The Urinary System: Structure, Function, and Homeostasis

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The Urinary System

Overview and Functions

The urinary system is essential for removing waste products from the body, regulating blood composition, and maintaining homeostasis. It consists of the kidneys, ureters, urinary bladder, and urethra.

Excretion of Wastes: Kidneys filter out nitrogenous wastes, toxins, drugs, ions, and hormones from the blood, forming urine.
Regulatory Functions: The kidneys produce renin (activates angiotensin for blood pressure regulation), erythropoietin (EPO) (stimulates red blood cell production), and convert vitamin D to calcitriol.
Homeostasis: Control blood volume, blood pressure, and blood pH.

Organs of the Urinary System

Kidneys: Filter blood and produce urine.
Ureters: Muscular tubes that transport urine from kidneys to the bladder.
Urinary Bladder: Stores urine until excretion.
Urethra: Conducts urine from the bladder to the outside of the body.

Kidney Structure and Blood Supply

Internal Anatomy

Renal Cortex: Outer region of the kidney.
Renal Medulla: Deeper region, composed of renal pyramids.
Renal Pelvis: Collects urine from calyces and channels it to the ureter.
Calyces: Collect urine from the pyramids.

Blood Supply

Kidneys receive 20–25% of resting cardiac output.
Renal artery: Supplies blood to the kidney.
Renal vein: Drains blood from the kidney.

Nephrons: Structure and Function

Types of Nephrons

Cortical Nephrons: Most common; short nephron loops.
Juxtamedullary Nephrons: Long loops that extend deep into the medulla.

Main Structures

Glomerular Capsule (Bowman's Capsule): Collects filtrate from blood.
Renal Tubule: Modifies filtrate into urine; consists of the proximal convoluted tubule (PCT), nephron loop, and distal convoluted tubule (DCT).

Renal Corpuscle

Glomerulus: Spherical capillary bed specialized for filtration.
Glomerular Capsule: Double-layered cup with visceral (podocytes) and parietal layers; filtration slits create a porous membrane.

Renal Tubule and Associated Capillaries

Renal Tubule: Extends from the glomerular capsule to the collecting duct.
Collecting Ducts: Collect urine from multiple nephrons.
Peritubular Capillaries: Low-pressure, porous capillaries for reabsorption and secretion; surround the renal tubule.

Urine Formation and Characteristics

Three Main Processes

Glomerular Filtration: Water and solutes smaller than proteins are forced out of the blood into the capsule by blood pressure. Plasma proteins and blood cells are too large to pass through.
Tubular Reabsorption: Water, glucose, amino acids, and ions are reabsorbed into the blood, mainly in the PCT. Most reabsorption is active, some is passive.
Tubular Secretion: Wastes (H+, K+, creatinine, drugs) are secreted from the blood into the filtrate.

Nitrogenous Wastes in Urine

Urea: End product of amino acid breakdown.
Uric Acid: From nucleic acid catabolism.
Creatinine: From muscle metabolism of creatine.
Ammonium (NH3+): From amino acid catabolism.

Urine Volume and Composition

1.0 to 1.8 liters of urine produced in 24 hours.
Filtrate: Contains everything in plasma except proteins and cells.
Urine: What remains after reabsorption; contains wastes, ions, and water.
Abnormal Constituents: Glucose, plasma proteins, RBCs, hemoglobin, WBCs, bile (not normally found in urine).

Ureters, Urinary Bladder, and Urethra

Ureters

Muscular tubes connecting kidneys to the bladder.
Transport urine via peristalsis, pressure, and gravity.

Urinary Bladder

Muscular, expandable sac for urine storage.
Two ureteral openings and one urethral opening (internal urethral orifice).
Internal urethral sphincter controls urine release.
In males, the prostate is located just below the bladder.

Urethra

Thin-walled tube carrying urine out of the body.
Females: Only urine passes through.
Males: Carries urine and sperm; divided into prostatic, membranous, and spongy urethra.

Micturition (Urination)

Voiding or emptying of the bladder.
Controlled by two sphincters: internal (smooth muscle) and external (skeletal muscle).
Bladder stretch receptors trigger reflex contractions when filled (200–500 mL).
Voluntary relaxation of sphincters allows urination.

Fluid, Electrolyte, and Acid-Base Balance

Roles of the Kidneys

Excrete nitrogenous wastes.
Maintain water and electrolyte balance.
Regulate blood pH.

Water Balance

Body Water Content: Females ~50%, males ~60%.
Compartments: Intracellular fluid (ICF, ~67%), extracellular fluid (ECF, ~33%).
Water intake (food, fluids, metabolism) must equal output (lungs, sweat, feces, urine).
Urine output is the only regulated factor.

Hormonal Regulation

Antidiuretic Hormone (ADH): Increases water reabsorption in the kidneys.
Aldosterone: Increases sodium reabsorption; water follows sodium.
Both hormones act on the DCT and collecting ducts.

Electrolyte Balance

Electrolyte changes cause water to shift between compartments.
For each Na+ reabsorbed, a Cl- follows; water follows salt.

Renin-Angiotensin Mechanism

Drop in kidney blood pressure triggers renin release.
Renin activates angiotensin, which increases BP by stimulating ADH and aldosterone, increasing Na+ reabsorption, thirst, and vasoconstriction.

Acid-Base Balance

Blood pH must remain between 7.35 and 7.45.
Alkalosis: pH above 7.45; Acidosis: pH below 7.35.
Kidneys regulate pH by secreting or reabsorbing H+ and HCO3-.
Buffer systems (bicarbonate, phosphate, protein) temporarily limit pH changes.

Renal Mechanisms for pH

When blood pH rises: HCO3- is secreted, H+ is reabsorbed.
When blood pH falls: HCO3- is reabsorbed, H+ is secreted.
Urine pH varies from 4.5 to 8.0.

Path of Urine Through the Urinary Tract

Glomerular Capsule
Proximal Convoluted Tubule (PCT)
Nephron Loop
Distal Convoluted Tubule (DCT)
Collecting Duct
Minor Calyx
Major Calyx
Renal Pelvis
Ureter
Urinary Bladder
Internal Urethral Sphincter
Prostatic Urethra
External Urethral Sphincter
Spongy Urethra
Urethral Orifice

Key Equations and Concepts

Filtration Equation:
Bicarbonate Buffer System: