The Urinary System

The Urinary System

Overview of the Urinary System

  • The urinary system is essential for filtering nitrogenous wastes from the blood and regulating water, electrolyte, and acid-base balances.

Functions of the Urinary System

  • WHAT

    • Filters nitrogenous wastes from the blood.

    • Regulates water balance, electrolyte balance, and acid-base balance.

  • HOW

    • The kidneys filter blood through nephrons, actively secreting or reabsorbing electrolytes as directed by hormones.

  • WHY

    • The kidneys maintain proper blood pressure, crucial for delivering oxygen and nutrients to tissues and facilitating adequate blood filtration.

Importance of the Kidneys

  • Kidneys are considered homeostatic organs, maintaining the purity and constancy of internal fluids, much like sanitation workers managing clean water and waste disposal.

  • Daily, the kidneys filter gallons of fluid from blood, processing it to eliminate wastes and excess ions via urine while returning necessary substances to the blood.

  • Main responsibilities include:

    • Eliminating nitrogenous wastes, toxins, and drugs.

    • Regulating blood volume, and maintaining balances of water and salts, acids and bases.

Additional Regulatory Functions of Kidneys

  • Renin Production: Helps regulate blood pressure.

  • Erythropoietin Release: Stimulates red blood cell production in the bone marrow.

  • Vitamin D Activation: Converts inactive vitamin D to its active form in the body.

Anatomy of the Urinary System

  • Major Components: Kidneys, ureters, urinary bladder, urethra.

  • Functions of Other Organs: Ureters transport urine from kidneys to the bladder; the bladder stores urine temporarily; the urethra conveys urine outside the body.

Kidneys

Location and Structure

  • Location: Retroperitoneal position (behind the peritoneum) in the superior lumbar region, extending from T12 to L3 vertebrae. The right kidney is slightly lower due to the liver.

  • Size: Each kidney is approximately 12 cm (5 inches) long, 6 cm (2.5 inches) wide, and 3 cm (1 inch) thick, resembling a large bar of soap.

  • Surface Anatomy: Has a convex lateral surface and a renal hilum (indentation for structures entering and exiting).

Protective Layers of the Kidney

  1. Fibrous Capsule: Transparent layer that encloses and protects the kidney.

  2. Perirenal Fat Capsule: Fatty tissue that cushions the kidney.

  3. Renal Fascia: Dense fibrous connective tissue anchoring the kidney and adrenal gland.

Homeostatic Imbalance

  • Ptosis: A condition where kidneys drop to a lower position due to loss of surrounding fatty tissue (can kink ureters).

  • Hydronephrosis: A condition that occurs when urine backs up due to kinked ureters, potentially damaging kidney tissue.

Internal Kidney Structure

  • Regions of the Kidney:

    • Renal Cortex (Bark): Outer light-colored region.

    • Renal Medulla: Darker inner region consisting of renal pyramids and renal columns.

    • Renal Pelvis: Flat, funnel-shaped structure at the hilum, continuous with the ureter.

    • Calyces: Extensions of the renal pelvis that collect urine from the renal pyramids.

Blood Supply to the Kidneys

  • Blood Flow: Approximately 25% of total blood supply passes through kidneys each minute.

  • Renal Artery: Supplies blood to each kidney, branching into:

    • Segmental Arteries

    • Interlobar Arteries (traversing renal columns)

    • Arcuate Arteries at the cortex-medulla junction

    • Cortical Radiate Arteries supplying the renal cortex.

  • Venous Return: Renal veins trace a path reversing the arterial flow, culminating in the inferior vena cava.

Nephrons: Functional Units of the Kidneys

Structure of Nephrons

  • Nephrons are the structural and functional units of the kidneys, responsible for urine formation. Each kidney contains over a million nephrons.

  • Components of a Nephron:

    1. Renal Corpuscle: Contains a glomerulus (knot of capillaries) surrounded by a glomerular capsule (Bowman's capsule).

    • Podocytes: Specialized cells with foot processes forming filtration slits around the glomerulus.

