Unit 14: Human Urinary System — Short Questions

Kidneys perform excretion by filtering nitrogenous wastes such as urea, along with extra water and salts, out of the blood and eliminating them as urine. At the same time, kidneys act as osmoregulatory organs: they maintain the water-to-salt balance of body fluids by adjusting how much water and salts are reabsorbed or excreted, producing dilute urine when body fluids are hypotonic and concentrated urine when body fluids are hypertonic or the body is dehydrated. Since the same nephron processes (filtration, reabsorption, secretion) achieve both waste removal and fluid/salt balance simultaneously, kidneys are justified as both excretory and osmoregulatory organs.

The human urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra.

Glomerular filtration (ultrafiltration) is the physical process in which the high blood pressure within the glomerular capillaries (caused by the efferent arteriole being narrower than the afferent arteriole) forces water, urea, glucose, amino acids, vitamins, and salts (Na⁺, K⁺, Cl⁻) out of the blood into the Bowman's capsule. About one-fifth (20%) of the fluid portion of blood is filtered this way to form the glomerular filtrate, while plasma proteins and blood cells, being too large, stay in the blood.

Glomerular capillaries are located within the Bowman's capsule and are designed for filtration; they operate under high blood pressure (due to the narrow efferent arteriole) which forces water and small solutes out of blood into the nephron. Peritubular capillaries surround the renal tubules and are designed for reabsorption and secretion; they operate under low hydrostatic pressure, pulling essential nutrients (glucose, ions) and water back into the blood from the tubules, while also secreting specific toxins into the tubule.

The three main processes are: (i) glomerular filtration, in which blood is filtered under pressure into Bowman's capsule; (ii) selective reabsorption, in which useful substances like glucose, amino acids, salts, and water are returned from the renal tubule to the peritubular capillaries; and (iii) tubular secretion, in which the distal convoluted tubule adds substances like hydrogen ions, ammonium, potassium ions and drug residues from blood into the filtrate.

Chronic kidney failure develops over a long period and is irreversible; it is usually caused by underlying conditions like diabetes, high blood pressure, or kidney disease such as chronic infection or inflammation of the glomeruli. Acute kidney failure occurs suddenly and often resolves with treatment; it may be caused by kidney infection, a blockage in the urinary tract (due to kidney stones, bladder cancer, or an enlarged prostate), certain medications such as NSAIDs, or blood clots in the renal arteries or arterioles.

The afferent arteriole carries blood into the glomerulus and is wider, while the efferent arteriole carries blood away from the glomerulus and is narrower. This difference in diameter creates the high pressure within the glomerular capillaries needed for filtration. After the glomerulus, the efferent arteriole goes on to form the peritubular capillaries (and vasa recta in juxtamedullary nephrons).

Chronic renal failure develops over a longer period of time and is irreversible; it is often caused by diabetes, high blood pressure, or chronic kidney disease such as infection or inflammation of the glomeruli. Acute renal failure (kidney injury) occurs suddenly and often resolves with treatment; it may result from kidney infection, blockage of the urinary tract, certain medications such as NSAIDs, or blood clots in renal arteries/arterioles.

In haemodialysis, blood is withdrawn from the body through a catheter in an artery, passed through an external dialyzer containing a semipermeable (cellulose) membrane bathed in dialysate, and the cleaned blood is returned through a catheter in a vein. In peritoneal dialysis, no blood is removed from the body; instead, 1.5–2 litres of dialysate are infused into the patient's own abdominal cavity through a catheter and left there for 4–6 hours, during which the peritoneal membrane itself acts as the filter, after which the dialysate is drained and discarded, repeated several times a day.

The renal cortex is the outer portion of the kidney, making up about one-third of its mass, and contains most of the renal corpuscles and convoluted tubules. The renal medulla is the inner portion, making up about two-thirds of the kidney, and consists of cone-shaped renal pyramids containing the loops of Henle and collecting ducts.

Kidney donation can be a life-saving gift for patients with end-stage kidney disease, offering them a functioning kidney in place of long-term dependence on dialysis, which carries its own risks such as infections, hypotension, anaemia, and emotional strain. Donors can be related, unrelated, or deceased, with careful matching of blood group, tissue type, and immune compatibility ensuring success. Importantly, donating a kidney does not have significant long-term health risks for the donor — studies show donors generally experience no negative impact on their overall health or lifespan — which makes living donation a safe and valuable option. Increasing kidney donation can greatly benefit the many patients affected by kidney failure.