The Urinary System: Structure and Function (Chapter 25, Part I)

The Urinary System

Overview

The urinary system is essential for maintaining the body's internal environment by filtering blood, removing metabolic wastes, and regulating fluid and electrolyte balance. This system includes the kidneys, ureters, urinary bladder, and urethra, each with specialized functions in urine formation and excretion.

Kidney Functions

Primary Functions

• Filtration of Blood: The kidneys filter blood to excrete metabolic wastes, toxins, and excess ions in urine, while recovering essential electrolytes and metabolites back into the bloodstream.

• Regulation of Blood Volume and Chemical Composition: Kidneys regulate blood volume (and thus blood pressure) and maintain the chemical makeup of blood, especially mineral ions.

• Maintenance of Water, Salt, and Acid-Base Balance: The kidneys help maintain homeostasis by balancing water and salt levels, as well as acids and bases in the body.

Secondary Functions

• Gluconeogenesis: During prolonged fasting, the kidneys can produce glucose from non-carbohydrate sources.

• Hormone Production: The kidneys produce renin (for blood pressure regulation) and erythropoietin (stimulates red blood cell production).

• Vitamin D Activation: The kidneys convert inactive vitamin D into its active form, which is essential for calcium absorption.

Example: In chronic kidney disease, the inability to regulate electrolytes and produce erythropoietin can lead to anemia and imbalances in blood chemistry.

Urinary System Structures

Major Components

• Kidneys (paired): Form urine from blood plasma for excretion.

• Ureters (paired): Transport urine from the kidneys to the bladder.

• Urinary Bladder: Serves as a temporary storage reservoir for urine.

• Urethra: Transports urine from the bladder to the exterior of the body.

Example: A blockage in the ureter can lead to hydronephrosis, a condition where urine backs up into the kidney, causing swelling and potential damage.

Kidney Location & External Anatomy

Position and Structure

• Retroperitoneal Position: Kidneys are located in the superior lumbar region, extending from the T12 to L3 vertebrae, behind the peritoneum.

• Size: Each adult kidney is about 12 cm x 6 cm (size of a soap bar).

• Asymmetry: The right kidney is slightly lower than the left due to the large right lobe of the liver.

• Surfaces: Kidneys have a convex (lateral) and concave (medial) surface.

• Renal Hilum: A vertical cleft on the medial surface serving as the entry/exit point for ureters, renal blood vessels, lymphatics, and nerves.

Supporting/Protective Tissue Layers

Superficial to Deep Layers

• Renal Fascia: Outermost layer of dense fibrous connective tissue anchoring the kidney and adrenal gland to surrounding structures.

• Perirenal Adipose Capsule: Middle fatty mass that cushions the kidney and attaches it to the posterior body wall.

• Fibrous Capsule: Innermost transparent connective tissue layer that prevents infection from spreading to the kidney.

Example: Loss of perirenal fat (as in extreme starvation) can cause the kidney to drop (nephroptosis), potentially kinking the ureter.

Internal Anatomy of the Kidney

Major Regions

• Renal Cortex: The light-colored, granular superficial region containing nephrons, which are the urine-forming structures.

• Renal Medulla: The dark-red inner region divided into several sections called renal pyramids.

• Renal Pyramids: Parallel bundles of collecting tubules extending away from the cortex, carrying urine to the collecting point. The base faces the cortex, and the apex (papilla) points toward the pelvis.

• Renal Columns: Inward extensions of cortical tissue that separate the pyramids.

• Renal Lobe: Consists of a medullary pyramid and its surrounding cortical tissue, separated by renal columns. There are typically 7-8 lobes per kidney.

Renal Pelvis and Calyces

• Renal Pelvis: A flat, funnel-shaped tube adjacent to the hilum and continuous with the ureter. It collects urine from the calyces.

• Major Calyces: 2 or 3 large urine-collecting regions formed from minor calyces, continuous with the renal pelvis.

• Minor Calyces: Multiple small collecting areas associated with a renal pyramid papilla that drain urine.

Urine Flow (from lateral to medial):

• Collecting ducts → Pyramid papillae → Minor calyces → Major calyces → Renal pelvis → Ureter

Urine is propelled by smooth muscle contraction (peristalsis) toward the bladder.

Blood and Nerve Supply

Renal Circulation

• Blood Flow: Renal arteries deliver approximately 25% (1,200 mL) of systemic cardiac output to the kidneys each minute.

• Arterial blood flows into the kidneys and venous return follows similar pathways.

Renal Plexus

• A network of autonomic nerve fibers and ganglia innervates the kidney and its associated ureter.

• Regulates kidney blood flow by adjusting renal artery diameter and controls the lumen size of glomerular arterioles.

Example: Sympathetic stimulation can constrict renal arteries, reducing urine output during stress.

Nephrons

Structure and Function

The nephron is the structural and functional unit of the kidney, responsible for urine formation. Each kidney contains about 1 million nephrons.

• Nephron = Renal Corpuscle + Renal Tubules

Renal Corpuscle

• Glomerulus: A cluster of glomerular capillaries where filtration occurs.

• Bowman's Capsule (Glomerular Capsule): A cup-shaped structure that surrounds the glomerulus and collects the filtrate.

Renal Tubules

• Proximal Convoluted Tubule (PCT)

• Nephronic Loop (Loop of Henle)

• Distal Convoluted Tubule (DCT)

Example: Damage to the glomerulus (as in glomerulonephritis) impairs filtration and can lead to kidney failure.

Glomerulus: Capillary Endothelium

Filtration Barrier

• The glomerulus consists of a cluster of very porous (fenestrated) capillaries supported by an underlying basement membrane.

• Fenestrations allow a solute-rich, protein-free solution to pass from blood through the filter membrane into Bowman's capsule.

• Filtrate: Blood plasma with raw materials that pass through the filter membrane and are processed into urine by the renal tubules.

Glomerular Capsule: Capsule Epithelium

Layers of Bowman's Capsule

• Parietal Layer: The external structural layer of flat squamous epithelium, providing support but not involved in filtration.

• Visceral Layer: The internal functional layer composed of branched, octopus-like epithelial cells called podocytes, which are involved in filtrate formation.

• Podocyte Foot Processes: These processes are very close to the glomerular capillaries and form filtration slits (pores) between them, creating the filtration barrier.

Example: Damage to podocytes can lead to proteinuria (protein in urine), as the filtration barrier is compromised.