bladder, and urethra.
• Kidneys filter blood to remove waste (urea, creatinine), maintain salt/water balance, and regulate blood pressure via hormones.

Nephron

• Each kidney contains approximately 1 million nephrons.
• Nephrons perform filtration, reabsorption, secretion, and excretion.

Filtration in the Glomerulus

• Blood enters the glomerulus via the afferent arteriole into a capillary knot.
• High pressure forces plasma (minus proteins) into Bowman's capsule, forming filtrate.
• Filters: Water, salts, glucose, amino acids, urea.
• Blood cells and large proteins remain in the blood during filtration.

Pathway Through a Nephron

• Glomerulus filters blood into Bowman's capsule (filtration).
• The Proximal Convoluted Tubule (PCT) reabsorbs most water, glucose, amino acids, and Na⁺, and secretes some toxins/drugs into filtrate.
• The Descending loop of Henle is permeable to water but impermeable to salts, allowing water reabsorption (osmosis) and filtrate concentration.
• The Ascending loop of Henle is impermeable to water but actively pumps Na⁺ and Cl⁻ into interstitial fluid, diluting the filtrate.
• The Distal Convoluted Tubule (DCT) reabsorbs Na⁺, regulated by aldosterone, and secretes more wastes like H⁺ and K⁺.
• The Collecting duct adjusts final water content based on ADH and drains urine into the renal pelvis, then the ureter, and finally the bladder.

Filtration & Reabsorption of Water and Solutes

• 99% of filtered water is reabsorbed back into the blood.
• Most essential solutes like glucose, Na⁺, and amino acids are reabsorbed in the PCT.
• The secretion of wastes into tubules helps remove toxins that were not filtered at the glomerulus.

Countercurrent Flow and Loop of Henle

• The loop of Henle sets up a countercurrent multiplier, with the descending limb losing water and the ascending limb pumping out salts.
• This creates a salty medullary gradient, which pulls water out of the collecting duct.
• Kidneys produce concentrated urine when needed.

Role of ADH (Antidiuretic Hormone)

• ADH is produced by the hypothalamus and released by the posterior pituitary.
• This increases the permeability of the collecting duct to water.
• High blood osmolarity (dehydration) triggers ADH release.
• ADH results in more water reabsorbed and less, more concentrated urine.
• Without ADH, dilute urine is produced, while with ADH, concentrated urine is produced.

Summary of Structures & Function

• Glomerulus: Filters blood under pressure.
• Bowman's Capsule: Collects filtrate.
• PCT: Reabsorbs water, Na+, glucose, and amino acids.
• Loop of Henle: Sets up a salt gradient for water reabsorption.
• DCT: Reabsorbs Na+, secretes H+, K+.
• Collecting Duct: Reabsorption is controlled by ADH.
• ADH: Hormone that reduces water loss via urine.