Urinary System 1 & 2
Urinary System: General Concepts
Purpose of the urinary system (Urinary System 1 & 2): to maintain body fluid volume and composition within normal limits and support homeostasis.
Key roles include osmoregulation, removal of nitrogenous wastes from cellular metabolism, and excretion of toxins.
Close association with the reproductive system (often referred to as the urogenital/urinogenital system).
Also described as the renal system (renalis = of the kidneys).
Learning outcomes (from Course Materials):
Name and identify the structures of the urinary system.
Name and identify gross and microscopic anatomical features of the kidney.
Describe kidney functions and the structure and function of the nephron.
Explain how urine concentration and volume are regulated.
Reference texts: Saladin (Chapter 23) and McKinley (Chapter 24).
Kidney and urinary system functions (Overview):
Homeostasis
Water balance
Electrolyte balance
Removal of nitrogenous wastes
Removal of toxins
Blood pH balance (including excretion of H⁺ and reabsorption of HCO₃⁻ to maintain pH ~ 7.4)
Blood pressure regulation (renin)
Red blood cell production (erythropoietin)
Osmoregulation: balancing water and dissolved solutes (Na⁺, Cl⁻, K⁺, Ca²⁺, HCO₃⁻)
Water gain sources: food, drink, metabolic water
Water loss sources: urination, defecation, evaporation, breathing, sweating
Tip: revise osmosis
Osmoregulation and the Kidneys
The kidneys respond to fluid intake:
High fluid intake → dilute urine with conservation of salts
Low fluid intake → concentrated urine with water conservation
Ability to concentrate urine to ≈ 4\times\text{blood osmotic concentration}
Purpose: excrete wastes with minimal water loss; kidneys conserve water and regulate plasma osmolarity.
Excretion of nitrogenous wastes (overview):
Ammonia (NH₃): highly toxic, highly soluble in water, diffuses rapidly, must be excreted in large volumes of very dilute solutions when water is available.
Urea: highly soluble in water, ~100,000× less toxic than NH₃, can be stored in a concentrated solution; requires water for disposal.
Uric acid: complex, relatively non-toxic, largely insoluble in water; excretion minimizes water loss but requires more energy; energy cost offset by water savings.
The Urinary System: Anatomy and Organs
Organs and their basic roles:
Kidneys: produce urine
Ureters: transport urine to the bladder
Bladder: stores urine
Urethra: conveys urine outside the body
Adrenal glands are located on top of the kidneys; kidneys and vessels arranged retroperitoneally.
Anatomical references (general orientation):
Kidneys lie retroperitoneally; related vessels include the renal artery (from the aorta) and renal vein (draining to the inferior vena cava).
The renal fascia, peritoneum, and surrounding structures position the kidneys in the posterior abdominal wall near vertebral level ~L2.
The urinary system operates alongside the reproductive system (hence terms like urogenital/urinogenital).
Gross Anatomy of the Kidney
Key external features:
Fibrous capsule
Renal cortex (outer layer)
Renal medulla (inner region; contains renal pyramids)
Renal pelvis (funnel for urine flowing to the ureter)
Major calyx and Minor calyx (drain into renal pelvis)
Renal papilla (apex of renal pyramid projecting into minor calyx)
Renal column (cortical tissue between pyramids)
Nephron units reside within the cortex and medulla; collecting ducts extend toward the renal papilla.
Vascular arrangement:
Renal artery supplies each kidney (branch off the aorta).
Kidney filtrate is processed and drained by the renal vein into the inferior vena cava.
Kidneys receive ~20–25% of cardiac output while constituting <1% of body weight.
Daily filtrate through renal capillaries: ≈ 1{,}100\text{–}2{,}000\text{ L}.
Daily filtrate produced by kidneys: ≈ 180\text{ L/day}; if excreted completely, would dehydrate the body.
The kidneys refine filtrate by concentrating urea and returning water and solutes to the blood, yielding ≈ 1.5\text{ L} of urine per day.
