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The urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra.
The kidneys are bean-shaped organs located in the retroperitoneal space, on either side of the vertebral column, between the T12 and L3 vertebrae. They are partially protected by the rib cage and surrounded by perirenal fat, which provides cushioning. They are responsible for urine formation and play critical roles in regulating blood pressure, electrolyte levels, and hormone production. The ureters are approximately 10 to 12 inches long tubes connecting each kidney to the bladder. Peristaltic movements of the ureter muscles propel urine into the bladder. The urinary bladder is a hollow muscular organ situated in the pelvis, posterior to the pubic symphysis, capable of temporarily storing up to 600 mL of urine. The urethra carries urine from the bladder to the outside of the body. In males, it ends at the tip of the penis, whereas in females, it terminates just above the vaginal opening.
Functions of the Urinary System
Urine formation: The kidneys filter waste products from metabolic processes to form urine. These wastes include urea and ammonia (from amino acid deamination), creatinine (from creatine phosphate degradation), uric acid (from nucleic acid catabolism), and urobilin (from hemoglobin breakdown). Collectively, these nitrogen-containing waste products are known as nitrogenous wastes. Urine also eliminates foreign substances, such as drugs and environmental toxins.
Ionic composition of blood and its regulation: Contribution to the regulation of blood ionic composition is another key function of kidneys. They manage this task by controlling amounts of several ions - chiefly sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl−), and phosphate (HPO42−) - excreted in the urine.
Regulation of blood pH: The kidneys also regulate blood pH by modulating the excretion of hydrogen ions (H+) and the reabsorption or generation of bicarbonate ions (HCO3−). Bicarbonate acts as a primary buffer in the blood, maintaining acid-base balance by neutralizing excess H⁺ ions.
Adjustment of blood volume: Adjustment of blood volume is another essential kidney function. They manage this by either conserving or eliminating water in urine. Changes in blood volume will subsequently affect blood pressure.
Enzyme secretion: The kidneys secrete renin, which also regulates blood pressure. Renin catalyzes the conversion of angiotensinogen into angiotensin I, initiating the renin-angiotensin-aldosterone system (RAAS). This pathway promotes vasoconstriction and sodium retention, increasing blood pressure and volume.
Maintenance of osmolarity: The kidney also maintains blood osmolarity, which is typically around 300 milliosmoles per liter (mOsm/liter). This balance is achieved through the differential action of antidiuretic hormone (ADH) and tubular reabsorption mechanisms, ensuring osmotic equilibrium between body compartments.
Hormone production: The kidneys also participate in hormone production, generating two key hormones - Calcitriol, the biologically active form of vitamin D that causes calcium homeostasis, and erythropoietin, which stimulates the creation of red blood cells.
Regulation of blood glucose: Kidneys participate in regulating blood glucose levels. They use glutamine for gluconeogenesis, which releases glucose into the blood to help keep blood glucose levels stable.
