Ch 8Excretion — Class 11 Zoology, concept-first.

Animal life first arose in the sea roughly 700 million years ago, and the earliest forms - much like modern sponges - had every cell bathed directly in seawater.

Every animal cell must get rid of ammonia, the toxic by-product of breaking down amino acids for energy. How an animal does this depends heavily on how much water it has to spare.

In humans, excretion of nitrogenous waste is the job of a compact, four-part system: a pair of kidneys that do the actual filtering, a pair of ureters that carry the urine away from each kidney, a sin…

A kidney is wrapped in three protective layers - an outer renal fascia, a perirenal fat capsule, and an inner fibrous capsule - that anchor it in place and cushion it from injury.

Each kidney contains close to a million nephrons, and every nephron has two main parts: a filtering unit called the renal corpuscle, and a long renal tubule leading away from it.

Before urine can be formed, the body first has to generate the waste it is designed to remove. When cells break down amino acids for energy, the amino group is released as ammonia, which is far too to…

Glomerular filtration is the first and entirely passive step of urine formation. Blood arriving at the glomerulus through the wide afferent arteriole is under an unusually high pressure - about 55 mmH…

Of the roughly 180 litres of filtrate produced daily, about 99% has to be reclaimed by the tubules, since it still carries glucose, amino acids, most of the water and most of the ions the body needs -…

Tubular secretion runs in the opposite direction to reabsorption: rather than recovering substances from the filtrate, the tubular cells actively move certain substances out of the peritubular blood a…

Mammalian kidneys can produce urine roughly four times more concentrated than the original glomerular filtrate, and this depends entirely on a counter-current arrangement set up by the long loops of H…

How much urine the kidneys make, and how concentrated it is, is not left to chance - it is continuously adjusted by a small set of hormones responding to changes in blood volume, blood pressure and bl…

Antidiuretic hormone (ADH), also called vasopressin, is the kidney's main water-conservation signal. Osmoreceptors in the hypothalamus continuously monitor blood volume and ionic concentration; when t…

The renin-angiotensin-aldosterone system (RAAS) is the kidney's main blood-pressure-and-sodium-conservation pathway, centred on the juxtaglomerular apparatus (JGA) - a specialised patch of tissue at t…

Atrial natriuretic factor (ANF), also called atrial natriuretic peptide, is released by the muscle cells of the heart's atria when they are excessively stretched by a high volume of blood returning to…

Micturition, or urination, is the process that finally releases the urine that the nephrons have spent hours producing.

The kidneys are the principal excretory organs, but they are not the only ones - several other organs quietly remove wastes of their own kind every day.

The excretory system, like any organ system, can malfunction in several distinct ways, and the disorders it is prone to range from ordinary infections to progressive organ failure.

A urinary tract infection typically begins when bacteria from the faeces are introduced into the urethra, most often because of improper toilet habits.

Renal failure is the loss of the kidney's ability to excrete metabolic wastes, and it shows up as an accumulation of urea in the blood together with a marked drop in urine output.

Glomerulonephritis, also known as Bright's disease, is an inflammation of the glomeruli in both kidneys simultaneously.

Uremia is the clinical name for a build-up of urea and other non-protein nitrogenous wastes - including uric acid and creatinine - in the blood, and it is a direct consequence of the kidneys losing th…

Renal calculi, commonly called kidney stones (or nephrolithiasis), are hard, stone-like masses that form when soluble crystals - mainly salts of sodium oxalate and certain phosphates - precipitate out…

When a patient's own kidneys can no longer clear urea and other toxic wastes from the blood, an external machine can temporarily take over that job through a process called haemodialysis.

Kidney transplantation is the ultimate, permanent correction for kidney failure, involving the surgical transfer of a healthy kidney from a donor into a patient whose own kidneys have failed.

Chapter-end questions on nephron structure and filtration, glomerular filtration forces and GFR, tubular reabsorption and secretion, hormonal regulation of kidney function, micturition, the role of ot…

Sample papers and previous-year board questions for this subject.