Urinary System

URINARY SYSTEM

Overview of the Urinary System

• Major Organs:

  • Kidneys

  • Ureters

  • Urinary bladder

  • Urethra

Primary Functions of the Urinary System

• Maintains optimal blood properties:

  • Regulation of water and electrolyte balance

  • Regulation of acid-base balance

• Excretion processes:

  • Removal of metabolic wastes, excess water, and electrolytes

  • Excretion of bioactive substances and drugs

• Endocrine functions:

  • Secretion of renin for blood regulation

  • Secretion of erythropoietin for red blood cell production

  • Conversion of vitamin D to calcitriol, its active form

  • Gluconeogenesis during periods of starvation

Structure of Kidneys

• Kidneys Anatomy:

  • Hilum: Concave medial border; entry point for nerves, blood vessels, lymphatics, and ureters

  • Renal Pelvis: Upper end of ureter which expands and divides into major calyces, further dividing into minor calyces

  • Outer Renal Cortex: Darker outer region with corpuscles and tubules

  • Inner Renal Medulla: Composed of aligned tubules; contains renal pyramids separated by renal columns

Nephrons: The Functional Units

• Nephrons:

  • Approximately 1.25 million per kidney; responsible for urine formation

  • Components include:

    • Renal corpuscle

    • Proximal convoluted tubule (PCT)

    • Loop of Henle

    • Distal convoluted tubule (DCT)

    • Connecting tubules

Renal Corpuscle and its Function

• Renal Corpuscle: Site of blood filtration

  • Composed of:

    • Glomerulus: A tuft of capillaries within the renal cortex

    • Bowman’s Capsule: Epithelial capsule surrounding Glomerulus

  • Functions:

    • Filtration of blood into the capsular space

Tubules in Nephrons

• Proximal Convoluted Tubule (PCT):

  • Located in the cortex; lined with cuboidal epithelium

  • Functions:

    • Reabsorbs 2/3 of water, electrolytes, and all organic nutrients

    • Active secretion of organic anions and cations

    • Hydroxylation of Vitamin D and production of erythropoietin

• Loop of Henle:

  • U-shaped, composed of thin descending limb and thick ascending limb

  • Key for concentrating urine and maintaining a hyperosmotic interstitium

• Distal Convoluted Tubule (DCT):

  • Key Features:

    • Less reabsorption compared to PCT

    • Sites for aldosterone regulation of Na+ reabsorption

  • Contains Macula Densa and Juxtaglomerular Cells (release renin)

Collecting Ducts and Urine Formation

• Collecting Ducts:

  • Merge to deliver urine to minor calyx

  • Principal Cells: Rich in aquaporins; regulate water reabsorption affected by ADH

  • Intercalated Cells: Maintain acid-base balance by secreting H+ or HCO3

Blood Circulation through the Kidneys

• Circulatory Pathway:

  • Renal arteries > Segmental arteries > Interlobar arteries > Arcuate arteries > Afferent arterioles leading to Glomerulus

  • Blood drains via efferent arterioles then forms peritubular capillaries and vasa recta for nutrient distribution

  • Final drainage through arcuate veins to interlobar veins to renal veins

Renal Functions: Filtration, Secretion, and Reabsorption

• Filtration: Blood components filtered into nephron

• Tubular Secretions: Movement from tubular cells to tubules

• Tubular Reabsorption: Substances move from tubules back into the blood

Ureters, Bladder, and Urethra

• Ureters: Transport urine from renal pelvis to urinary bladder

• Urinary Bladder: Stores urine; lined with transitional epithelium that protects from hypertonic urine

• Urethra: Muscular tube that expels urine from the bladder; longer in males and has three segments:

  • Prostatic urethra

  • Membranous urethra

  • Spongy urethra (within the penis)