Composition and Functions of Urine

Urine Composition

• Urine is a liquid by-product with a clear and amber color (pigmented urobilin)
• Specific gravity of urine is 1020-1030
• pH of urine is 4.5-8
• Daily production of urine is 1000ml-1500ml
• Composition of urine includes:
  • Water (96%)
  • Urea (2%)
  • Uric acid
  • Creatinine
  • Ammonia
  • Sodium
  • Potassium
  • Chlorides
  • Phosphates
  • Sulphates
  • Oxalates

Water Balance and Urine Output

• Balance between fluid intake and output is controlled by kidneys
• Smallest volume required to excrete waste products is 500ml/day
• More than 500ml of urine production is controlled by ADH (Antidiuretic Hormone)
• Increased blood osmotic pressure stimulates osmoreceptors, which stimulate posterior pituitary and release ADH, reducing blood osmotic pressure
• ADH increases water reabsorption
• Increased blood volume stimulates stretch receptors in the heart, releasing ANP (Atrial Natriuretic Peptide), which reduces the reabsorption of water and sodium and inhibits ADH secretion

RAAS (Renin-Angiotensin-Aldosterone System)

• RAAS regulates electrolyte balance and blood pressure
• Low renal blood flow due to low blood volume, low blood pressure, or low blood sodium triggers the secretion of Renin from kidneys
• Renin converts Angiotensinogen to Angiotensin I, which is then converted to Angiotensin II by ACE
• Angiotensin II increases blood pressure through vasoconstriction
• Angiotensin II also stimulates the secretion of Aldosterone from the adrenal cortex
• Aldosterone increases sodium and water reabsorption, increases potassium excretion, and increases blood sodium and blood volume, leading to an increase in blood pressure

Urine Flow Pathway

• Urine flows through the following pathway:
  • Distal convoluted tubules
  • Collecting ducts
  • Large papillary ducts
  • Renal pelvis
  • Major calyces
  • Minor calyces

Hormonal Influence on Urine Formation

• Parathyroid hormone increases blood calcium levels
• Calcitonin lowers blood calcium levels
• ADH increases water reabsorption by increasing the permeability of distal convoluted tubules and collecting ducts
• Aldosterone increases the reabsorption of sodium and water and excretion of potassium
• ANP (Atrial Natriuretic Peptide) increases the reabsorption of sodium and water from proximal convoluted tubules and collecting ducts

Formation of Urine

• 3 processes occur in the formation of urine:
  • Filtration
  • Reabsorption
  • Secretion
• Reabsorption:
  • Occurs mainly in the proximal convoluted tubule
  • Reabsorbs water, glucose, amino acids, sodium, and other substances
  • Epithelial lining has microvilli to increase surface area
• Proximal Convoluted Tubule:
  • Reabsorbs 65% of Na+, 65% of H2O, 90% of filtered bicarbonate, 50% of Cl- and K+
• Loop of Henle:
  • Creates a gradient of increasing Na ion concentration towards the end of the loop within the interstitial fluid of the renal pyramid
  • Reabsorbs 25% of Na+, 15% of water, and 40% of K+
• Distal Convoluted Tubule:
  • Under the influence of Aldosterone, reabsorbs Na⁺ and secretes K⁺
  • Regulates pH by secreting H⁺ when pH of the plasma is low
  • Only 10% of the filtered NaCl and 20% of water remains
• Collecting Duct:
  • Allows for the osmotic reabsorption of water