General study notes: kidneys are retroperitoneal organs, with the adrenal glands sitting on their superior poles; the renal hilum houses vessels, nerves, and the ureter.
Nephron: Functional Unit of the Kidney
Each kidney contains roughly 1.2\times 10^{6} nephrons.
Nephron structure:
Renal corpuscle: glomerulus + Bowman’s capsule
Renal tubule: proximal convoluted tubule (PCT) → Loop of Henle → distal convoluted tubule (DCT) → collecting duct (CD)
Collecting duct(s) drain several nephrons into papillary ducts, minor calyx, major calyx, renal pelvis, ureter.
Nephron function:
Nephrons extract filtrate from blood and refine it into urine
Filtrate formation and solute/water handling occur along the tubule with selective reabsorption and secretion.
Glomerular Filtration and Filtration Membrane
Glomerular filtration creates a plasma-like filtrate from blood plasma; starts at the renal corpuscle.
Filtration membrane consists of three barriers:
Endothelium of glomerular capillaries
Basement membrane
Filtration slits between podocyte extensions (pedicels)
Filtration process:
Afferent arteriole diameter > efferent arteriole diameter, generating high hydrostatic pressure in the glomerulus.
Small solutes pass through slits; larger components (RBCs, most plasma proteins) remain in blood.
Components that pass into the capsular space/tubule: water, electrolytes, glucose, amino acids, fatty acids, vitamins, urea, uric acid, creatinine.
Filtration barriers prevent passage of blood cells, plasma proteins, large anions, and protein-bound minerals/hormones; most molecules > ~8 nm cannot pass.
Filtration membrane damage can lead to proteinuria and hematuria.
Bowman’s capsule and glomerulus have distinct poles:
Vascular pole: afferent enters, efferent exits
Urinary pole: where renal tubule begins
Podocytes: visceral layer with foot processes wrapping glomerular capillaries; filtration slits highly selective.
Glomerular Filtration Rate (GFR) and Regulation
GFR is the volume of filtrate formed by both kidneys per day.
Typical GFR values:
Female: ≈ 150\text{ L/day}
Male: ≈ 180\text{ L/day}
GFR is about 30–35× the total blood volume in the body; about 99% of filtrate is reabsorbed; urine output is ≈ 1.5\text{ L/day} (often cited as 1–2 L/day).
If GFR is too high:
Filtrate passes through tubules too rapidly for adequate reabsorption → dehydration and electrolyte depletion.
If GFR is too low:
Wastes are reabsorbed, leading to azotemia (elevated nitrogenous wastes in blood).
Regulation of GFR: autoregulation of nephrons maintains stable GFR amidst arterial flow changes; sympathetic stimulation (e.g., during exercise or shock) constricts afferent arterioles, reducing GFR and urinary output to redirect blood to vital organs.
The Nephron: Tubular Anatomy and Function
The nephron tubule components and their epithelial features:
Proximal convoluted tubule (PCT): located in cortex; long; simple cuboidal epithelium with prominent microvilli; abundant mitochondria; surrounded by peritubular capillaries; primary site of reabsorption.
Loop of Henle: descending limb (thick/thin segments) and ascending limb; descending limb highly permeable to water (thin segment); thick ascending limb has active salt transport; medullary architecture underpins concentration gradient.
Distal convoluted tubule (DCT): cortex; simple cuboidal epithelium; lacks brush border (no microvilli).
Collecting ducts: medulla to papilla; simple cuboidal epithelium; site of final urine concentration; water permeability regulated by ADH.
Blood flow and tubule flow:
Peritubular capillaries surround PCT and DCT; vasa recta surround the loop of Henle in juxtamedullary nephrons to maintain medullary osmolality gradients.
Proximal Convoluted Tubule (PCT) – Structure and Function
PCT characteristics:
Cortex location; long tubule with abundant microvilli; large surface area for absorption.
