Urinary System: Structure, Function, and Regulation

Urinary System

Introduction

The urinary system is essential for maintaining homeostasis by excreting waste products and regulating the composition of blood. Its primary organ, the kidney, forms urine, while other structures serve for storage and transport.

• Primary function: Excretion of waste products and excess substances (e.g., water, ions).

• Location: Kidneys are retroperitoneal (behind the peritoneum).

Kidney Function

Excretion

• Ions: Excretes Na+, K+, Ca2+, Cl-, HPO42-, H+. Retains HCO3- (bicarbonate) to buffer H+.

• Water: Regulates blood volume, pressure, and osmolarity.

• Nitrogenous waste: Removes NH3+, urea (from protein metabolism).

• Other wastes: Drugs, toxins, and metabolic byproducts (often after liver processing).

Hormone Production

• Renin-Angiotensin-Aldosterone (RAA) system: Regulates blood pressure.

• Calcitriol: Regulates Ca2+ levels.

• Erythropoietin: Stimulates red blood cell production.

Gluconeogenesis

• Kidneys can produce glucose from glutamine (an amino acid), especially during fasting.

Anatomy of the Urinary System

Kidney Structure

• Renal capsule: Dense irregular connective tissue, outer layer.

• Adipose capsule: Protective fat layer; helps hold kidney in place. Nephroptosis ("floating kidney") can occur if this is insufficient.

• Renal fascia: Anchors the kidney.

• Cortex: Outer region; contains renal columns (extensions).

• Medulla: Inner region; contains renal pyramids.

Nephron

The nephron is the functional unit of the kidney. Each kidney contains about 1 million nephrons.

• Renal corpuscle: Includes the glomerulus (capillary network) and Bowman's capsule (glomerular capsule).

• Renal tubule: Consists of the proximal convoluted tubule, loop of Henle (descending and ascending limbs), and distal convoluted tubule. Collecting and papillary ducts follow.

All nephron segments are lined by a single layer of cells, but cell type and thickness vary.

Types of Nephrons

• Cortical nephrons: 80–85% of nephrons; short loops of Henle; produce dilute urine.

• Juxtamedullary nephrons: Long loops of Henle extend deep into medulla; produce concentrated urine.

Nephron Function

Urine formation involves three main processes: filtration, reabsorption, and secretion.

Glomerular Filtration

• Produces glomerular filtrate (portion of plasma entering the capsular space).

• About 16–20% of plasma is filtered (150–180 L/day); over 99% is reabsorbed.

Filtration Barriers

• Fenestrated capillaries: Large pores allow most substances except blood cells.

• Basal lamina: Retains large proteins.

• Podocytes: Filtration slits are the final barrier.

Filtered substances: H2O, glucose, vitamins, amino acids, small plasma proteins, NH3, urea, ions.

Net Filtration Pressure (NFP)

• Filtration is driven by pressure gradients:

  • Glomerular blood hydrostatic pressure (GBHP): Blood pressure in capillary.

  • Capsular hydrostatic pressure (CHP): Pressure in capsule.

  • Blood colloid osmotic pressure (BCOP): Osmotic gradient between blood and filtrate.

Equation:

$\text{NFP} = \text{GBHP} - \text{CHP} - \text{BCOP}$

Regulation of Filtration Rate

• Filtration rate is kept constant despite blood pressure changes by:

  • Adjusting blood flow to glomerulus

  • Changing surface area of glomerular capillaries

• Renal autoregulation:

  • Myogenic mechanism: Stretching of afferent arteriole triggers contraction.

  • Tubuloglomerular feedback: Juxtaglomerular apparatus constricts afferent arteriole as GFR increases.

• Neural regulation: Sympathetic stimulation causes vasoconstriction, reducing GFR.

• Hormonal regulation:

  • Angiotensin II: Vasoconstriction (RAA pathway).

  • Atrial natriuretic peptide (ANP): Increases capillary surface area.

Tubule Function

Most filtrate is reabsorbed in the tubules; secretion also occurs here.

Proximal Convoluted Tubule (PCT)

• Reabsorbs large amounts of water and solutes (driven by Na+ movement via symporters).

