Overview
This lecture covers the anatomy, function, and regulation of the urinary (renal) system, focusing on the kidney and processes involved in urine formation and homeostasis.
Urinary System Anatomy & Function
- The urinary system includes the kidneys, ureters, bladder, and urethra.
- Kidneys receive 20-25% of cardiac output and regulate blood volume, electrolytes, acid-base balance, remove waste, activate vitamin D, and produce hormones (EPO, renin).
- The nephron, the kidney’s functional unit, filters blood to form urine; each kidney has over 1 million nephrons.
Nephron Structure & Filtrate Pathway
- Blood enters via afferent arteriole into the glomerulus (capillary tuft).
- Filtrate passes through Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT), and collecting duct.
Glomerular Filtration
- Glomerular filtration separates large proteins/cells (stay in blood) from water, ions, and small molecules (enter filtrate).
- The filtration membrane consists of fenestrated capillary endothelium, basement membrane, and podocyte filtration slits.
- Net filtration pressure (NFP) is determined by hydrostatic and osmotic pressures; typical NFP ≈ 10 mmHg.
Regulation of GFR
- GFR (glomerular filtration rate) is regulated by blood pressure, autoregulation (myogenic and tubuloglomerular feedback), and sympathetic nervous system.
- Autoregulation maintains constant GFR via afferent arteriole constriction (high GFR) or dilation (low GFR).
- Sympathetic stimulation decreases GFR to reduce urine output during stress.
Tubular Reabsorption
- Reabsorption returns needed substances (water, ions, glucose, amino acids) from filtrate to blood, mainly in the PCT.
- 99% of water, all nutrients, and most sodium are reabsorbed.
- Transport occurs via passive (diffusion, osmosis) and active (Na+/K+ pump, co-transport) mechanisms; T-max defines the maximum reabsorption capacity for substances.
- Glucose has a high reabsorption threshold; excess appears in urine only if plasma levels are abnormally high.
Tubular Secretion & Urine Concentration
- Secretion moves unwanted substances (H+, K+, toxins, drugs) from blood into filtrate, mainly in DCT and collecting duct.
- Acid and potassium secretion help maintain pH and electrolyte balance; aldosterone increases sodium reabsorption and potassium secretion.
- Loop of Henle creates an osmotic gradient (countercurrent mechanism) for water reabsorption; ADH increases water reabsorption in collecting duct.
- Urine concentration depends on hydration status and hormone levels (aldosterone, ADH).
Plasma Clearance & Urination
- Plasma clearance is the rate a substance is removed from plasma via filtration, reabsorption, and secretion.
- Creatinine clearance estimates GFR; glucose normally has zero clearance (fully reabsorbed), acids are secreted and cleared quickly.
- Bladder stretch receptors trigger urination reflex; voluntary control is via the external urethral sphincter.
Key Terms & Definitions
- Nephron — functional unit of the kidney, filters and regulates blood.
- Glomerulus — capillary tuft in nephron for blood filtration.
- Bowman's Capsule — surrounds glomerulus, collects filtrate.
- GFR (Glomerular Filtration Rate) — rate of filtrate formation in kidneys.
- Tubular Reabsorption — movement of substances from filtrate back to blood.
- Tubular Secretion — movement of substances from blood into filtrate.
- T-max (Transport Maximum) — maximal reabsorption/secretion rate for a substance.
- Aldosterone — adrenal hormone increasing sodium reabsorption and potassium secretion.
- ADH (Antidiuretic Hormone) — hormone increasing water reabsorption in collecting duct.
Action Items / Next Steps
- Redraw the nephron and annotate locations of filtration, reabsorption, and secretion.
- Review summaries/diagrams in the textbook for step-by-step nephron processes.
- Prepare questions for clarification on any unclear mechanisms before next class.