Excretory Products and their Elimination Test 58

Solution

Correct answer is option B.
Explanation for correct option:

• Podocytes are a type of cells in the inner walls of the Bowman's capsule of nephrons in the kidneys that wrap around the capillaries of the glomerulus. 
• The long processes or foot projections of the podocytes wrap around the capillaries and leave slits between them.
• Blood is filtered through these slits, each known as a slit diaphragm or filtration slit. 

Metanephric kidney filters waste from the blood but excretes them to the outside through a pair of tubes, the ureters. In the embryo, the wastes are excreted directly into the amniotic fluid. The metanephric kidney is the final adult kidney of reptiles, birds, and mammals.

On the basis of development, kidneys are of 3 types.
(1) Pronephric kidney: Develop from anterior part (pronephros) of nephrotome plate. Its nephrons are simple tubular in shape. Nephrons are not differentiated. For example, cyclostomes and tadpole of the frog.
(2) Mesonephric kidney: Develop from middle part (mesonephros) of nephrotome plate and remaining part of nephrotome is destroyed. Only Bowman's capsule is found in nephrons while remaining part is simple tubular. For example, most of the fishes and adult Amphibians.
(3) Metanephric Kidney: Develops from posterior part (metanephros) of nephrotome while remaining part is destroyed. Nephrons are well differentiated into Bowman's capsule,  PCT, DCT and loop of Henle's eg. Reptile, Aves, Mammals.

During filtration, blood enters the afferent renal arteriole and flows into the glomerulus where blood components such as water and nitrogenous waste will move towards the inside of the glomerulus, and non-filterable components such as cells will exit via the efferent arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate. The next step is reabsorption, during which molecules and ions will be reabsorbed back into the circulatory system. What remains in the glomerulus after this process has taken place is known as the tubular fluid and this is what will pass through to the collecting duct to form urine. During secretion, some substances such as hydrogen ions, creatinine, and drugs will be removed from the blood through the peritubular capillary network into the distal convoluted tubule, or collecting duct. The end product of all these processes is urine; a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorption. It is removed from the body through the process of excretion.

• The kidney is present in the abdominal segment. 
• It has two major parts the outer cortex and the inner medulla. 
• The medial border of the kidney is concave in the centre and convex toward either extremity; it is directed forward and a little downward. 
• Its central part presents a deep longitudinal fissure, bounded by prominent overhanging anterior and posterior lips. 
• This fissure is a hilum, that transmits the vessels, nerves, and ureter.

The distal convoluted tubules of a number of adjacent nephrons open into a common collecting duct or tubule. Collecting ducts (present in medullary pyramids) are long tubules, which traverse through the medulla in the pyramids and opens in the papilla of the medulla (in rabbit) or papilla of the individual medullary pyramid (in human). 

The length of the loop of Henle is the determining factor with regard to how concentrated urine an animal can produce. In species which produce very concentrated urine, almost all of their nephrons have very long loops of Henle. However, in animals, such as beavers, where dehydration is not really an issue, have a very short loop of Henle and excrete dilute urine.

Correct option -D