• 100% of glucose and amino acids reabsorbed.

• 65% of water and Na+ reabsorbed.

• H+ secreted (Na+/H+ antiporter).

• Passive diffusion of Cl-, K+, Ca2+, Mg2+ follows water reabsorption.

• NH3 and urea filtered and secreted.

Loop of Henle

• Reabsorbs 20–30% of Na+, K+, Ca2+; 15% of water.

• Descending limb: Reabsorbs water by osmosis.

• Thick ascending limb: Na+/K+/2Cl- symporters; impermeable to water.

Distal Convoluted Tubule (DCT)

• Na+/Cl- symporters for reabsorption.

• Ca2+ reabsorption (stimulated by parathyroid hormone, PTH).

• 15% of water reabsorbed.

Collecting Duct

• 90–95% of water and solutes reabsorbed by this point.

• Na+ moves in via leak channels.

• K+ may be secreted or reabsorbed depending on blood levels.

• H+ may be excreted.

Hormonal Regulation of Tubule Function

Renin–Angiotensin–Aldosterone (RAA) System

• Renin: Released by juxtaglomerular apparatus in response to low blood pressure; converts angiotensinogen to angiotensin I, then to angiotensin II.

• Angiotensin II: Vasoconstricts afferent arterioles (reduces filtrate formation), enhances Na+ and Cl- reabsorption in PCT, stimulates aldosterone secretion.

• Aldosterone: Stimulates Na+ and Cl- reabsorption in DCT (increases water reabsorption).

Antidiuretic Hormone (ADH)

• Released by posterior pituitary in response to low blood pressure or volume.

• Increases water permeability of collecting duct cells by increasing water channels (aquaporins).

• Promotes water reabsorption, concentrating urine.

Atrial Natriuretic Peptide (ANP)

• Secreted by the heart in response to increased blood pressure.

• Decreases production of ADH and aldosterone.

• Inhibits Na+ reabsorption in PCT and DCT (decreases water reabsorption).

Other Components of the Urinary System

Ureters

• Transport urine from kidneys to bladder by gravity, hydrostatic pressure, and peristalsis.

• Inner mucosa secretes mucus to protect ureteral cells.

Bladder

• Muscular, extensible sac; holds 700–800 mL of urine.

Micturition Reflex

• Bladder fullness detected by stretch receptors.

• Micturition center in sacral spinal cord; reflex usually triggered at ~400 mL.

• High degree of conscious control.

Incontinence

• Lack of voluntary control over urination.

Urethra

• Females: ~1.5 inches long.

• Males: 6–8 inches long.

Clinical and Applied Aspects

Diuretics

• Decrease water reabsorption in kidneys; increase urination rate.

• Examples: Alcohol (inhibits ADH), caffeine (inhibits Na+ reabsorption), antihypertensive drugs.

Dialysis

• Hemodialysis: Blood passes through a selectively permeable membrane; wastes diffuse into dialysis solution.

• Continuous ambulatory peritoneal dialysis: Peritoneum acts as dialysis membrane; dialysis fluid introduced into peritoneal cavity.

Osmoregulation and Seawater

• Seawater is ~3.5% salt; concentrated human urine is ~2% salt.

• It takes almost twice the volume of urine to excrete the salt from an equivalent volume of seawater.

Summary Table: Key Features of Nephron Segments

Segment	Main Function	Key Transporters/Processes	Hormonal Influence
	Proximal Convoluted Tubule (PCT)	Bulk reabsorption of water, ions, nutrients	Na+ symporters, Na+/H+ antiporters	None
Loop of Henle	Concentration of urine, reabsorption of water and ions	Na+/K+/2Cl- symporters (thick ascending limb)	None
Distal Convoluted Tubule (DCT)	Fine-tuning of ion reabsorption	Na+/Cl- symporters	Aldosterone, PTH
Collecting Duct	Final water and ion reabsorption, acid-base balance	Water channels (aquaporins), Na+ leak channels	ADH, Aldosterone, ANP

Example: In dehydration, ADH levels rise, increasing water reabsorption in the collecting duct and producing concentrated urine.