The Urinary System and Excretion

Overview of Excretory Systems

The excretory system is responsible for removing metabolic waste products from the body, maintaining homeostasis by regulating water, electrolyte, and acid-base balance. Multiple organs contribute to excretion, each eliminating specific waste products.

• Lungs: Remove CO2, water vapor, and heat.

• Gastrointestinal (GI) Tract: Eliminates solid wastes, secretions, water, heat, CO2, and salts.

• Skin: Excretes heat, water, CO2, and salts via sweat.

• Liver: Removes bile pigments (e.g., bilirubin).

• Kidneys: Main organs for excretion of water, urea, salts, and incidental removal of CO2 and heat.

Example: The kidneys filter blood to remove urea, a toxic byproduct of protein metabolism, while the lungs expel carbon dioxide produced during cellular respiration.

Excretory Organs and Waste Products

Gross Anatomy of the Urinary System

Urinary Tract Structure

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra, which work together to produce, transport, store, and eliminate urine.

• Kidneys: Filter blood and form urine.

• Ureters: Muscular tubes (~27 cm long) that transport urine from the kidneys to the bladder via peristalsis.

• Urinary Bladder: Muscular sac that stores urine; features a trigone area formed by the openings of the ureters and urethra.

• Urethra: Conducts urine from the bladder to the exterior; differs in length and structure between males and females.

Histology of the Urinary Tract

• Ureters:

  • Mucosa: Transitional epithelium with rugae

  • Muscularis: Smooth muscle (longitudinal and circular layers)

  • Adventitia: Fibrous connective tissue

• Urinary Bladder:

  • Mucosa: Transitional epithelium with rugae

  • Submucosa

  • Muscularis: Three layers (inner and outer longitudinal, middle circular)

  • Adventitia: Fibrous coat

  • Internal sphincter: Smooth muscle; external sphincter: Skeletal muscle

• Urethra:

  • Epithelium transitions from transitional to stratified squamous

  • Female: ~4 cm; Male: ~15–20 cm (prostatic, membranous, penile portions)

Physiology of Micturition (Urination)

Neural Control of Urination

Micturition is the process of expelling urine from the bladder. It involves both involuntary and voluntary neural mechanisms.

• Stretch receptors in the bladder wall send signals to the spinal cord when the bladder fills.

• Motor output from the spinal cord causes relaxation of the internal sphincter and contraction of the bladder muscle (detrusor).

• Urge to urinate is perceived as pressure at the external sphincter, which is under voluntary control.

• Relaxation of the external sphincter allows urine to be expelled; normal urination is a passive process.

Kidney Structure and Blood Flow

Gross Anatomy of the Kidney

• Hilum: Entry/exit site for vessels and ureter

• Capsule: Outer fibrous covering

• Cortex: Outer region containing renal corpuscles and convoluted tubules

• Medulla: Inner region with renal pyramids and columns

• Pyramids: Cone-shaped structures containing loops of Henle and collecting ducts

• Calyx, Papilla, Pelvis: Collect and funnel urine toward the ureter

Renal Blood Flow

Blood supply to the kidney is highly organized to facilitate filtration and reabsorption.

• Renal artery → Interlobar artery → Arcuate artery → Interlobular artery → Afferent arteriole → Glomerular capillaries → Efferent arteriole → Peritubular capillaries (or vasa recta in juxtamedullary nephrons) → Interlobular vein → Arcuate vein → Interlobar vein → Renal vein

• The glomerulus is a unique capillary bed fed and drained by arterioles, forming an arterial portal system.

The Nephron: Structure and Function

Types of Nephrons

• Cortical Nephrons: ~7/8 of all nephrons; short loops of Henle; no vasa recta.

• Juxtamedullary Nephrons: ~1/8 of all nephrons; long loops of Henle; associated with vasa recta; crucial for concentrating urine.

Nephron Anatomy

• Renal Corpuscle: Glomerulus + Bowman's capsule

• Renal Tubule: Proximal convoluted tubule (PCT) → Loop of Henle → Distal convoluted tubule (DCT) → Collecting duct (CD)

Urine Formation: Processes and Regulation

1. Glomerular Filtration

Filtration of blood occurs in the glomerulus due to high hydrostatic pressure. The filtration barrier consists of fenestrated capillaries, a basement membrane, and podocytes with slit pores.

• Only small solutes and water pass into Bowman's capsule; proteins and blood cells are retained.