    1. Renal Tubule: Includes the proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT) - about 3 cm long.

    • Cortical Nephrons: Located primarily in the cortex, most common.

    • Juxtamedullary Nephrons: Located at the cortex-medulla junction with loops extending into the medulla.

Blood Supply in Nephrons

  • Afferent Arteriole: Feeds blood into the glomerulus, has a larger diameter than the efferent arteriole, creating high pressure for filtration.

  • Efferent Arteriole: Drains blood from the glomerulus, leading to the peritubular capillaries, which are adapted for absorption, as they are low-pressure vessels.

  • Glomerulus vs. Peritubular Capillaries: Glomeruli are specialized for filtration, while peritubular capillaries are adapted for nutrient reabsorption.

Urine Formation

Processes of Urine Formation

  1. Glomerular Filtration: Passive process where fluid moves from blood into the glomerular capsule, forming filtrate (plasma without proteins). Normal glomerular pressure is required for efficient filtration.

    • Conditions Affecting Filtration: Low blood pressure can lead to oliguria (100-400 ml/day) or anuria (<100 ml/day).

  2. Tubular Reabsorption: Begins as filtrate enters the PCT. Useful substances (water, glucose, amino acids, ions) are reabsorbed back into the blood. This process can be both passive (osmosis) and active (membrane carriers).

  3. Tubular Secretion: Transports unwanted substances (H+, K+, creatinine) from the blood into the tubular fluid, aiding in urine formation.

Nitrogenous Wastes in Urine

  • Common Nitrogenous Wastes:

    • Urea: End product of protein metabolism.

    • Uric Acid: Product of nucleic acid metabolism.

    • Creatinine: Byproduct of creatine metabolism in muscle tissues.

Characteristics of Urine

Composition

  • Urine is primarily water (99%), but contains nitrogenous wastes and excess solutes (salt, urea).

  • Normal Volume: Approximately 1.0 to 1.8 liters produced per day.

  • Color: Ranges from pale yellow (dilute) to deep yellow (concentrated) due to urochrome.

  • Odor: Usually aromatic, but can change based on certain foods, medications, or diseases.

  • pH: Typically around 6.0, varies with diet and health status.

Specific Gravity

  • Specific gravity of urine ranges from 1.001 to 1.035, reflecting its concentration compared to distilled water, which has a specific gravity of 1.0.

Abnormal Urinary Components

  • Substances typically not found in urine include glucose, blood proteins, red blood cells, and white blood cells. The presence of these may indicate underlying conditions like diabetes or infection.

Ureters, Urinary Bladder, and Urethra

Ureters

  • Structure: Two slender tubes, 25-30 cm (10-12 inches) long.

  • Function: Transport urine from kidneys to bladder via peristalsis.

Urinary Bladder

  • Description: Collapsible muscular sac for urine storage; located posterior to the pubic symphysis.

  • Volume: Can hold over 500 ml (1 pint) of urine; expands as urine accumulates.

Urethra

  • Function: Conveys urine from bladder to outside; length differs between sexes (20 cm in males; 3-4 cm in females).

  • Sphincters: Internal (involuntary) and external (voluntary) sphincters control urine release.

Micturition (Voiding)

  • Process: Involves the contraction of the bladder wall and relaxation of the internal sphincter, stimulated by stretch receptors. The external sphincter is voluntarily controlled.

Common Homeostatic Imbalances

  • Incontinence: Inability to control external sphincter, often due to emotional stress, childbirth pressure, or neurological issues.

  • Urinary Retention: Inability to expel urine, often requiring catheterization for relief.

Fluid, Electrolyte, and Acid-Base Balance

Kidney's Role in Blood Composition

  1. Excretion of Nitrogenous Wastes.

  2. Maintaining Water Balance: Governed by antidiuretic hormone (ADH).

  3. Electrolyte Balance: Regulated by hormones like aldosterone.

  4. Maintaining Proper Blood pH.