Biology

The Human Excretory System maintains homeostasis by regulating body fluid volume, ion concentration, pH, and elimination of metabolic wastes.

1. Parts:

• a pair of kidneys
• a pair of ureters
• a urinary bladder
• a urethra

2. The kidney:

• dark purplish – immense blood supply
• shaped like a bean
• located at both sides of the vertebral column
• in the back of the abdominal cavity
• easily detached
• held in position by connective tissue and protected by a layer of fat
  • capsule: fibrous connective tissue
  • renal hilum: the renal artery enters and the renal vein and ureter leave
  • cortex
  • medulla: made up of renal pyramids
  • pelvis: central cavity, leads into the ureter

3. The nephron:

• functional unit of kidney
• 1-2 million / kidney
• Functions: filters, reabsorbs, excretes
• Parts:
  • Renal corpuscle – in the cortex
    • capillary knot = glomerulus (branched capillaries)
    • Bowman’s capsule (double-walled, cup-shaped structure around the glomerulus)
  • Renal tubule
    • proximal convoluted tubule
    • loop of Henle
      • descending limb
      • hairpin turn
      • ascending limb
    • distal convoluted tubule

4. Production of urine

1. glomerulus

Blood plasma is filtered through the glomerulus into the Bowman’s capsule 

=> (ultra)FILTRATE = protein-free blood plasma

• water, glucose, amino acids, salts, urea
• 180 litres / day

• Why? Arteriole entering the glomerulus has greater diameter than the arteriole leaving the glomerulus -> high blood pressure -> pushes blood plasma through the wall of the capillary
• Passive movement – depends only on pressure

(8kPa in capillary knot, 3kPa in capsule)

• No reabsorption

1. proximal convoluted tubule

• filtrate 180 l -> urine 5 l / day
• reabsorption: -> tubule cells -> capillaries
  • almost 100% glucose
  • 80% Na+, K+ actively
  • Vitamins
  • amino acids
  • 75% water – passively by osmosis

1. loop of Henle

• descending limb: the filtrate becomes more concentrated, because water passes
• hairpin turn: filtrate is the most concentrated here
• ascending limb: wall of tubule is impermeable to water -> sodium reabsorption is not followed by water diffusion -> filtrate becomes less concentrated

1. d) distal convoluted tubule

• permeable to water -> water leaves, volume is reduced
• reabsorption of water and ions depend on water and ion concentration of blood
• place where hormones take effect

1. collecting ducts

• water reabsorption
• URINE:
• no sugar (<- diabetes mellitus)
• no proteins
• urea
• ions

-> pelvis -> ureter(smooth muscle - peristalsis) -> urinary bladder: stores – pressure on wall, sphincter (smooth + voluntary muscle) -> urethra

Secretion:

• by tubule cells into the tubule (descending limb, distal t.)
• secreted substances diffuse from blood capillaries into tubule cells
• H+, K+, aspirin, penicillin, drugs -> drug testing for athletes (muscle-building steroids)

5. Hormonal regulation

- ADH (or vasopressin) antidiuretic hormone – from pituitary gland -> stimulates water reabsorption in the distal convoluted tubules, by making the tubule more permeable to water. Alcohol inhibits ADH -> more urine

- aldosterone: from adrenal cortex promotes Na+ reabsorption in distal tubules

6. diseases

Kidney failure:

Artificial kidney – dialysis - only filtration

Kidney transplant

Nephritis: inflammation of the kidney due to bacterial contamination

Cystitis: urinary bladder inflammation – frequent, painful urination, pus in urine

Urinary incontinence - involuntary leakage of urine

Kidney stones:

• due to contamination, abnormal metabolism, too concentrated urine
• made up of Ca-phosphate, Ca-oxalate, urea

• stone develops in collecting ducts -> gets into the pelvis and grows -> enters the ureter – sudden, painful cramp
• treatment:
  • surgical
  • get the patient pass the stone ( high fluid intake, ultrasound)