Overview
This lecture explains excretion in humans and animals, focusing on the structure and function of the human excretory system, kidney function, urine formation, hormonal regulation, and related disorders.
Excretory Products and Types
- Metabolic processes produce wastes such as carbon dioxide, ammonia, urea, uric acid, water, and excess ions.
- Excretion is the process of removing metabolic wastes from the body.
- Ammonia, urea, and uric acid are nitrogenous wastes; ammonia is most toxic and needs the most water to excrete.
- Ammonotelic animals (e.g., bony fish) excrete ammonia; ureotelic animals (e.g., mammals) excrete urea; uricotelic animals (e.g., birds) excrete uric acid.
Evolution and Structure of Excretory Systems
- Primitive animals have simple excretory organs like flame cells (protonephridia).
- More advanced animals evolved organs such as nephridia, Malpighian tubules, green glands, and kidneys.
- Human excretory system includes kidneys, ureters, urinary bladder, and urethra.
Structure of the Human Kidney
- Kidneys are bean-shaped, reddish-brown organs, protected partially by the ribs.
- Each kidney contains about a million nephrons, the functional units.
- Nephron consists of the glomerulus and renal tubule (Bowman's capsule, PCT, Henle’s loop, DCT, collecting duct).
- Cortical nephrons have short Henle’s loops; juxtamedullary nephrons have long Henle’s loops.
Urine Formation
- Urine formation includes glomerular filtration, reabsorption, and secretion.
- Glomerular filtration occurs in the glomerulus; about 125 ml/min filtered (GFR).
- Reabsorption and secretion occur in the PCT, Henle’s loop, DCT, and collecting duct.
- Counter-current mechanism in Henle’s loop and vasa recta concentrates urine.
Hormonal Regulation of Kidney Function
- ADH (vasopressin) enhances water reabsorption when fluid levels are low.
- JGA releases renin when blood pressure/GFR falls; triggers angiotensin and aldosterone, increasing blood pressure and sodium reabsorption.
- Heart releases ANF when blood pressure is high, reducing sodium/water via vasodilation and inhibiting renin-angiotensin system.
Disorders and Diagnostics
- Normal urine output is 1-1.5 liters/day, acidic (pH 6), yellowish, and may indicate metabolic disorders.
- Glycosuria indicates diabetes mellitus; ketonuria indicates ketone presence.
- Uremia (high urea in blood) may require hemodialysis or kidney transplantation.
- Renal calculi (kidney stones) and glomerulonephritis are other kidney disorders.
Key Terms & Definitions
- Excretion — elimination of metabolic waste from the body.
- Nephron — basic functional unit of the kidney.
- Glomerular Filtration Rate (GFR) — rate at which blood is filtered in the glomeruli.
- Antidiuretic Hormone (ADH) — hormone that increases water reabsorption in kidneys.
- Renin-Angiotensin Mechanism — hormonal system regulating blood pressure and kidney function.
- Atrial Natriuretic Factor (ANF) — hormone from the heart that reduces blood sodium and water.
- Uremia — accumulation of urea in the blood.
- Hemodialysis — artificial removal of waste from the blood.
Action Items / Next Steps
- Review kidney structure diagrams and nephron parts.
- Complete assigned reading on excretory system disorders.
- Prepare for questions on hormonal regulation of kidney function.