Describe the structure of a nephron. 


A nephron is the functional unit of the kidney.
Each nephron compreises of two parts – the glomerulus and the renal tubule.
 
The different parts of a nephrons are : 

Glomerulus - is a tuft of capillaries formed by the afferent arteriole – a fine branch of renal
artery. Blood from the glomerulus is carried
away by an efferent arteriole.

Renal tubule - The renal tubule begins with a double walled cup-like structure called Bowman’s capsule, which encloses the glomerulus.


Malphigian body - The Glomerulus alongwith Bowman’s capsule, is called the malpighian body or renal corpuscle.

PCT - The tubule continues further to form a highly coiled network – proximal convoluted tubule 

Loop Of Henle - A hairpin shaped Henle’s loop forms the next part of the tubule which has a
descending and an ascending limb.

DCT - The ascending limb continues as another highly coiled tubular region called distal
convoluted tubule (DCT).

Collecting Duct - The DCTs of many nephrons open into a straight tube called collecting duct, many of which converge and open into the renal pelvis through medullary pyramids in the calyces.

The Malpighian corpuscle, PCT and DCT
of the nephron are situated in the cortical
region of the kidney whereas the loop of Henle
dips into the medulla. 

Peritubular capillaries - The efferent arteriole emerging from the glomerulus forms a fine
capillary network around the renal tubule called the peritubular capillaries.

Vasa recta - A minute vessel of this network runs parallel to the Henle’s loop forming a ‘U’ shaped vasa recta. 

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Describe briefly mechanism of urine formation in human kidney.


Urine formation in kidney can be describe in 3 headings :

1. Ultrafiltration : It is the filtration which occurs under high blood pressure. The afferent arteriole supplies blood to glomerulus and efferent arteriole collects blood from glomerulus. The afferent arteriole has more breath than the capillaries of glomerulus. Therefore, the pressure in glomerulus is two and half times more than ordinary capillary bed. Thus smaller molecules of glucose, water, salts, ammonia etc. diffuse from glomerulus into Bowman's capusle. And this process is called ultrafiltration. 

2. Selective reabsorption : It is the process by which useful materials are absorbed by the blood from glomerular filtrate. The tubular  epithelial cells in different segments of nephron perform this either by active or passive  mechanisms. For example, substances like glucose, amino acids, Na+, etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron

3. Selective secretion : The metabolic waste products like ammonia, creatine, H+ ions etc. present in the blood diffuse into distal convoluted tubule DCT. This process is called selective secretion. The tubular cells secrete substances like H+, K+ and ammonia into the filtrate. Tubular secretion helps in the maintenance of ionic and acid base balance of body fluids.

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Describe the excretory organs in protozoans, sponges and coelentrates.

Protozoans : Protozoans have no excretory organs. The metabolic waste products i.e. ammonia is passed out by diffusion through the cell membrane in surrounding water. They carry out the process of excretion with the help of contractile vacuoles.

Sponges : Ammonia is the main excretory product in sponges. It diffuses from cells into the water filling the canals and surrounding the sponge. From the canal system, the waste matter is carried away by the outgoing current of water. 

Coelentrates : Coelentrates have no excretory organs. They pass out metabolic waste by diffusion from all cells. They are ammonotelic i.e. their excretory product is ammonia. The waste water entering coelentron leaves when its water is renewed.

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 Describe the structure of Nephron.

Nephron : A nephron is a structural and functional unit of kidney. It is differentiated into four regions Bowman's capusle, proximal convoluted tubule, loop of Henle and distal convoluted tubule (DCT).  Describe the structure of Nephron.

Fig. Uriniferous tubule

(i) Bowman's capsule : It is a double walled cup containing bunch of parallel capillaries called glomerulus. The glomerulus rests on basement membrane. It is a continuous layer. The Bowman's capsule and glomerulus together are called renal corpuscle or malpighian body. The endothelial cells lining the glomerulus have pores  between them. The cells of inner wall of Bowman's capsule are called podocytes have gaps called slit pores 25nm wide between them.

The glomerulus is supplied by an afferent arteriole and is drained by a relatively narrow efferent arteriole. The latter forms peritubular capillary network around the rest of nephron.

(ii) Proximal convoluted tubule (PCT) : It starts from the neck of the Bowman's capsule.

(iii) Loop of Henle : It is a U-shaped segment of the nephron located in the renal medulla. It consists of two straight parallel limbs.

(iv) Distal convoluted tubule (DCT) : Like PCT, it is greatly twisted and lies in the renal cortex. DCT continues into a collecting tubule.

Collecting tubes or ducts : These are larger tubes, each receiving the collecting tubule of several nephrons. They join each other forming larger ducts of bellini. These open into calyces, which lead into the pelvis.

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Describe the various nitrogenous waste products.

Nitrogenous waste products : Ammonia , urea, uric acid are some nitrogenous waste products excreted by our body. 

1. Ammonia : It is formed by the deamination of amino acids during the protein metabolism. Ammonia is highly toxic. It is highly soluble in water and needs a large amount of water for its excretion. The animals which excret ammonia are called ammonotelic organisms. For expmple bony fishes , aquatic amphibians and insects.

 2. Urea : It is less toxic than ammonia. Most mammals, fishes and amphibians excrete their nitrogenous waste products in the form of urea. Process of excretion of urea is called ureotelism and such animals are called ureotelic. It needs less water for excretion, hence helps in the conservation of water. Animals like mammals, terrestrial amphibians and marine fishes excrete urea. 

3. Uric acid : Reptiles and birds excrete uric acid and are called uricotelic. The uric acid is is excreted in the foprm of pellet and paste with minimum loss of water. 

 

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