Unit 14: Human Urinary System — Long Questions

Each kidney is a reddish-brown, bean-shaped organ with an adrenal gland on its top, and both are surrounded by a fibrous renal capsule. The kidney has a convex side and a concave side; a depressed area on the concave side, the renal hilum, is where the renal artery enters and the renal vein and ureter leave — this arrangement allows blood supply in and filtered blood plus urine out at one point.

Internally, the kidney has two main regions. The outer renal cortex (about one-third of kidney mass) contains most of the renal corpuscles and convoluted tubules, making it the main site of glomerular filtration and early reabsorption. The inner renal medulla (about two-thirds of kidney mass) is made of cone-shaped renal pyramids containing the loops of Henle and collecting ducts; its role is to build the salt gradient (counter-current mechanism) that concentrates urine. The renal pelvis, a funnel-shaped structure at the centre, collects urine from the pyramids and channels it into the ureter.

Thus, the structural division of the kidney into cortex, medulla and pelvis directly matches the functional sequence of filtration, reabsorption/concentration, and collection/elimination of urine.

Each kidney has about one million nephrons, its functional units. A nephron has two main parts: a renal corpuscle and a renal tubule.

The renal corpuscle consists of the glomerulus, a capillary network derived from an afferent renal arteriole, surrounded by the Bowman's capsule, a cup-shaped, double-walled sac — this is the site of glomerular filtration.

The renal tubule leads away from Bowman's capsule and has three connected parts: (i) the proximal convoluted tubule, highly coiled and dipping into the medulla, where most reabsorption occurs; (ii) the loop of Henle, a U-shaped structure with a descending limb (permeable to water) continuing toward the medulla and an ascending limb (impermeable to water, actively transports salts) returning toward the cortex; and (iii) the distal convoluted tubule, again highly coiled, lying near the renal corpuscle, which is the main site of tubular secretion and hormone-regulated reabsorption.

The distal convoluted tubules of many nephrons drain into a single collecting duct; collecting ducts merge into larger papillary ducts that discharge into the renal pelvis.

Nephrons are of two types: cortical nephrons (about 85%), located mainly in the renal cortex with short loops of Henle; and juxtamedullary nephrons, originating near the medulla with long loops of Henle extending deep into the medulla, important for concentrating urine.

Urine formation in the nephron involves three major processes.

(i) Glomerular filtration: Because the efferent arteriole is narrower than the afferent arteriole, blood in the glomerular capillaries is under high pressure. This pressure forces water, urea, glucose, amino acids, vitamins, and salts (Na⁺, K⁺, Cl⁻) from the blood into Bowman's capsule. About 20% of the fluid portion of blood is filtered this way, forming the glomerular filtrate; plasma proteins and blood cells are too large to filter and remain in the blood.

(ii) Selective reabsorption: Useful substances in the filtrate are returned from the renal tubule into the peritubular capillaries. Most reabsorption (about 75% of water, plus glucose, amino acids, and salts like sodium, potassium, calcium) occurs in the proximal convoluted tubule, largely by active transport and osmosis. More water is reabsorbed in the descending limb of the loop of Henle, while the ascending limb actively transports out salts but is impermeable to water. Some water and salts are also reabsorbed in the distal convoluted tubule, and the collecting duct is the final site of water reabsorption.

(iii) Tubular secretion: The epithelial cells of the distal convoluted tubule take substances such as hydrogen ions, ammonium, potassium ions, and metabolic toxins or drug residues (e.g., penicillin) from the surrounding blood and secrete them into the filtrate, helping maintain the blood's acid-base balance.

From the collecting ducts, urine flows through the renal pelvis into the ureter, then to the urinary bladder, and is eliminated through the urethra.

Concentration of urine is regulated by two coordinated mechanisms.

Counter-current mechanism The descending limb of the loop of Henle is permeable to water but not to salts, while the ascending limb is impermeable to water but actively transports salts (Na⁺, with Cl⁻ following) out into the surrounding interstitial fluid. As filtrate flows down the descending limb, water is drawn out by osmosis because the surrounding fluid is saltier; as filtrate flows up the ascending limb, more salt is pumped out, keeping the surrounding medullary fluid highly salty. When urine later passes down the collecting duct through this salty interstitial environment, water moves out by osmosis into the blood via the vasa recta, concentrating the urine.

