Urinary System Overview and Kidney Functions

Urinary System Overview

• The urinary system consists of organs that filter blood, remove waste, and regulate fluid balance
• Kidneys filter blood and produce urine
• Ureters transport urine from kidneys to bladder
• Urinary bladder stores urine
• Urethra carries urine outside the body

Kidney Functions

• Excretion of nitrogenous wastes (e.g., urea, uric acid, creatinine)
• Regulation of blood volume and pressure (via renin)
• Regulation of erythrocyte production (via erythropoietin)
• Regulation of vitamin D activation
• Maintenance of electrolyte balance
• Maintenance of acid-base balance

Kidney Structure

• Retroperitoneal position (behind parietal peritoneum)
• Level of T12 to L3 vertebrae (right kidney is slightly lower)
• Three protective layers: fibrous capsule, perirenal fat capsule, renal fascia
• Renal cortex (outer region), renal medulla (deeper region with renal pyramids), renal pelvis (medial region)
• Renal hilum: a medial indentation where structures enter/exit the kidney (ureters, renal blood vessels, nerves)
• Kidneys have a complex blood supply (one-quarter of total blood supply passes through each minute)
• Blood supply enters via renal artery, branches into arterioles, then capillaries
• Blood leaves via renal vein

Nephrons

• Structural and functional units of the kidney
• Contains over a million nephrons per kidney
• Two main structures: renal corpuscle and renal tubule
• Renal corpuscle: glomerulus (knot of capillaries) and glomerular (Bowman's) capsule
• Renal tubule: proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT)

Urine Formation

• Three processes: glomerular filtration, tubular reabsorption, tubular secretion
• Glomerular filtration: Nonselective passive process (forces water and small solutes from blood into glomerular capsule)
• Tubular reabsorption: Selective process (reabsorbs water, glucose, amino acids, and ions)
• Tubular secretion: Reabsorption in reverse; moves substances from blood into filtrate (e.g., hydrogen ions, creatinine)
• Urine characteristics: clear and pale to deep yellow, slightly acidic, sterile, slightly aromatic

Ureters

• Slender tubes (25-30 cm) connecting kidneys to bladder
• Continuous with renal pelvis
• Enter posterior aspect of bladder
• Run behind peritoneum
• Peristalsis aids urine transport

Urinary Bladder

• Smooth, collapsible muscular sac
• Stores urine temporarily
• Trigone: a triangular region of the bladder base with openings for ureters and urethra
• Three layers: detrusor muscle (smooth muscle), mucosa (transitional epithelium), external fibrous connective tissue
• Capacity: about 500 ml

Urethra

• Thin-walled tube carrying urine from bladder to exterior
• Length varies (females: 3-4 cm, males: 20 cm)
• Location in females: anterior to vaginal opening; in males: travels through prostate and penis
• Two sphincters: internal (involuntary, smooth muscle) and external (voluntary, skeletal muscle)

Micturition (Voiding)

• Emptying of the urinary bladder
• Two sphincters control urine release
• Impulses cause bladder contractions and relaxation of sphincters to expel urine

Fluid, Electrolyte, and Acid-Base Balance

• Kidneys regulate water balance, electrolyte balance, and acid-base balance of the blood
• Blood composition depends on diet, cellular metabolism, and urine output

Hormonal Control

• Antidiuretic hormone (ADH): regulates water reabsorption in the kidneys
• Aldosterone: regulates electrolyte balance, particularly sodium and potassium

Development and Problems

• Kidneys start developing early in embryonic life and begin excreting urine by the 3rd month
• Common congenital issues: polycystic kidney and hypospadias
• Common urinary system problems (adults): Infections (often from Escherichia coli), urinary retention, incontinence, kidney failure