Summary
Highlights
The urinary system primarily excretes metabolic waste, regulating fluid and electrolytes, and maintaining acid-base balance. Key waste products include urea, creatinine, and uric acid, filtered by the kidneys. The kidneys adjust water and electrolyte reabsorption and excretion to maintain fluid balance and blood pH by excreting hydrogen ions and reabsorbing bicarbonate. The system also regulates blood volume and pressure through the renin-angiotensin-aldosterone system and aids in red blood cell and vitamin D synthesis.
The kidneys are vital organs that continuously filter blood, removing waste, conserving useful materials, and forming urine. They regulate fluid and electrolytes (sodium, potassium, calcium, chloride), maintain acid-base balance, control blood pressure via the renin-angiotensin-aldosterone system, and produce hormones like renin and erythropoietin. The kidney’s external structures include the protective renal capsule and the hilum, where vessels and the ureter enter and exit. Internal structures include the cortex (filtration site), medulla (contains renal pyramids for urine concentration), and renal pelvis (collects urine).
Each kidney contains over a million nephrons, the microscopic units where urine formation occurs. Key parts include the glomerulus (initial filtration site), Bowman's capsule (collects filtrate), proximal convoluted tubule (bulk reabsorption of water, glucose, amino acids, and electrolytes), loop of Henle (concentrates urine), distal convoluted tubule (secretion of waste and electrolyte regulation), and collecting duct (final urine formation and water reabsorption control by ADH). The two types of nephrons are juxtamedullary nephrons (for water conservation) and cortical nephrons (distributed throughout the cortex).
The ureters are muscular tubes (25-30 cm long) that transport urine from the kidneys to the urinary bladder via peristalsis, allowing urine flow against gravity. They have three layers: mucosa (stretches), smooth muscle (peristalsis), and adventitia (anchors). The urinary bladder is a hollow, expandable muscular organ that temporarily stores urine, allowing for controlled urination. Its structure includes a transitional epithelium lining and the detrusor muscle, essential for bladder filling and urine expulsion. The urethra is the final exit tube for urine, connecting the bladder to the outside. It features an involuntary internal urethral sphincter preventing leakage and a voluntary external urethral sphincter for conscious control over urination. The urethra is shorter in females, making them more prone to infection.
Urine formation involves three main processes: filtration (in the glomerulus), tubular reabsorption (returning essential substances to the blood), and secretion (removing additional waste into the tubule). Regulation of urine concentration and volume involves the renin-angiotensin-aldosterone system, antidiuretic hormone (ADH), and atrial natriuretic hormone. The micturition reflex, activated by bladder distention, coordinates urination with the nervous system. Urinalysis is a diagnostic procedure that checks urine color (normally yellow to clear), clarity (should be clear), odor (should be faint), pH (4.5-8), specific gravity (1.005-1.030), and absence of glucose or protein, which can indicate health issues like dehydration, infection, or diabetes.