BIO 304 · Human Anatomy & Physiology
Kidney Anatomy & Glomerular Filtration
Urinary System · Module 15
A reference for the Kidney Anatomy & GFR video. Each kidney has about a million nephrons. Each nephron starts with a glomerulus that filters plasma into a tubule. Filtration rate is the most important number in nephrology.
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- Identify the gross and microscopic anatomy of the kidney and nephron.
- Explain the three layers of the filtration barrier and what they restrict.
- Calculate net filtration pressure and describe the regulation of GFR.
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Kidney & Nephron Anatomy
Gross anatomy
- Cortexouter; houses most of the nephron
- Medullainner; renal pyramids; collecting ducts
- Renal pyramidscone-shaped; apex (papilla) drips into minor calyx
- Calyces & renal pelviscollect urine; funnel into ureter
- Ureter → bladder → urethratransports urine out
- Renal artery / veinenter/exit at hilum; ~20% of cardiac output goes to kidneys
Nephron parts (in order)
- Glomeruluscapillary tuft; site of filtration
- Bowman capsulecup around glomerulus; receives filtrate
- Proximal convoluted tubule (PCT)cortex; massive reabsorption
- Descending loop of Henleinto medulla; permeable to water
- Ascending loop of Henlepermeable to ions, not water; sets up gradient
- Distal convoluted tubule (DCT)back in cortex; fine-tunes ions
- Collecting ductfinal concentration; ADH-regulated water reabsorption
Two nephron types
- Cortical (~85%)short loops; do most filtration
- Juxtamedullarylong loops dipping deep into medulla; concentrate urine
Filtration barrier (3 layers)
- Fenestrated capillary endotheliumlets out water and small solutes; blocks cells
- Basement membraneshared GBM; charge and size barrier; blocks most proteins
- Podocyte filtration slitsfoot processes wrap glomerular capillaries; slit diaphragms
GFR & Regulation
What gets filtered
- Waterfreely filtered
- Small solutes (Na+, K+, glucose, urea, amino acids)freely filtered
- Albuminnormally not filtered (charge + size); appearance in urine = problem
- Cellsnot filtered (RBCs, WBCs)
Net filtration pressure
- Glomerular hydrostatic pressure (~55 mmHg)pushes fluid out
- Bowman capsule hydrostatic pressure (~15 mmHg)pushes back in
- Glomerular oncotic pressure (~30 mmHg)pulls fluid back in
- Net pressure (~10 mmHg)55 - 15 - 30 = 10 mmHg outward
GFR (glomerular filtration rate)
- Normal adult~125 mL/min, ~180 L/day
- Almost all reabsorbed~1.5 L/day leaves as urine
- Best clinical measurecreatinine clearance estimates GFR
Regulation of GFR
- Myogenic autoregulationafferent arteriole constricts when BP rises (smooth muscle stretch reflex)
- Tubuloglomerular feedbackmacula densa senses Na+/Cl- at DCT; signals afferent arteriole
- Sympathetic nervous systemsevere drop in BP → afferent constriction → GFR drops
- Renin-angiotensin (RAAS)low BP → renin from JG cells → angiotensin II → constricts efferent arteriole → maintains GFR
Juxtaglomerular apparatus
- Macula densacells of DCT next to afferent arteriole; chemoreceptors
- JG (granular) cellsmodified smooth muscle of afferent; release renin; baroreceptors
Other kidney functions
- Erythropoietin (EPO)production in response to low O₂
- Activates vitamin Dfinal hydroxylation step
- Acid-base balancereabsorbs HCO₃-; excretes H+ as needed
- Glucose homeostasisgluconeogenesis during fasting
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