• The net filtration pressure is determined by the balance of capillary blood pressure (favors filtration), fluid pressure in Bowman's capsule, and capillary osmotic pressure (both oppose filtration).

• Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute by both kidneys.

2. Tubular Reabsorption

Most filtrate is reabsorbed in the PCT via active and passive transport mechanisms. Water reabsorption follows osmotic gradients established by solute movement.

• Na+: Passively enters PCT cells, actively pumped out by Na-K ATPase.

• Cl-: Follows Na+ passively.

• Water: Follows solute reabsorption by osmosis.

• Glucose and Amino Acids: Reabsorbed via Na+-dependent co-transporters.

• Tubular Maximum (Tm): Maximum rate of reabsorption due to limited transporter proteins.

3. Loop of Henle: Countercurrent Multiplier

The loop of Henle establishes a concentration gradient in the medulla, essential for water reabsorption and urine concentration.

• Descending Limb: Permeable to water, not solutes; concentrates filtrate.

• Ascending Limb: Impermeable to water, actively transports NaCl out; dilutes filtrate.

• Countercurrent flow and differential permeability create a hyperosmotic medullary interstitium.

4. Vasa Recta: Countercurrent Exchange

The vasa recta preserve the medullary osmotic gradient by absorbing water and solutes without dissipating the gradient.

• Descending vasa recta absorb solutes; ascending vasa recta release solutes and absorb water.

• Prevents medullary edema and maintains concentration gradient.

5. Distal Convoluted Tubule (DCT) and Collecting Duct (CD)

• DCT: Reabsorbs NaCl (stimulated by aldosterone), passively reabsorbs water, secretes NH3 and H+, reabsorbs HCO3- (pH regulation).

• Collecting Duct: Impermeable to water unless antidiuretic hormone (ADH) is present; reabsorbs urea, further increasing medullary osmolarity.

Hormonal Regulation of Kidney Function

Juxtaglomerular Apparatus (JGA)

• Juxtaglomerular (JG) Cells: Specialized smooth muscle cells in afferent arteriole; secrete renin in response to low blood pressure.

• Macula Densa: Specialized DCT cells; sense Na+ concentration, regulate renin release.

• Lacis Cells: May produce erythropoietin (stimulates RBC production) in response to low O2.

• Extraglomerular Mesangium: Connective tissue supporting glomerular vessels.

Renin-Angiotensin-Aldosterone System (RAAS)

• Renin: Converts angiotensinogen to angiotensin I.

• Angiotensin-Converting Enzyme (ACE): Converts angiotensin I to angiotensin II.

• Angiotensin II: Stimulates aldosterone secretion, increases arteriolar constriction (raises blood pressure).

• Aldosterone: Increases Na+ reabsorption in DCT, raising blood volume and pressure.

• Atrial Natriuretic Factor (ANF): Inhibits Na+ reabsorption; produced by atria in response to increased blood volume.

Acid-Base Balance and the Kidneys

Sources of Acid Gain and Loss

• Acid Gain:

  • Generation of H+ from CO2

  • Metabolism of proteins and organic compounds (lactic acid, amino acids, phosphoric acid)

  • Loss of bicarbonate in diarrhea or urine

• Acid Loss:

  • Loss of H+ in vomitus

  • Excretion of H+ in urine; reabsorption and new production of bicarbonate in kidney

Renal Regulation of Acid-Base Balance

• H+ is secreted and HCO3- is reabsorbed in the PCT and DCT.

• Bicarbonate is freely filtered; kidneys control reabsorption and new production to maintain pH.

• No reabsorption of H+ in the kidney; only secretion.

• Blood buffers: H2CO3, proteins, hemoglobin, phosphate.

• Urine buffers: Phosphate, bicarbonate, ammonia (NH3).

Acid-Base Disorders

• Respiratory Acidosis/Alkalosis: Caused by changes in CO2 elimination (e.g., hypoventilation or hyperventilation).

• Metabolic Acidosis/Alkalosis: Caused by changes in H+ or HCO3- due to metabolic processes or renal dysfunction.

Key Equations

• Glomerular Filtration Rate (GFR): Where is the filtration coefficient, is glomerular capillary pressure, is Bowman's space pressure, and is oncotic pressure in glomerular capillaries.

• Renin-Angiotensin-Aldosterone System:

• Bicarbonate Buffer System:

Additional info: Understanding the mechanisms of urine concentration and acid-base regulation is essential for diagnosing and treating renal and metabolic disorders.