Overview
This lecture introduces the human excretory system, highlights the kidneys and nephron, and explains how urine is produced and processed.
Introduction to the Excretory System
- The excretory system removes metabolic wastes and maintains osmotic balance in the body.
- Major metabolic wastes include carbon dioxide and nitrogenous wastes like urea from protein breakdown.
- Organs involved: skin (excretes water/substances), liver (detoxifies, produces urea), lungs (excrete CO₂), and kidneys (main focus).
Urinary System Components
- The urinary system includes two kidneys (filter blood), two ureters (drain urine), one bladder (stores urine), and one urethra (expels urine).
- Kidneys filter blood to produce urine, removing waste and maintaining fluid/salt balance.
Structure and Function of the Nephron
- Nephrons, about a million per kidney, are the functional units that filter blood and form urine.
- The nephron starts at the glomerulus (capillaries) surrounded by Bowman’s capsule; blood pressure forces filtrate into the capsule.
- Filtrate contains water, salts, glucose, amino acids, urea, and other substances.
Nephron Processing: Filtrate Journey
- In the proximal tubule, NaCl, water, glucose, amino acids, and bicarbonate are reabsorbed; H⁺ and NH₄⁺ are secreted, aiding pH regulation.
- Descending limb of the loop of Henle: water is reabsorbed by osmosis; filtrate becomes concentrated as solutes remain.
- Ascending limb: NaCl is reabsorbed (thin segment by diffusion, thick by active transport); filtrate becomes dilute.
- Distal convoluted tubule: more secretion of H⁺, NH₄⁺, K⁺ occurs; NaCl, water, and bicarbonate reabsorbed; pH regulation continues.
- Collecting duct: final adjustment of water and NaCl reabsorption, regulated by hormones; urine concentration varies by hydration status.
- Some urea diffuses back into interstitial fluid from the collecting duct.
Additional Details
- Diuretics increase urine water content and treat conditions like high blood pressure.
- Dialysis (hemodialysis or peritoneal) is used if kidneys fail to filter blood.
- Nephrology is the medical specialty focused on kidney function.
Key Terms & Definitions
- Excretory system — removes metabolic wastes and maintains osmotic balance.
- Nephron — the functional unit of the kidney; filters blood and forms urine.
- Glomerulus — a cluster of capillaries where filtration begins.
- Bowman’s capsule — surrounds the glomerulus and collects filtrate.
- Filtrate — fluid filtered from blood containing waste and useful substances.
- Reabsorption — movement of substances from filtrate back into the body.
- Secretion — movement of substances from the body into the filtrate.
- Diuretics — medications that increase urine output.
- Dialysis — artificial blood filtration for kidney failure.
Action Items / Next Steps
- Review nephron part functions and the process of urine formation.
- Explore further the mechanisms of active/passive transport in the nephron.
- Read more about hormonal control in water reabsorption (ADH/aldosterone).