Hormonal control Antidiuretic hormone (ADH), produced by the hypothalamus and released from the posterior pituitary, is triggered when osmoreceptors in the hypothalamus detect high blood salt concentration (dehydration). ADH makes the collecting duct walls more permeable to water, so more water moves from filtrate into blood, producing concentrated (hypertonic) urine and conserving water. When blood salt concentration falls, ADH secretion stops, the collecting duct becomes less permeable to water, and dilute (hypotonic) urine is excreted.

Aldosterone, secreted by the adrenal cortex when blood pressure drops or blood sodium is low, acts on the distal convoluted tubule and collecting duct to increase sodium reabsorption into blood; water follows osmotically, which also helps concentrate urine and raise blood volume/pressure. Aldosterone also promotes potassium excretion.

Together, the counter-current mechanism creates the salt gradient, while ADH and aldosterone control how much water and sodium are actually reabsorbed, determining the final concentration of urine.

Causes Urinary/kidney stones are hard, crystalline mineral deposits that form small "pebbles" in the kidney or urinary tract. About 70% of patients have calcium oxalate stones, caused by hypercalciuria (high urinary calcium) and hyperoxaluria (high urinary oxalate). About 10% have uric acid stones, caused by hyperuricemia (increased uric acid in blood); such stones form when urine is persistently acidic. High concentrations of cysteine and phosphates in urine also cause stones (e.g., calcium phosphate stones, associated with alkaline urine). Continuous dehydration increases the risk of kidney stone formation in general.

Treatments Treatment usually involves lithotripsy, i.e., breaking the stones. (i) Extracorporeal Shockwave Lithotripsy (ECSWL) uses a lithotripter to generate shockwaves from outside the body that break the stone into small pieces, which then pass out in the urine. (ii) For larger stones, Percutaneous Nephrolithotripsy (PNL) is used: a tube is inserted from the patient's back into the kidney, a nephroscope (small camera) visualizes the stone, ultrasound equipment breaks it, and the fragments are grasped and removed with special equipment.

Renal dialysis is used to treat kidney failure by artificially filtering wastes from blood. There are two types.

Haemodialysis A catheter is inserted into an artery, usually in the arm, and blood is circulated through a dialyzer — a filter containing a semipermeable (cellulose) membrane — alongside a special fluid called dialysate. As blood passes through, wastes, excess water, and salts move out of the blood across the membrane and dissolve in the dialysate. The cleaned blood is then returned to the body through a second catheter in a vein.

Peritoneal dialysis This uses the peritoneal membrane lining the patient's abdominal cavity as the filter. About 1.5–2 litres of dialysate are infused into the abdomen through a catheter and left there for 4–6 hours. During this time, waste products and excess fluid from the blood capillaries of the peritoneum pass into the dialysate, which is then drained out and discarded. This is repeated several times a day for adequate clearance of wastes and fluid.

Problems associated with dialysis It requires access to the bloodstream, increasing infection risk. Hypotension (low blood pressure) is a common complication of haemodialysis. Muscle cramps can occur due to changes in salt levels. Dialysis can cause anaemia, leading to fatigue and weakness. Removing too much fluid too quickly can cause dehydration or low blood pressure. Dialysis can also be emotionally challenging, causing anxiety, depression, or a feeling of being overwhelmed by the treatment's demands.

Principles Kidney transplant is a surgical procedure in which a person's non-functioning kidney is replaced with a healthy kidney from a donor, offered to individuals with end-stage kidney failure. It requires careful evaluation, matching (blood group, tissue type, immune compatibility), and monitoring of both donor and recipient for the best outcome. Potential donors can be: a related donor, genetically related to the recipient (parent, sibling, or child) and usually the best match; an unrelated donor, not genetically related (e.g., spouse, friend, co-worker), which can still succeed if blood group and tissue type match; or a deceased donor, whose organs are donated after death and carefully matched to recipients by blood group, tissue type, and immune compatibility.

Problems The recipient's immune system may recognize the transplanted kidney as foreign and attack it, causing organ rejection even while the patient takes immunosuppressant medications. A suppressed immune system also raises the risk of infections after transplant. The immunosuppressant drugs given to prevent rejection can themselves cause side effects such as high blood pressure, weight gain, and an increased risk of infections and cancers.