The Urinary System

Kidney Anatomy

The kidneys are vital organs responsible for filtering blood, removing waste, and maintaining homeostasis. Their structure is specialized to support these functions.

• Cortex: The outer region of the kidney, containing most nephrons.

• Medulla: The inner region, organized into renal pyramids.

• Nephron: The functional unit of the kidney, responsible for urine formation. There are two types:

  • Cortical nephron: Located mostly in the cortex.

  • Juxtamedullary nephron: Extend deep into the medulla, important for concentrating urine.

• Glomerular (Bowman's) Capsule: The initial filtering component of the nephron.

Flow of Urine

1. Glomerular (Bowman's) Capsule

2. Proximal Convoluted Tubule (PCT)

3. Loop of Henle (thin and thick segments)

4. Distal Convoluted Tubule (DCT)

5. Collecting Duct

6. Minor Calyx

7. Major Calyx

8. Renal Pelvis

Flow of Blood

1. Afferent arteriole

2. Glomerular capillaries

3. Efferent arteriole

4. Peritubular capillaries or vasa recta

5. Veins

Functions of the Kidneys

• Filtration: Removal of fluids and waste from blood.

• Absorption: Reclaiming essential substances back into the blood.

• Secretion: Adding additional wastes to the filtrate.

• Regulation: Maintains water, salt, acid-base balance, and blood pressure (via renin).

• Hormone Production: Erythropoietin (stimulates RBC production), activation of vitamin D.

Kidney Physiology – Urine Formation

The nephron filters and processes about 180 L of fluid per day, with only about 1% excreted as urine. The entire blood volume is filtered every 36–45 minutes.

Phases of Urine Formation

1. Glomerular Filtration: Passive, nonselective process where water and small solutes pass through the filtration membrane.

2. Reabsorption: Movement of substances from filtrate back into blood, mainly in the PCT and loop of Henle.

3. Secretion: Active transport of additional wastes from blood into the filtrate, mainly in the DCT and collecting duct.

Glomerular Filtration

• Filtration Membrane: Three layers:

  • Fenestrated capillary endothelium (blocks blood cells and large proteins)

  • Basement membrane (negatively charged, repels anions, allows cations and neutral molecules)

  • Visceral layer (podocytes) – traps proteins, phagocytizes them

• Filtration Pressure: Determined by opposing forces:

  • Glomerular hydrostatic pressure (~55 mm Hg) pushes filtrate out

  • Colloid osmotic pressure (~30–32 mm Hg) and capsular hydrostatic pressure (15 mm Hg) oppose filtration

Net Filtration Pressure (NFP):

• Formula: Example:

Glomerular Filtration Rate (GFR)

• Amount of fluid filtered per minute (normal adult: 125 mL/min)

• Factors affecting GFR:

  • Total surface area for filtration

  • Filtration membrane permeability

  • Net Filtration Pressure (NFP)

• Regulation:

  • Constriction of afferent arterioles decreases GFR; dilation increases GFR

  • Constriction of efferent arterioles increases GFR; dilation decreases GFR

  • Controlled by the sympathetic nervous system

Tubular Reabsorption

• Returns useful substances to blood; both active and passive mechanisms

• Active Reabsorption:

  • Moves substances against gradients (e.g., glucose, amino acids, vitamins, most ions)

  • Co-transported with Na+; limited by carrier/enzyme saturation (tubular maximum)

  • Plasma proteins reabsorbed by pinocytosis, broken down to amino acids

• Passive Reabsorption:

  • Driven by electrochemical and osmotic gradients

  • Water follows Na+ by osmosis; solutes follow by solvent drag

  • Anions (Cl-, HCO3-, PO43-) follow electrochemical gradients

• Substances Not Reabsorbed: Creatinine (not reabsorbed), urea (40–50% reabsorbed), uric acid (incompletely reabsorbed)

Nephron Segments and Reabsorption

Tubular Secretion

• Occurs mainly in distal and collecting tubules

• Functions:

  • Disposal of drugs and toxins

  • Elimination of wastes (urea, uric acid)

  • Removal of excess K+ (regulated by aldosterone)

  • Regulation of blood pH (secretion of H+, retention of HCO3-)

Regulation of Urine Concentration and Volume

Osmolarity measures solute concentration in solution. The kidneys use countercurrent mechanisms to maintain body fluid osmolarity and produce urine of varying concentration.

Osmolarity

• Definition: Number of solute particles per liter of water

• Formula:

Countercurrent Mechanism

• Describes how kidneys maintain solute concentration gradients

• Involves filtrate in tubules and blood in vasa recta flowing in opposite directions

• Proximal tubule filtrate is iso-osmotic with plasma (300 mosm/L)

• Loop of Henle as Countercurrent Multiplier:

  • Descending limb: Permeable to water, not NaCl; water leaves, increasing filtrate osmolarity up to 1200 mosm/L

  • Ascending limb: Impermeable to water; actively transports Na+ and Cl- out, diluting filtrate to ~100 mosm/L by the time it reaches the cortex

• Vasa Recta: Maintains osmotic gradient by passive exchange of water and NaCl, ensuring medullary concentration is preserved

Urine Dilution and Concentration

• Without ADH: Distal and collecting tubules are impermeable to water; dilute urine is excreted

• With ADH: Tubules become permeable to water; water is reabsorbed into hyperosmotic medulla, resulting in concentrated urine and decreased urine volume

Key Hormones in Renal Regulation

Example: Diabetes Insipidus

In diabetes insipidus, ADH is deficient or the kidneys do not respond to it, resulting in the excretion of large volumes of dilute urine and risk of dehydration.

Additional info: The countercurrent mechanism is essential for the kidney's ability to produce urine of varying concentration, allowing the body to conserve water during dehydration or excrete excess water when hydrated.