Excretory Products and Their Elimination

Animals accumulate ammonia, urea, uric acid, CO2, water and ions like Na+, K+, Cl-, phosphate, sulphate, etc., either by metabolic activities or by other means.

The process of excreting ammonia is called ammonotelism and organisms that excrete it are ammonotelic.
Similarly, the organisms that excrete urea are called ureotelic.

Excretory structures of various organisms are:

Protonephridia or flame cells: Platyhelminthes, rotifers, some annelids and the cephalochordate- Amphioxus
Nephridia: Earthworms and other annelids
Malphigian tubules: Insects like cockroaches
Antennal gland or green gland: Crustaceans like prawns

Human excretory system comprises a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
Kidneys are situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
Each kidney has an opening, called hilum.
Inside the kidney, there are two zones: cortex and medulla.
Nephron is the basic functional unit of kidney.
Glomerulus and the renal tubules are the two parts of nephron.
The glomerulus receives its blood supply from an afferent arteriole of the renal circulation.
The renal tubule begins with Bowman’s capsule.
Glomerulus along with Bowman’s capsule is called malpighian body or renal corpuscle. After passing the tubule, the filtrate continues to the collecting duct system.
The components of the kidney tubules are proximal convoluted tubule, Henle’s loop, and distal convoluted tubule.
Urine formation occurs as the result of three distinct processes: Glomerular filtration, tubular reabsorption, and tubular secretion that take place in different parts of the nephron.
The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR).
When fluids flow in opposite directions in parallel and adjacent tubes of Henle’s loop, it forms a counter current.

Regulation of kidney function: When the osmotic pressure of blood changes, osmoreceptors gets activated. As a result, water diffusion into and out of the osmoreceptor cells changes. That is, they expand when the blood plasma is more dilute and contract with higher concentration.
This causes an afferent neural signal to be sent to the hypothalamus, which increases or decreases vassopressin (ADH) secretion from the posterior pituitary to return blood concentration to normality.
Decreased blood pressure stimulates the kidneys to secrete rennin which splits the plasma protein angiotensinogen to angiotensin I.
Angiotensin I is converted to angiotensin II, which causes vasoconstriction and stimulates the adrenal cortex to secrete aldosterone.
Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule, which leads to an increase in blood pressure and GFR.

When urine is stored in the bladder, a voluntary signal is given by the central nervous system (CNS), which is initiated by the stretching of the urinary bladder. This is called micturation.

Except kidney there are some other organs which help in the elimination of excretory wastes. They are: skin, lungs, liver etc.

Disorders of excretory system are Uremia, Renal calculi, Glomerulonephritis, Diabetes insipidus etc.

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