The Urinary Tract

The urinary system is the body's master regulator. While most people think of it as simply producing urine, its true importance lies in maintaining the precise chemical balance that every cell in the body depends on. The urinary system: - Filters waste from the blood — removing urea (from protein breakdown), creatinine (from muscle metabolism), drugs, and toxins - Regulates fluid balance — controlling how much water the body retains - Controls electrolyte levels — sodium, potassium, calcium, and others - Maintains pH — keeping blood pH between 7.35 and 7.45 (slightly alkaline) - Produces hormones — erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure) The system consists of two kidneys, two ureters, the bladder, and the urethra.

The kidneys are two bean-shaped organs, each about the size of a fist (10–12 cm), located in the posterior abdomen on either side of the vertebral column. They sit retroperitoneally — behind the peritoneum (the membrane lining the abdominal cavity) — so they are not in the abdominal cavity proper. The right kidney sits slightly lower than the left because the liver occupies the upper right abdomen. External structure Each kidney is enclosed in three layers of protective tissue: - Renal capsule — a tough fibrous capsule directly around the kidney - Perirenal fat — a cushion of fat providing protection against trauma - Renal fascia — an outer fibrous sheet anchoring the kidney to surrounding structures The renal hilum is the indentation on the medial (inner) side of each kidney — the point of entry and exit for the renal artery, renal vein, and ureter. Internal structure Cut a kidney open and you see three distinct regions: 1. Renal cortex — the outer layer, reddish-brown. Contains the glomeruli (filtration units) and convoluted tubules. 2. Renal medulla — the inner layer, composed of triangular renal pyramids. Contains the loops of Henle and collecting ducts. 3. Renal pelvis — the funnel-shaped cavity at the centre, collecting urine. It divides into major and minor calyces (cups) that collect urine from each pyramid. The pelvis narrows into the ureter. Blood supply The kidneys receive approximately 20–25% of the heart's entire output — about 1.2 litres of blood per minute. This enormous blood flow reflects their role as filters. Blood enters via the renal arteries (branches of the aorta) and leaves via the renal veins (draining into the inferior vena cava).

Each kidney contains approximately 1 million nephrons — the microscopic functional units that actually produce urine. Understanding the nephron is the key to understanding kidney disease and the drugs that treat it. Each nephron consists of: 1. Renal corpuscle (in the cortex) - Glomerulus — a knot of capillaries surrounded by Bowman's capsule - Bowman's capsule — a cup-shaped structure that collects the filtrate Blood enters the glomerulus under high pressure, forcing water, salts, glucose, urea, and small molecules through the capillary walls into Bowman's capsule — a process called filtration. Large molecules (proteins) and blood cells are too big to pass and remain in the blood. The filtered fluid is called glomerular filtrate — about 180 litres are produced per day. 2. Proximal convoluted tubule (PCT) (in the cortex) Reabsorbs the majority of the filtrate — about 65–70% of water, all glucose, all amino acids, and most electrolytes. If glucose appears in the urine (glycosuria), the blood glucose is so high it exceeds the PCT's reabsorption capacity — the threshold is about 10 mmol/L. 3. Loop of Henle (dips into the medulla) Creates a concentration gradient in the medulla that allows the kidney to produce concentrated urine. The descending limb is permeable to water; the ascending limb is impermeable to water but actively transports sodium out. This counter-current multiplier mechanism is what allows urine to be up to 4 times more concentrated than blood plasma. 4. Distal convoluted tubule (DCT) (in the cortex) Fine-tunes electrolyte balance — particularly sodium and potassium — under hormonal control. Aldosterone (from the adrenal gland) acts here to increase sodium reabsorption and potassium excretion. ACE inhibitors and ARBs (blood pressure drugs) work partly through this system. 5. Collecting duct Multiple nephrons drain into a collecting duct that passes through the medulla into a calyx. ADH (antidiuretic hormone, also called vasopressin) acts on collecting duct cells to make them permeable to water — allowing water to be reabsorbed when the body is dehydrated. Without ADH (as in diabetes insipidus), the collecting duct remains impermeable and vast quantities of dilute urine are produced.

The ureters Two muscular tubes (25–30 cm) that carry urine from the renal pelvis to the bladder. Peristaltic contractions push urine down regardless of gravity. The ureters enter the bladder at an oblique angle — this creates a flap-valve effect that prevents urine from refluxing back up when the bladder contracts. There are three natural narrowings in each ureter where kidney stones commonly become lodged: 1. The junction between the renal pelvis and ureter (pelviureteric junction, PUJ) 2. Where the ureter crosses the pelvic brim 3. Where the ureter enters the bladder wall (vesicoureteric junction, VUJ) A stone stuck at any of these points causes renal colic — excruciating, colicky (wave-like) flank pain radiating to the groin, often with haematuria (blood in urine). The urinary bladder A hollow, muscular organ in the pelvis that stores urine. Its wall contains the detrusor muscle — smooth muscle that contracts to expel urine. The internal surface is lined with transitional epithelium (urothelium) — specialised cells that can stretch as the bladder fills without being damaged. The normal bladder holds 300–500 ml before the urge to urinate becomes compelling. The internal urethral sphincter (involuntary smooth muscle) and external urethral sphincter (voluntary skeletal muscle) keep the bladder closed. The desire to urinate is felt when stretch receptors in the bladder wall signal to the brain. The urethra The tube that carries urine from the bladder to the outside. There is a significant anatomical difference between males and females: - Female urethra — short, about 4 cm. Proximity to the anus and vagina makes ascending urinary tract infections (UTIs) common. The urethral opening (urethral meatus) is anterior to the vaginal opening. - Male urethra — long, about 20 cm. Passes through the prostate gland and the penis. Less prone to UTIs because of the length. The prostate surrounds the urethra — when it enlarges (benign prostatic hyperplasia), it compresses the urethra causing urinary obstruction.

Urinary tract infections (UTIs) — bacterial infection most commonly in the bladder (cystitis). Symptoms: dysuria (painful urination), frequency, urgency, cloudy/offensive urine. More common in women. Ascending infection to the kidneys causes pyelonephritis — fever, loin pain, and systemic illness. Renal calculi (kidney stones) — crystals forming in urine (commonly calcium oxalate). Small stones pass spontaneously causing intense colic. Larger stones require intervention (lithotripsy or surgery). Stones composed of uric acid form in patients with gout. Acute kidney injury (AKI) — sudden deterioration in kidney function. Three categories based on cause: - Pre-renal — reduced blood flow to kidneys (dehydration, heart failure, haemorrhage) - Renal — direct kidney damage (glomerulonephritis, drugs like NSAIDs, contrast dye) - Post-renal — obstruction to urine outflow (kidney stone, enlarged prostate) Chronic kidney disease (CKD) — gradual, irreversible loss of kidney function. Often caused by diabetes (diabetic nephropathy) or hypertension. Measured by eGFR (estimated glomerular filtration rate). End-stage kidney disease requires dialysis or transplantation. Benign prostatic hyperplasia (BPH) — non-cancerous enlargement of the prostate compressing the urethra. Causes hesitancy, poor stream, incomplete emptying, and nocturia (waking at night to urinate). Very common in men over 60.