Lined with simple cuboidal epithelium; abundant mitochondria provide ATP for active transport.
Primary function:
Reabsorbs ≈ 65\% of glomerular filtrate (water and solutes) back into blood.
Also secretes certain substances into tubular fluid for disposal in urine.
Reabsorption mechanics:
Two routes:
Transcellular route: through epithelial cells; involves active and passive transport of solutes.
Paracellular route: between cells through leaky junctions; water follows solutes via solvent drag.
Water reabsorption is mostly obligatory, driven by solute reabsorption; water follows active transport of solutes.
Solvent drag: water carries dissolved solutes through tight junctions.
Loop of Henle – Thin and Thick Segments
Loop of Henle structure:
Descending limb: thin segment, simple squamous epithelium; highly permeable to water; reabsorbs water.
Ascending limb: thin segment (if present) and thick segment; thick segment lined with simple cuboidal epithelium with many mitochondria; active transport of salts; reabsorption of Na⁺, K⁺, Cl⁻.
Function:
Creates a concentration gradient from cortex to medulla (medullary osmotic gradient) that enables water reabsorption in the collecting duct.
Countercurrent multiplier concept (illustrated):
The more salt pumped out of the ascending limb, the saltier the interstitial fluid of the medulla becomes.
The more water that leaves the descending limb, the saltier the filtrate in the tubule becomes.
Higher medullary osmolarity increases water reabsorption in the descending limb and collecting duct.
Vasa Recta and Medullary Osmotic Gradient
Vasa recta structure and function:
Capillaries run in a countercurrent arrangement to the loop of Henle: descending vasa recta lose water and gain salt, while ascending vasa recta gain water and lose salt as they rise.
This countercurrent exchange maintains the medullary osmolality gradient essential for concentrating urine.
Collecting Duct and Water Conservation
Collecting duct characteristics:
Location: medulla to cortex; permeable to water in the medullary portion.
Function: concentrates urine by reabsorbing water under the influence of antidiuretic hormone (ADH).
Antidiuretic hormone (ADH):
ADH rises when blood osmolarity increases or blood volume decreases.
Increases water permeability of the collecting duct and distal tubule, concentrating urine.
In cases of overhydration, ADH secretion decreases, producing larger volumes of dilute urine.
Clinical correlation: Diabetes insipidus involves high urine output (polyuria) and thirst (polydipsia) due to deficient ADH or kidney insensitivity to ADH.
Urea Recycling and Medullary Osmolarity
Urea handling:
Glomerular filtration continually adds urea to filtrate.
Thick ascending limb and DCT are relatively impermeable to urea, increasing its concentration in the filtrate.
Collecting duct is permeable to urea; urea leaks out into the medullary interstitium, aiding the osmotic gradient.
Some urea travels back to the descending limb, creating a loop that helps maintain medullary osmolarity.
This constant recycling maintains the high osmolarity of the deep medulla.
Inter-Species Variation in Urine Concentration
Renal medulla and loop length correlate with concentrating ability:
Beavers: longer loops, can concentrate urine ~2× plasma.
Humans: loops of Henle moderately long; concentrate urine ~4× plasma.
Desert mammals (e.g., camels): very long loops; urine ~8× plasma.
Australian hopping mouse: can concentrate urine ~22× plasma.
Implication: longer loops of Henle increase medullary osmotic gradient, enabling greater water reabsorption and more concentrated urine.
Ureters and Bladder – Structure and Function
Ureters:
Mucosa lined by transitional epithelium (urothelium) with lamina propria.
Muscularis composed of 2–3 layers of smooth muscle; no true submucosa.
Peristaltic contractions (peristaltic waves) propel urine from renal pelvis to bladder.
Presence of transitional epithelium allows distension with urine flow.
Urinary bladder:
Muscular sac located on the floor of the pelvic cavity.
Capacity ≈ 500\text{ mL}, maximum 700\text{–}800\text{ mL}.
