Urinary System Functions and Structures

• The urinary system performs several functions related to waste elimination, regulation, and hormone production.
• It filters blood and removes metabolic wastes, forming urine.
• It regulates ion balance, adjusting levels of Na⁺, K⁺, and Ca²⁺.
• The system maintains acid-base balance by altering H⁺ and HCO₃⁻ levels.
• It regulates blood pressure by adjusting blood volume and releasing renin.
• The urinary system eliminates hormones and drugs from the body.
• It activates vitamin D, converting it to calcitriol for calcium regulation.
• It regulates erythropoiesis by secreting erythropoietin (EPO) in response to low O₂, stimulating RBC production.
• Gluconeogenesis occurs in the urinary system, producing glucose in fasting/starvation conditions.

Structures & Functions

• The nephron, located in the renal cortex and medulla, is the functional unit of the kidney.
• The nephron contains the renal corpuscle:
  • The glomerulus is a capillary network filtering blood.
  • Bowman’s capsule receives filtrate from the glomerulus.
• The nephron contains the renal tubule:
  • The proximal convoluted tubule (PCT) reabsorbs nutrients, ions, and water.
  • The loop of Henle consists of the descending limb (water reabsorption) and ascending limb (salt reabsorption).
  • The distal convoluted tubule (DCT) regulates ion balance.
  • The collecting duct is responsible for the final urine concentration.
• Ureters transport urine from the kidneys to the bladder via peristalsis.
• The urinary bladder can store ~1L of urine.
• The trigone is a funnel-like area directing urine to the urethra.
• The urethra transports urine outside the body.
• The female urethra is short (~4 cm) and only for urine.
• The male urethra is longer (~18-20 cm) and carries urine and semen.

Kidney Structure & Location of Key Parts

• The cortex is the outer layer that contains nephrons.
• The medulla is the inner region that contains renal pyramids.
• Renal pyramids are cone-shaped structures in the medulla.
• Renal columns are extensions of the cortex between pyramids.
• The renal pelvis is a funnel-like structure collecting urine from calyces.
• Minor calyces collect urine from pyramids, merge into major calyces, and drain into the renal pelvis.
• The hilum is the indentation where the ureter, renal artery, and renal vein enter/exit.
• The renal artery and vein supply blood to and drain blood from the kidney.

Nephron Structures & Functions

• The afferent arteriole brings blood into the glomerulus.
• Blood is filtered in the glomerulus.
• Bowman’s capsule captures filtrate from the glomerulus.
• The efferent arteriole takes filtered blood away from the glomerulus.
• The proximal convoluted tubule reabsorbs nutrients, ions, and water.
• The loop of Henle includes the descending limb, where water exits and filtrate concentrates, and the ascending limb, where Na⁺ and Cl⁻ exit and filtrate dilutes.
• The distal convoluted tubule further adjusts ion balance.
• The collecting duct makes final water/ion adjustments and contributes to urine formation.
• Peritubular capillaries reabsorb substances into the blood.

Urine Formation

• Blood is filtered at the glomerulus, and filtrate enters Bowman’s capsule.
• Large molecules (proteins, RBCs) remain in the blood during glomerular filtration.
• Essential substances (water, glucose, amino acids) are reabsorbed into the blood, mainly in the PCT, where 65% of reabsorption occurs.
• Water follows Na⁺ by osmosis during tubular reabsorption.
• Waste products and excess ions (H⁺, K⁺, drugs) are actively secreted into tubules, helping maintain pH and electrolyte balance.

Hormonal Regulation in the DCT & Collecting Duct

• Aldosterone, from the adrenal cortex, increases Na⁺ reabsorption, causing water to follow and increasing blood volume/pressure.
• Antidiuretic hormone (ADH), from the posterior pituitary, increases water reabsorption through aquaporins and produces concentrated urine.
• Atrial natriuretic peptide (ANP), from the heart, opposes aldosterone, increasing Na⁺ and water excretion and lowering blood pressure.
• Parathyroid hormone (PTH), from the parathyroid gland, increases Ca²⁺ reabsorption.

Additional Notes

• Urine is 95% water and 5% solutes.
• Urine pH ranges from 4.5 to 8.0.
• Normal urine volume is 1-2 L/day.
• The micturition (urination) reflex is triggered when the bladder stretches (200-300 mL urine), activating baroreceptors.
• The micturition center in the pons signals the detrusor muscle to contract and internal/external sphincters to relax, allowing urine to exit the body.