Excretory Products and their Elimination: common doubts, answered
The questions students ask most often about Excretory Products and their Elimination, each with a short answer. For the full chapter, read the Excretory Products and their Elimination notes.
Nitrogenous wastes
Read this section in the notes →What is the difference between ammonotelic, ureotelic and uricotelic animals?
Ammonotelic animals excrete ammonia, which is very toxic and needs a lot of water, as in many bony fishes and aquatic amphibians. Ureotelic animals excrete urea, as in mammals, many terrestrial amphibians and marine fishes. Uricotelic animals excrete uric acid, which needs the least water, as in reptiles, birds and insects.
Where is urea made in the body?
Urea is made in the liver. Ammonia formed during the breakdown of amino acids is highly toxic, so the liver converts it into the less toxic urea, which is released into the blood and filtered out by the kidneys. The kidneys do not make urea, although some animals keep a little urea in the kidney matrix to help maintain the osmolarity they need.
Excretory structures in animals
Read this section in the notes →What excretory structures do different animals have?
Flatworms like Planaria have protonephridia, or flame cells, and earthworms have nephridia. Insects such as cockroaches have Malpighian tubules, and prawns have green or antennal glands. Vertebrates have kidneys. The structures differ, but all of them help in the removal of nitrogenous waste and in osmoregulation.
Human excretory system
Read this section in the notes →What is the position and size of the human kidney?
The human kidneys are bean-shaped organs, one on each side of the vertebral column, located between the levels of the last thoracic and the third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each is about 10 to 12 cm long, 5 to 7 cm wide and weighs 120 to 170 grams.
What are the columns of Bertini?
The columns of Bertini are extensions of the kidney cortex that pass between the medullary pyramids. The medulla itself is divided into cone-shaped pyramids that project into the calyces. This structure gives a firm arrangement in which the cortex surrounds and also penetrates the medulla.
The nephron
Read this section in the notes →What are the parts of a nephron?
A nephron has a glomerulus, which is a tuft of capillaries, Bowman's capsule, the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, which opens into a collecting duct. The glomerulus with Bowman's capsule makes up the Malpighian body, or renal corpuscle. Each kidney has nearly a million nephrons.
What is the difference between cortical and juxtamedullary nephrons?
In cortical nephrons the loop of Henle is short and extends only a little into the medulla, and the vasa recta is absent or highly reduced. In juxtamedullary nephrons the loop is long and runs deep into the medulla, with a well-developed vasa recta. Juxtamedullary nephrons are important for concentrating urine.
Glomerular filtration
Read this section in the notes →What is the difference between blood flow to the kidney and GFR?
The kidneys receive and filter about 1100 to 1200 mL of blood per minute, about one-fifth of the output of each ventricle, but the glomerular filtration rate is only about 125 mL of filtrate per minute, which is 180 litres a day. GFR is thus the volume of filtrate formed, not the blood volume reaching the kidney.
What does the glomerular filtrate contain?
The filtrate contains almost all plasma constituents except proteins, since the filtration barrier has three layers: the glomerular endothelium, the basement membrane and the epithelium of Bowman's capsule, whose podocytes leave tiny filtration slits. Blood cells and large proteins are held back, so the filtrate is protein-free.
Reabsorption and secretion
Read this section in the notes →How much of the filtrate is reabsorbed?
Nearly 99 per cent. The kidneys filter about 180 litres a day but excrete only about 1.5 litres of urine, so almost all the filtrate is reabsorbed. Reabsorption occurs by both active and passive transport, and the PCT reabsorbs nearly all the nutrients and 70 to 80 per cent of electrolytes and water.
What is tubular secretion?
Tubular secretion is the process in which the tubule cells add substances such as hydrogen ions, potassium ions and ammonia from the blood to the filtrate. It helps maintain the ionic and acid-base balance of the body fluids. It is the opposite direction to reabsorption, which returns useful substances to blood.
Function of the tubules
Read this section in the notes →What is the difference between the descending and ascending limbs of the loop of Henle?
The descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate becomes concentrated as it moves down. The ascending limb is impermeable to water but allows electrolytes to leave, so the filtrate becomes dilute as it moves up. Do not give the ascending limb water permeability.
Counter current mechanism
Read this section in the notes →How does the counter current mechanism concentrate urine?
The flow of filtrate in the two limbs of Henle's loop is in opposite directions, as is the flow in the vasa recta, which sets up a gradient in the medulla. The interstitial concentration rises from about 300 to 1200 mOsmol per litre, mainly through sodium chloride and urea, so water can leave the collecting duct, allowing urine four times more concentrated than filtrate.
Regulation of kidney function
Read this section in the notes →What does ADH do?
ADH, or vasopressin, is released from the neurohypophysis and increases water reabsorption in the distal tubule and collecting duct. So less urine is formed and it is more concentrated, preventing diuresis. When body fluid volume rises, ADH release falls and the urine becomes more dilute.
How does the renin-angiotensin mechanism work?
When blood pressure or flow to the glomerulus falls, the juxtaglomerular cells release renin, which converts angiotensinogen to angiotensin I and then to angiotensin II. Angiotensin II raises blood pressure and stimulates the adrenal cortex to release aldosterone, which promotes sodium and water reabsorption. ANF, on the other hand, causes vasodilation and acts as a check.
Micturition
Read this section in the notes →How does the urinary bladder empty?
Micturition is a reflex. Stretching of the bladder wall by urine sends signals to the central nervous system, which sends motor messages back to cause the contraction of bladder muscles and relaxation of the urethral sphincter, releasing urine. The act is under voluntary control as well, in adults.
Role of other organs in excretion
Read this section in the notes →Which organs other than the kidney help in excretion?
The lungs remove large amounts of carbon dioxide, about 200 mL per minute, and water. The liver secretes bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs in bile. The skin removes sweat, containing sodium chloride, small amounts of urea and lactic acid, and sebaceous glands remove sterols and waxes.
Disorders of the excretory system
Read this section in the notes →What does glucose or ketone bodies in urine indicate?
Glucose in urine, glycosuria, and ketone bodies, ketonuria, indicate diabetes mellitus. Normal urine is 1 to 1.5 litres a day with a pH of about 6.0 and contains about 25 to 30 grams of urea daily. Analysis of urine therefore helps in diagnosing metabolic disorders and kidney malfunction.
How does haemodialysis work in kidney failure?
Blood from an artery is pumped to a dialysing unit after adding heparin, which stops clotting. Nitrogenous wastes pass across a cellophane membrane into a dialysing fluid, which has the same composition as plasma minus the nitrogenous wastes. Cleaned blood, with anti-heparin added, is returned to a vein.
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