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Define Glomerular Filtration Rate (GFR)

The amount of filtrate during urine formation in the glomerulus of the kidney per minute is called glomerular filtration rate. In a healthy individual the GFR is approximately 125 ml/ minute.


Explain the autoregulatory mechanism of GFR.

There is an auto regulatory mechanism present in the kidney to regulate the glomerular filtration rate. This mechanism is carried out by juxta glomerular apparatus (JGA) which is a special sensitive region formed by cellular modification of the distal convoluted tubule and the different arteriole at the location of their contact. A fall in the GFR will activate the JG cells to release renin which converts the angiotensinogen in blood to angiotensin I and later into angiotensin II. The angiotensin II is a powerful vaso constrictor which increases the glomerular blood pressure and there by increase the GFR. The Angiotensin II also activates the adrenal cortex to release aldosteron which causes the reabsorption of Na+ and water from the distal parts of the tubule. This also results in the increase in blood pressure and GFR.


Name the following:

(a) A chordate animal having flame cells as excretory structures

(b) Cortical portions projecting between the medullary pyramids in the human kidney

(c) A loop of capillary running parallel to the Henle’s loop.

(a) Amphioxus

(b) Renal column

(c) Vasa recta.


Give a brief account of the counter current mechanism.

Production of concentrated urine is aided by the Henle’s loop and Vasarecta. The flow of filtrate in both the henle’s loop is in opposite direction and also in the vasa recta this flow is called as counter current pattern. The vasa recta is a capillary parallel to the henle’s loop. The Nacl which is transported by the ascending limb of the Henle’s loop is exchanged with the descending limb of vasa recta. In the same way small amounts of urea enter the thin segments of the ascending limb of Henle’s loop and again it is transported back to the interstitium. This exchange of materials facilitated by the Henle’s loop and vasa recta is called counter current mechanism.


Describe the role of liver, lungs and skin in excretion.

Role of Liver in Excretion
The largest gland in out body is the liver. Its secretions include bilirubi, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these secretions are passed out along with the digested waste material.

Role of Lungs in Excretion
The Lungs is the main respiratory organ. It excretes the waste gas CO2 by exhalation and also small quantities of water.

Role of Skin in Excretion
The skin is the largest receptor organ covering the entire body. It contains the sweat and sebaceous gland. The secretions of these glands are sent out of the body. The sweat secreted by the sweat gland contains NaCl, small amount of urea, lactic acid etc. These waste materials are excreted along with the sweat.

The sebaceous glands excrete substance like sterols, hydrocarbons and waxes through sebum.


Explain micturition

The Urinary bladder collects the urine formed by the nephron and it stores the urine till a voluntary signal is given by the central nervous system. As the wall of the urinary bladder is stretched by the increase in the volume of the collected urine the signal is passed to the CNS. The CNS in return passes on motor messages to start the contraction of the smooth muscles of the bladder and simultaneously there is relaxation of the urethral sphincter which causes the release of urine. This process involved in the release of urine is called micturition.


What is meant by the term osmoregulation?

Osmoregulation is the regulation of ions and the fluid volume inside the body.


Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?

Aquatic animals are ammonotelic as the ammonia is generally excreted by diffusion across the body surface. Whereas in terrestrial animals ammonia which is toxic cannot circulate in the blood till it reach the excretory system, it is converted into lesser toxic nitrogenous wastes like urea and uric acid by the liver and released into the blood from where it is filtered and excreted out by the kidneys. Hence terrestrial animals are either ureotelic or uricotelic.


What is the significance of juxta glomerular apparatus (JGA) in kidney function?

The juxta glomerular apparatus in the kidney is a autoregulatory mechanism of GFR. It plays a major role in the regulation of GFR. When the GFR falls, the decrease activates the juxta glomerular cells to release renin which can stimulate the glomerular blood flow. This there by brings back the glomerular filtration rate to normal.

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