Test: Excretory Products and their Elimination - Assertion and Reasoning - NEET MCQ

Ureotelism is defined as the urinary elimination of nitrogen mainly as urea. Aquatic mammals like whales and seals are said to be ureotelic animals because their major nitrogenous waste product is urea. As a matter of fact, ammonia is the basic nitrogenous catabolite of protein but since ammonia is highly toxic to the animals, therefore, its concentration must be kept very low in the blood.

Sharks are ureotelic, they are not ammonotelic animals. Ureotelic animals excrete urea instead of ammonia as the major nitrogenous waste product. These include man and all other mammals, terrestrial and semi-aquatic amphibians such as toads and frogs, cartilaginous fishes (elasmobranchs) such as sharks and sting rays. Sharks need to avoid water loss from body, thus they cannot excrete ammonia, as it requires enough water to be eliminated. In order to retain so much urea in their blood, their blood osmotic pressure approaches that of sea water. This minimises water loss from their body to adjust to the concentrated saline water of the sea.

Ammonia is a type of the basic nitrogenous catabolite of proteins, that is highly soluble in water and highly toxic to the animal. Therefore, its concentration must be kept very low in the blood. Due to this ammonia should be removed as rapidly from the body as it is formed. A large volume of water is required by the animals to dissolve ammonia and remove it from the body. So, its elimination in urine involves considerable loss of water from the body

When sufficient water is available, earthworms excrete ammonia. Because a large volume of water is needed by the animal to dissolve ammonia and eliminate it from, the body. Whereas in drier environment the animal removes urea as it needs only considerable amount of water for the excretion of urea because urea is very soluble in water.

In birds and reptiles, ureters and the rectum open into a common sac is known as the cloaca (as there is no chamber for urine and faces) for these two that stores both, and reabsorbs water from them and ultimately excretes these white and brownish black material along with aqueous fluid.

Urea is more toxic than uric acid and less toxic to ammonia.
Ammonia > Urea > Uric acid
Urea formation takes place in kidneys.

In vertebrates, accessory excretory organs are the lungs, liver and skin are referred as because besides the urinary system, these organs also participate in the removal of waste products from the body. The liver helps in the excretion of cholesterol, bile pigments (bilirubin and biliverdin), inactivated products of steroid hormones, some vitamins and many drugs. These are carried by the bile to the intestine and are eliminated with the faeces. It has no role in urine secretion.

Urinary bladder and ureters of excretory system are lined by transitional epithelium because it is a stretchable epithelium, hence the urinary bladder and ureters may be considerably stretched without getting torn when they are filled with urine. Ureters are thin muscular tubes which emerge from the hilum of each kidney. Urine enters the ureters from the renal pelvis and is conducted along the ureters by peristaltic waves on their walls. Ureters from both the kidneys finally open into urinary bladder which is a hollow muscular sac. In this way urine from both the kidneys is drained into the urinary bladder which stores it temporarily.

In vertebrates, the lungs, liver & skin are referred as accessory excretory organs because besides the urinary system these organs also participate in the removal of waste products from the body. The liver is the principal organ for the excretion of cholesterol, bile pigments (bilirubin and biliverdin) and inactivated products of steroid hormones, some vitamins and many drugs. It secretes these substances in the bile and indirectly helps by formation of urea through amino acids in ornithine cycle. They are carried by the bile to the intestine and are ultimately eliminated with the faeces. It has no role in secretion of urine.

Urea is the chief nitrogenous constituent of human urine, though it possesses small amount of ammonia. But when the urine is allowed to stand for sometime, bacterial degradation occurs and it leads to the production of ammonia from urine, and thus smells strongly.

Kidneys play an essential role in maintaining the concentration and osmotic pressure of blood. When water intake of an aminal is very high, the urine excreted has to be hypotonic i.e., dilute and lower in osmotic pressure than their blood in order to remove the excess of water contrary to this, when there is a threat of excessive water loss from the body; the urine needs to be hypertonic more concentrated and higher in osmotic pressure than their blood, to reduce the loss of water with urine. In this way, the osmotic concentration of the blood is maintained.

Deamination is the process of converting amino acid to keto acid with the release of NH₃. It occurs in the liver.

The nephrons are of two types on the basis of location. In majority of nephrons, the loop of Henle is too short, that extends only little into the medulla. These nephrons are called cortical nephrons and form 85% of the total nephrons. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. these nephrons are called juxtamedullary nephrons ehich form 15% of the total nephrons.

Cortical nephrons are situated in the renal cortex and consists of short loop of Henle and no vasa recta. Hence, they are not involved in concentration of urine. They control plasma volume when water supply is normal.

The term homeostasis (homeios same; stasis = standing) was introduced by Walter Cannon in 1932. It maintains the stability of the cell environment and this way provides the organism with a degree of independence of the environment; in order to achieve stability, mammals have two kidneys that perform excretory fuction and maintain urea level in blood.

Nephron is the principal functional unit of the kidney of vertebrates. Its number reaches to one million in each kidney. Filtration of blood takes place in the Malpighian body, later is composed of a tuft of capillaries called the glomerulus, and is together with the cupped end of the renal tubules, is called Bowman's capsule.Nephron is the principal functional unit of the kidney of vertebrates. Its number reaches to one million in each kidney. Filtration of blood takes place in the Malpighian body, later is composed of a tuft of capillaries called the glomerulus, and is together with the cupped end of the renal tubules, is called Bowman's capsule.

Renal thereshold of a substance is at its highest concentration in the blood, up to which it is totally reabsorbed from the glomerular filtrate. Renal threshold of glucose is about 180 mg per 100 ml. It is totally reabsored and does not appear in the urine so long as its blood level does not exceed 180 mg. But when its blood level exceeds 180 mg, some of the filtered glucose is left unabsorbed in the tubules and consequently appears in the urine. Some substances which are either totally reabsorbed actively or most of their amounts are reabsorbed actively are called high threshold substances. High threshold substances are excreted in the urine only when their blood concentration is considerably high, for example glucose and amino acids.

Kidneys play an essential role in maintaining the concentration and osmotic pressure (osmoconcentration) of blood. When water intake of an animal is very high, the urine excreted has to be hypotonic i.e., dilute and lower in osmotic pressure than their blood in order to remove the excess of water. Contary to this, when there is a threat of excessive water loss from the body; the urine needs to be hypertonic more concentrated and higher in osmotic pressure than their blood, to reduce the loss of water with urine. In this way, the osmotic concentration of the blood is maintained.

Descending limb is permeable to water but not to Na⁺. Consequently water moves out into interstitium and concentration of Na⁺ in tubular filtrate rises making the filtrate hypertonic. Ascending loop is impermeable to water but permeable to Na⁺ and makes the filtrate hypotonic.

Glomerular filtrate is the protein free fluid which is filtered from the blood of glomerular capilaries to the lumen of the Bowman's capsule. This process is called glomerular filtration. About one-fifth of the total volumne of plasma flowing through the kidneys is filtered out as the glomerular filtrate. The filtration occurs across the membrane made to the glomerular capillary wall and the inner memrane of the Bowman's capsule. The pores of this following membrane are impermeable to large molecules or particles. Large particles like blood cells and protein macromolecules do not normally enter into the glomerular filtrate. But smaller molecules like glucose, urea, creatinine, amino acids and mineral salts are filtered into the Bowman's capsule in concentrations more or less similar to their respective concentrations in the plasma. The filtrate therefore almost resembles the protein free plasma in composition and osmotic pressure.