Excretory Products and Their Elimination
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Excretory products and their elimination.
Ammonia is the most toxic forms and requires large amount of water for its elimination while uric acid being least toxic can be removed with minimum loss of water. Ammonotelic: process of excretion of ammonia.
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Urieotelic: process of excretion of urea.
Uricotelic: process of excretion of uric acid as pellets.
- Found in mammals, many terrestrial amphibians and marine fishes.
- Ammonia is converted into urea by liver and released into blood which again filtered by kidneys.
Uricotelic: process of excretion of uric acid as pellets.
- Found in reptiles, birds, land snails and insects.
The Human Excretory System:
Human excretory system consists of: (i) pair of kidneys.
(ii) pair of ureters.
(iii) a urinary bladder.
(iv) a urethra.
Diagram of Human excretory system is shown is Figure 1.
(ii) pair of ureters.
(iii) a urinary bladder.
(iv) a urethra.
Diagram of Human excretory system is shown is Figure 1.
Structure of Kidney:
Notch towards the center of the inner concave surface is called hilum through which blood vessels, ureter and nerves enter. Inside the hilum, there is a cavity called renal pelvis with projection called calyces. Region of kidney is divided into two zones: inner medulla and outer cortex.
Inner medulla is divided into few conical shaped masses called medullary pyramids, which project into the calyces. Outer medulla extends between the medullary pyramids in the form of renal columns and are called Coulums of Bertini.
Cross section of a kidney is shown is Figure 2.
Video for structure for Structure for Kidney.
Inner medulla is divided into few conical shaped masses called medullary pyramids, which project into the calyces. Outer medulla extends between the medullary pyramids in the form of renal columns and are called Coulums of Bertini.
Cross section of a kidney is shown is Figure 2.
Video for structure for Structure for Kidney.
Structure of Nephron:
Nephron is the structural and functional unit of kidney.
A nephron has two parts which are (i) Glomerulus and (ii) Renal tubules. (i). Glomerulus: is made up of cluster of blood capillaries formed from afferent arteriole and efferent arteriole. Afferent arteriole carries the blood into the glomerulus and Efferent arteriole carries the blood away from the glomerulus. The efferent arteriole will run parallel to the Henle’s loop forming a network of capillary called vasa recta. (ii) Renal tubules: consist of (a) Bowman’s capsule, (b) Proximal Convoluted Tubule (PCT) (c) Henle’s Loop and (d) Distal Convoluted Tubule (DCT) Bowmen’s capsule is the cup-like structure that encloses the glomerulus. Bowmen’s capsule and glomerulus are together called malpighian body or renal corpuscle. Diagram of malpighian body is shown in Figure 3. The bowmen’s capsule extends further to form highly coiled proximal convoluted tubule (PCT), which continues further to form “U-shaped” Henle’s loop. Henle’s loop, which consist of a descending limb and an ascending limb, is descended by another highly coiled tubular structure called the distal convoluted tubule (DCT), which opens into the collecting duct that opens into the renal pelvis through medullary pyramids in the calyces. The malpighian body, (PCT) and (DCT) are located in the cortical region and Henle’s loop is located in the medullary region. The nephron whose Henle’s loop extends very little into the medulla is called the cortical nephron. The nephron whose Henle’s loop extents deep into the medulla is called juxta medullary nephron. Diagram of glomerulus and renal tubules are shown in Figure 4. Video for Structure of Nephron.
Animation for Structure of Nephron.
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Urine Formation:
Urine formation involves 3 main processes (i) Glomerular Filteration or Ultra filteration,
(ii) Reabsorption and
(iii) Secretion.
(i) Glomerular Filteration (ultra filteration):
The glomerular capillary blood pressure causes the filtration of blood through three layers: endothelium of glomerular blood vessel, epithelium of Bowmen’s capsule and the basement membrane between the two layers. The epithelial cells of bowmen’s capsule called podocytes are arranged in such a way that it leaves minutes spaces called filtration slits through which all the components of plasma except large particles like proteins and RBCs pass through it and therefore, the filtration is called the ultra filtration. Glomerular filtration rate (GFR) is the amount of filtrate formed by the kidneys per minute. For a healthy individual, average GFR is 125 ml/day
(ii) Reabsorption: is the process of absorption of the filtrate by renal tubules either by active (glucose, amino acid, Na+) or passive transportation (urea and water). Volume of filtrate formed is 180 L/day and urine releases 1.5 L/day.
(iii) Secretion: is the process of secretion of substances like H+, K+, and NH3 into the filtrate to maintain pH, and acid and base balance of body fluid.
