The Urinary System: Structure and Function

The Urinary System

Introduction to the Urinary System

The urinary system is essential for maintaining homeostasis by removing metabolic wastes, regulating blood volume and pressure, controlling plasma ion concentrations, and contributing to pH balance. It consists of paired kidneys, ureters, the urinary bladder, and the urethra.

• Removes metabolic wastes via urine

• Osmoregulation: Regulates blood volume, blood pressure, and plasma ion concentrations

• pH homeostasis: Controls loss of hydrogen and bicarbonate ions

Organs of the Urinary System

Major Components and Their Functions

• Kidneys: Produce urine by filtering blood and removing wastes

• Ureters: Transport urine from kidneys to urinary bladder

• Urinary bladder: Temporarily stores urine prior to elimination

• Urethra: Conducts urine to the exterior; in males, also transports semen

Anatomical Position and Protection of the Kidneys

The kidneys are retroperitoneal organs located on either side of the vertebral column. The left kidney is slightly superior to the right. Each kidney is protected and stabilized by three layers:

• Fibrous capsule: Collagen fibers covering the outer surface

• Perinephric fat: Thick layer of adipose tissue surrounding the capsule

• Renal fascia: Dense, fibrous outer layer anchoring the kidney

Gross and Internal Anatomy of the Kidney

External and Internal Structures

The kidney has a distinct outer cortex and inner medulla. The medulla contains renal pyramids, which are separated by renal columns. The renal pelvis collects urine from the major calyces and channels it into the ureter.

• Renal cortex: Outer region containing nephrons

• Renal medulla: Inner region with renal pyramids

• Renal pelvis: Funnel-shaped structure collecting urine

• Hilum: Entry/exit site for renal artery, vein, and ureter

Pathway of Urine Flow

Urine is produced in the nephrons, passes through the collecting ducts, and drains into minor calyces, then major calyces, the renal pelvis, and finally the ureter.

Blood Supply and Innervation of the Kidneys

Renal Circulation

The kidneys receive about 20–25% of cardiac output. Blood enters via the renal artery, branches into smaller arteries, and is filtered in the glomerulus. Venous blood returns via the renal vein.

• Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus

• Efferent arterioles → peritubular capillaries/vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein

Renal Nerves

Sympathetic innervation regulates blood flow and urine formation by adjusting the diameter of renal arterioles and stimulating renin release.

Microscopic Structure: The Nephron

Nephron Anatomy and Types

Nephrons are the functional units of the kidney, each consisting of a renal corpuscle and a renal tubule. There are two main types:

• Cortical nephrons: 85% of nephrons, located mostly in the cortex, with short loops of Henle

• Juxtamedullary nephrons: 15% of nephrons, with long loops extending deep into the medulla, crucial for concentrating urine

Renal Corpuscle

The renal corpuscle consists of the glomerulus (a capillary network) and the glomerular (Bowman's) capsule. Filtration occurs here, producing a protein-free filtrate.

• Podocytes: Specialized cells with foot processes forming filtration slits

• Filtration membrane: Composed of fenestrated endothelium, basement membrane, and podocyte foot processes

Renal Tubule and Collecting System

The renal tubule consists of the proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT). The collecting system receives fluid from multiple nephrons and transports it to the renal pelvis.

• PCT: Reabsorbs water, ions, and nutrients

• Nephron loop: Establishes osmotic gradient for water reabsorption

• DCT: Secretion and selective reabsorption under hormonal control

• Collecting duct: Final adjustments to urine composition and volume

Summary Table: Main Structures of the Kidney

Structure	Function
Renal Cortex	Contains renal corpuscles and most of the nephron tubules
Renal Medulla	Contains renal pyramids and nephron loops
Renal Pelvis	Collects urine from calyces and channels it to the ureter
Hilum	Entry/exit for renal artery, vein, and ureter
Ureter	Transports urine to the bladder

Key Terms and Concepts

• Osmoregulation: Regulation of water and solute concentrations in the blood

• Filtration: Movement of water and solutes from blood into the nephron

• Reabsorption: Movement of substances from filtrate back into the blood

• Secretion: Movement of substances from blood into the filtrate

• Renin: Enzyme involved in blood pressure regulation

• ADH (Antidiuretic Hormone): Regulates water reabsorption in the collecting ducts