Anatomy & Physiology

• Urinary System


  • 2 Kidneys
  • 2 Ureters
  • Urinary bladder
  • Urethra
• Major Functions


  • Regulates red blood cell production by producing erythropoietin
  • Regulate blood volume and blood pressure by secreting enzyme renin
  • Role in activation of Vitamin D
• The Kidneys


  • Bean-shaped organs, reddish brown in color
  • About 11cm long x 6cm wide x 3cm thick
  • Location: either side of vertebral column positioned retroperitoneally against the back's deep muscle
  • Left kidney is 1.5 to 2.0 cm higher than the right kidney
• The Kidneys


  1. Renal sinus – the hollow chamber formed in the medial section of the kidney

  2. Renal pelvis – the funnel-shaped sac at the superior end of the ureter

  3. Renal medulla – region of the kidney consisting of conical masses (renal pyramids) appearing straited

  4. Renal cortex – granular region surrounding the medulla

  5. Nephrons – the functional units of the kidney

  6. There are about one million nephrons in a kidney
• Renal blood vessels


  1. Abdominal aorta → renal arteries → interlobar arteries → arcuate arteries → cortical radiate (interlobular) arteries → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries → cortical radiate (interlobular) veins → arcuate veins → interlobar veins → renal veins → inferior vena cava

  2. 20%-25% (1500 ml) of total cardiac output flows to the kidney per minute (person at rest)
• Renal corpuscle


  • Glomerulus / Glomerular capillaries – blood capillaries that filter fluids
  • Glomerular / Bowman's Capsule – cup-like sack with porous membrane around the glomerulus.
  • Renal Tubule – up to 50mm in length
  • Proximal convoluted tubule
  • Nephron loop – descending limb, Loop of Henle, ascending limb
  • Distal convoluted tubule
• Urine formation


  1. Glomerular Filtration - Blood, water and dissolved substances from the plasma are filtered in the glomerular capillaries towards the renal tubules.

  2. Average GFR in an adult is 120-125 ml/minute

  3. Allows molecules <3 nm to pass through

  4. Net filtration pressure – the resulting forces of hydrostatic pressure (of glomerular blood), colloid osmotic pressure of plasma and hydrostatic pressure (of the Bowman's capsule) allow filtration to happen.

  5. Can be affected by factors such blood volume, blood pressure, enzymes (e.g. renin-angiotensin-aldosterone-system, antidiuretic hormone, atrial natriuretic peptide)
• Urine formation


  1. Tubular Reabsorption - Substances from the glomerular filtrate move back from the tubules to the blood.

  2. Facilitated by lower blood pressure and more permeable walls of the peritubular capillaries

  3. Occurs throughout the renal tubules but mostly happens in the proximal convoluted tubule

  4. Substances reabsorbed: water, glucose, amino acids, creatinine, lactic acid, citric and ascorbic acid, phosphate, sulfate, calcium, potassium and sodium ions, chloride ions and other negatively charged ions
• Urine formation


  1. Tubular Secretion - The reverse process of moving some substances from the peritubular capillaries into the renal tubules.

  2. Substances secreted: Penicillin, histamine, phenobarbital, creatinine, ammonia, hydrogen ions

  3. Some potassium is secreted in the distal convoluted tubule

  4. Regulation of urine concentration and volume may be affected by hormones such as aldosterone and ADH (antidiuretic hormone)
• Concentration of Different substances

  • Sodium
  • Chloride
  • Bicarbonate
  • Potassium
  • Calcium
  • Magnesium
  • Phosphate
  • Sulfate
  • Glucose
  • Urea
  • Uric Acid
• Urine composition: 95% water usually contains urea and uric acid and traces of amino acids and different electrolytes
• Urine volume


  • 0.6 – 2.5 L/day
  • Can be affected by factors such as fluid intake, water loss from sweating and respiration, temperature, humidity, emotions (e.g. anxiety)
  • Normal: 50-60 ml/hr
  • Output of less than 30 ml/hr may indicate kidney problem
• Ureter


  1. From the distal convoluted tubules, urine passes through the collecting ducts then into the renal calyces and renal pelvis entering URETER.

  2. 25-30 cm long beginning in the renal pelvis

  3. Location: behind parietal peritoneum running parallel to vertebral column

  4. Layers: mucous coat (inner), muscular coat (middle), Fibrous coat (outer) – connective tissue

  5. Moves urine using muscular peristaltic waves from the renal pelvis toward the bladder

  6. Has a flap-like fold of membrane or valve that prevents backflow of urine from the bladder
• Urinary bladder


  • This is a spherical in shape, hollow and muscular organ that temporarily stores urine
  • Location: Neck of bladder is inferior to the prostate gland, Anterior to uterus and vagina
  • Rugae – folds in the inner wall that stretches when bladder is full
  • Trigone – triangular area in the internal floor formed by the 2 ureteral orifice and opening of the urethra
  • Layers: Mucous coat – composed of transitional epithelial cells, Submucous coat – connective tissues and elastic fibers, Muscular coat – bundles of smooth muscles cells forming the detrusor muscle, Detrusor muscle around the neck of bladder has stronger muscle tone to support the internal urethral sphincter in holding the urine in the bladder, Serous coat – parietal peritoneum; only coating the bladder's upper surface
• Micturition / Urination


  1. The process of expelling urine from the urinary bladder

  2. Triggered by stimulation of stretch receptors when bladder is filled

  3. Involves contraction of detrusor muscle, (reflex) relaxation of the internal urethral sphincter and voluntary relaxation of the external urethral sphincter --- controlled by the spinal reflexes

  4. Urge to urinate is felt when bladder contains about 150 ml urine, although it can hold up to 600 ml urine
• Urethra


  • Tube that allows urine to pass out of the body from urinary bladder and ends with external urinary orifice (urinary meatus)
  • Wall is thin, lined with mucous membrane, layer of smooth muscle and mucous glands (urethral glands)
  • Female: 3-4 cm long; anterior to vaginal opening; posterior to clitoris
  • Male: 20 cm long; works as part of both urinary and reproductive system; extends to the distal glans of penis