Definition
Excretion is the removal of metabolic waste products (like urea) from the body. Osmoregulation is the separate but related process of regulating the amount of water (and dissolved salts) in body fluids, keeping their concentration within a healthy range.
Along with excretion, kidneys also play a major role in osmoregulation: while removing urea and other wastes, they simultaneously adjust exactly how much water and which salts are reabsorbed back into the blood versus excreted, based on the body's current needs — this dual role is why the kidneys are central to both waste removal and fluid balance at once, not just one or the other.
Definition
Main parts of the human urinary system: two kidneys (filter blood, produce urine), two ureters (carry urine from kidneys to the bladder), the bladder (stores urine), and the urethra (carries urine out of the body).
Definition
A kidney has an outer renal cortex and an inner renal medulla. The renal medulla contains cone-shaped areas called renal pyramids.
The hilum is the depression on the concave side of the kidney, where the renal artery enters, and the renal vein and ureter leave.
The kidney controls the composition of blood by continuously filtering it (removing wastes and excess substances) and then selectively reabsorbing exactly what the body needs back into the blood, so that blood leaving the kidney has a carefully regulated composition.
Definition
Major parts of a nephron: the renal corpuscle (made of the glomerulus, a knot of capillaries, and Bowman's capsule, which surrounds it), the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT), and the collecting duct.
The renal corpuscle (glomerulus Bowman's capsule) is the structure responsible for filtering blood, via pressure filtration — high blood pressure inside the glomerulus's capillaries forces plasma (minus large components) out through the capillary walls and into Bowman's capsule, forming the filtrate. The renal tubule (PCT, loop of Henle, DCT) is the rest of the nephron, where that filtrate is processed further (reabsorption and secretion) before it becomes urine.
Definition
Blood cells and large plasma proteins are NOT present in the filtrate entering Bowman's capsule, since pressure filtration only lets small molecules (like water, glucose, amino acids, salts, and urea) pass through the capillary and capsule walls — anything too large to fit through these pores stays in the blood.
Example 1 — pressure filtration vs tubular secretion
Differentiate between pressure filtration and tubular secretion.
- Pressure filtration happens once, at the renal corpuscle: blood pressure forces small molecules out of the glomerulus's capillaries into Bowman's capsule, forming the filtrate — a largely passive, pressure-driven process that doesn't distinguish between useful and waste small molecules.
- Tubular secretion happens later, along the tubule (particularly the DCT): specific substances (like extra , , and some drugs/wastes not already filtered) are actively transported directly from the blood into the nephron's tubule fluid, adding to what will become urine.
Answer: Pressure filtration is a bulk, non-selective process at the start; tubular secretion is a later, selective, active-transport process adding specific wastes
Definition
Reabsorption of nutrients (glucose, amino acids) and much of the filtered water occurs mainly in the proximal convoluted tubule (PCT), through active transport and osmosis — the nephron ensures essential nutrients aren't lost in urine specifically because the PCT actively transports glucose and amino acids back into the blood so efficiently that, under normal conditions, approximately of the filtered glucose and amino acids are reabsorbed here, leaving essentially none in the urine.
Definition
The loop of Henle is important for balancing water and salts in the body because its long length (with descending and ascending limbs having different permeability to water and salt) builds up a strong salt concentration gradient deep in the renal medulla — this gradient is exactly what lets the collecting duct later reabsorb varying amounts of water by osmosis, depending on the body's hydration needs, so a longer loop of Henle allows a steeper gradient and so more concentrated urine when needed.
Definition
When there's excess water in the blood, the body produces less antidiuretic hormone (ADH), making the collecting duct less permeable to water — less water is reabsorbed, producing large volumes of dilute urine. When there's less water in the blood, more ADH is released, making the collecting duct more permeable to water — more water is reabsorbed back into the blood (using the concentration gradient the loop of Henle built up), producing a smaller volume of concentrated urine.
Definition
Kidney stones are hard mineral deposits that form inside the kidney, commonly from excess dietary calcium, oxalate (found in foods like spinach and chocolate), or excess salt/protein intake combined with insufficient water intake, letting minerals crystallize instead of staying dissolved.
Lithotripsy is a non-surgical treatment that uses focused shock waves (or sometimes lasers) directed from outside the body to break large kidney stones into much smaller fragments, which can then pass out of the body naturally through urine without needing invasive surgery.
Definition
Kidney failure is the loss of the kidneys' ability to adequately filter blood and remove waste, leading to a dangerous buildup of toxins and fluid in the body.
Chronic diseases like diabetes can lead to kidney failure because persistently high blood glucose levels damage the delicate blood vessels in the glomeruli over time, progressively impairing their ability to filter blood properly, eventually leading to permanent kidney damage.
Symptoms of kidney failure include fatigue, swelling (especially in the legs/ankles, from fluid retention), reduced urine output, and nausea. Treatments for kidney failure include dialysis (haemodialysis or peritoneal dialysis) and kidney transplantation.
Example 2 — haemodialysis vs peritoneal dialysis
Differentiate between haemodialysis and peritoneal dialysis.
- Haemodialysis: a procedure in which a machine is used to filter blood outside the body — blood is pumped through an external filtering device (an artificial membrane) that removes wastes and excess fluid, before the cleaned blood is returned to the patient.
- Peritoneal dialysis: uses the patient's own peritoneum (the membrane lining the abdominal cavity) as the filtering membrane — a special fluid is introduced into the abdominal cavity, wastes diffuse across the peritoneum into this fluid, which is then drained out.
Answer: Haemodialysis filters blood externally through a machine; peritoneal dialysis filters it internally using the patient's own peritoneum as the membrane
Definition
The primary purpose of haemodialysis is to artificially perform the kidney's filtering function — removing waste products and excess fluid from the blood — when the patient's own kidneys can no longer do this adequately.
Kidney transplantation replaces a failed kidney with a healthy donor kidney; a successfully transplanted kidney can function well for many years (commonly a decade or more, sometimes considerably longer, depending on many factors), often giving a much better quality of life than long-term dialysis.
Quick Recap
- ●Excretion (waste removal) vs osmoregulation (water/salt balance) — kidneys do both together
- ●Urinary system: kidneys ureters bladder urethra. Kidney: cortex medulla (renal pyramids) hilum
- ●Nephron: renal corpuscle (glomerulus Bowman's capsule, pressure filtration) PCT (reabsorbs glucose/amino acids) loop of Henle (builds salt gradient) DCT (tubular secretion) collecting duct
- ●ADH controls water reabsorption at the collecting duct: more ADH concentrated urine; less ADH dilute urine
- ●Kidney stones: mineral build-up (calcium, oxalate, low water intake); treated with lithotripsy (shock waves) if needed
- ●Kidney failure: often from diabetes/hypertension damaging glomeruli; treated with haemodialysis (external machine), peritoneal dialysis (patient's own peritoneum), or transplantation
