Urinary System | Physiology Of Urine Formation | RAS | Acid Base Balance | Mictirutional Reflex

The lecture begins by introducing the urinary system (also called the renal system) as the body's major excretory system, though other systems like the digestive and respiratory systems also play excretory roles. The main components — kidneys, ureters, urinary bladder, and urethra — are described structurally. The kidneys are bean-shaped, located on either side of the vertebral column between T12 and L3, and measure 10–12 cm in length, weighing 120–170 grams. Internally, the kidney has three layers: the outer cortex, inner medulla (containing medullary pyramids), and the renal pelvis.

The nephron is identified as the functional unit of the kidney, analogous to the neuron in the nervous system. Each nephron consists of a renal corpuscle (containing the glomerulus and Bowman's capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, and distal convoluted tubule). The loop of Henle is further divided into descending and ascending limbs. Two types of nephrons are discussed: cortical (85%) with short loops of Henle, and juxtamedullary (15%) with long loops.

The physiology of urine formation involves three steps. First, glomerular filtration: blood plasma (excluding RBCs, WBCs, platelets, and plasma proteins) is filtered from glomerular capillaries into Bowman's capsule, driven by hydrostatic pressure. The GFR is approximately 125 mL/min or 180 liters/day. Second, tubular reabsorption: about 99% of the filtrate is reabsorbed back into the blood — glucose, amino acids, sodium, potassium in the PCT; sodium and chloride in the loop of Henle; and sodium, bicarbonate, and water in the DCT. Third, tubular secretion: waste substances (like K+, H+, ammonia, urea) that escaped filtration are actively secreted from blood capillaries back into the tubular lumen for excretion.

The kidney's role in acid-base balance is explained as the most effective and final mechanism among the three systems (buffer, respiratory, renal). The kidney maintains pH by excreting H+ ions via the tubular cells (where CO2 + H2O → H2CO3 → HCO3⁻ + H+, with H+ secreted into the lumen via a hydrogen-sodium antiport pump) and by reabsorbing bicarbonate ions to neutralize excess acidity.

The renin-angiotensin system (RAS) is described as a hormonal mechanism activated during emergencies like blood loss. Renin (secreted by the kidney's juxtaglomerular apparatus) converts angiotensinogen (a liver protein) into angiotensin I, which is then converted to angiotensin II by ACE. Angiotensin II causes vasoconstriction (raising blood pressure), increases sympathetic activity, and stimulates aldosterone release from the adrenal cortex — which promotes Na+ and water reabsorption, restoring blood volume.

The micturition reflex refers to the process of urination. As the bladder fills, stretch receptors are activated, sending sensory signals to the brain. The brain responds via motor nerves, triggering bladder muscle contraction and urethral relaxation, leading to urine discharge. If not voluntarily released, the urge to urinate intensifies.

Finally, common urinary disorders are reviewed: polycystic kidney disease (cyst formation enlarging the kidney and reducing function), UTIs (bacterial infection of the urinary tract, more common in females, causing pain, blood in urine, or frequent urges), nephrotic syndrome (increased glomerular permeability causing protein loss in urine), urinary incontinence (loss of bladder control leading to frequent, uncontrollable urination), and kidney stones (mineral and salt deposits in the kidney, associated with low water intake, obesity, or high-salt/sugar diets).