Wall architecture: mucosa lined by transitional epithelium; lamina propria; muscularis (3 layers of smooth muscle).
As the bladder fills, it expands superiorly; rugae flatten; epithelium thins from 5–6 layers to 2–3 layers.
Urethra and sphincters:
Internal urethral sphincter (in males): thickening of detrusor muscle; involuntary control; compresses urethra to retain urine.
External urethral sphincter: skeletal muscle; voluntary control; located in the pelvic floor.
Pattern similar to internal/external anal sphincters.
Neural Control of Micturition
Neural circuit overview:
Stretch receptors in bladder wall detect fullness; signals travel to sacral spinal cord (S2–S4).
Reflex arc: contraction of detrusor muscle and relaxation of internal urethral sphincter; bladder empties.
Brain involvement allows voluntary control over the external urethral sphincter.
When the bladder is distended:
Signals initiate micturition; if timely, the brain can coordinate voluntary control to relax the external sphincter and allow urination.
Kidney Anatomy and Nephron Details (Revisited)
Nephron components and flow:
Renal corpuscle (glomerulus + Bowman’s capsule) filters blood plasma to form filtrate.
Renal tubule (PCT → Loop of Henle → DCT) refines filtrate into urine.
Collecting duct collects filtrate from multiple nephrons for final urine formation.
Filtrate flow sequence:
Bowman’s capsule → Proximal tubule → Descending limb of Loop of Henle → Ascending limb → Distal tubule → Collecting duct → Papillary duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Urinary bladder → Urethra.
Summary: Major Physiological Processes in Urine Formation
Four major stages of urine formation:
1) Glomerular filtration: plasma-like filtrate produced from blood plasma.
2) Tubular reabsorption: removal of useful solutes from filtrate back to blood.
3) Tubular secretion: addition of wastes from blood into filtrate.
4) Water conservation: reabsorption of water from urine to blood, concentrating wastes in urine.
Key abstractions:
Glomerular filtration rate (GFR) and its regulation are central to kidney function.
Autoregulation stabilizes GFR; sympathetic input can override autoregulation during stress to protect organs and maintain essential blood flow.
The nephron’s tubules have specialized epithelia dedicated to bulk reabsorption or secretion and are driven by solute gradients and hormonal controls (e.g., ADH).
Equations and Quantitative Details (LaTeX)
GFR values:
\text{GFR}_{\text{female}} \approx 150\ \text{L/day}
\text{GFR}_{\text{male}} \approx 180\ \text{L/day}
Urine production (typical):
\text{Urine daily} \approx 1.5\ \text{L} (range 1–2 L/day)
Concentration capability of kidney: approximately 4\times\text{blood osmotic concentration} in urine concentrating ability.
Urea handling gradient:
Urea contributes to medullary osmolarity via recycling between the collecting duct and loop of Henle.
References for Context
Saladin: Chapter 23; McKinley: Chapter 24 (for cross-reference)
General clinical context: renal filtration, GFR regulation, and tubule transport mechanisms are foundational to understanding renal physiology and disease states.
Quick Connections to Real-World Relevance
Renin release ties renal function to blood pressure regulation via the RAAS pathway.
Erythropoietin produced by kidneys links renal health to oxygen transport capacity in the body.
ADH abnormalities underlie disorders of water balance (e.g., diabetes insipidus).
Understanding glucosuria (glucose in urine) and the renal threshold helps explain pathology in diabetes mellitus (Type 1): when filtered glucose exceeds transport maximum (Tm), glucose appears in urine.
Urea recycling and medullary gradient underpin the body’s ability to conserve water in arid environments; mammals show substantial variation in this capacity depending on loop length.
Note: All indices, constants, and values above are drawn from the provided transcript of lecture slides and align with standard renal physiology concepts (nephron function, filtration, reabsorption, secretion, and urine concentration). Use the equations and values to reinforce memory and problem-solving in exam scenarios.