Video for Urine Formation.
(ii) Reabsorption and
(iii) Secretion.
(i) Glomerular Filteration (ultra filteration):
The glomerular capillary blood pressure causes the filtration of blood through three layers: endothelium of glomerular blood vessel, epithelium of Bowmen’s capsule and the basement membrane between the two layers. The epithelial cells of bowmen’s capsule called podocytes are arranged in such a way that it leaves minutes spaces called filtration slits through which all the components of plasma except large particles like proteins and RBCs pass through it and therefore, the filtration is called the ultra filtration. Glomerular filtration rate (GFR) is the amount of filtrate formed by the kidneys per minute. For a healthy individual, average GFR is 125 ml/day
(ii) Reabsorption: is the process of absorption of the filtrate by renal tubules either by active (glucose, amino acid, Na+) or passive transportation (urea and water). Volume of filtrate formed is 180 L/day and urine releases 1.5 L/day.
(iii) Secretion: is the process of secretion of substances like H+, K+, and NH3 into the filtrate to maintain pH, and acid and base balance of body fluid.
Video for Urine Formation.
Function of Tubules:
Proximal Convoluted Tubule (PCT):
Henle’s loop:
Distal Convoluted Tubule (DCT):
Collecting duct:
Video for Function of Tubules. |
Mechanism of Concentration of Filtrate:
Salt concentration is low at the outer medulla (300 mOsmol/L) and high at the lower medulla (1200 mOsmol/L). This concentration gradient is maintained by counter-current mechanism of the loop of Henle.
As the filtrate moves down through the descending loop, the salt concentration in outer medulla increases gradually. The filtrate will try to equalize its concentration with the concentration of the outer medium. Since the descending loop is permeable to water, water will diffuse passively out into the surrounding medium. The water is taken away by the vasa recta. As the filtrate moves up through the ascending loop, the outside salt concentration will gradually decrease. However, the ascending loop is permeable to salt while impermeable to water. Therefore, salt will diffuse outward into the medium while water cannot move in either direction. As a result, as the filtrate moves through the loop of Henle, water volume in the surrounding medium will increase and water volume in the loop will decrease. In this way the urine gets concentrated. Video for Concentration of Filtrate. |
Regulation of Kidney Function:
Kidney function is regulated by hormonal feedback mechanism involving hypothalamus, JGA and the heart.
Hypothalamus:
Excessive loss of fluid from the body will activate osmoreceptors which stimulate the hypothalamus to release Antidiuretic hormone (ADH) or vasopressin from neurohypophysis.
ADH or vasopressin will:
Juxtaglomerular apparatus (JGA):
JGA is a sensitive region formed by cellular modifications of DCT and afferent arteriole.
Fall in GFR will activate the juxtaglomerular (JG) cells to release renin which converts angeotnsinogen to angeotensin I and then to angeotensin II.
Angeotensin II:
Heart:
Increase in blood flow in atria of heart will cause the release of Atrial Natriuretic Factor (ANF), which being a vasodilator cause the dilation of blood vessels leading to decrease in GFR. ANF will also decrease the absorption of Na+ from the distal convoluted tubule and therefore, water will not be absorbed.
ANF mechanism is opposite to that of renin-angeotensin mechanism.
Hypothalamus:
Excessive loss of fluid from the body will activate osmoreceptors which stimulate the hypothalamus to release Antidiuretic hormone (ADH) or vasopressin from neurohypophysis.
ADH or vasopressin will:
- activate water reabsorption from collecting duct thereby increasing GFR. Increase in FR switches off the osmoreceptors which suppresses ADH release.
- ADH also being a vasoconstrictor will increase the blood pressure thereby increasing GFR.
Juxtaglomerular apparatus (JGA):
JGA is a sensitive region formed by cellular modifications of DCT and afferent arteriole.
Fall in GFR will activate the juxtaglomerular (JG) cells to release renin which converts angeotnsinogen to angeotensin I and then to angeotensin II.
Angeotensin II:
- will activate the adrenal cortex to release aldosterone which reabsorbs water and Na+ from DCT leading to increase in GFR.
- being a vasoconstrictor will increase the glomerular blood pressure and therefore GFR.
Heart:
Increase in blood flow in atria of heart will cause the release of Atrial Natriuretic Factor (ANF), which being a vasodilator cause the dilation of blood vessels leading to decrease in GFR. ANF will also decrease the absorption of Na+ from the distal convoluted tubule and therefore, water will not be absorbed.
ANF mechanism is opposite to that of renin-angeotensin mechanism.
Micturition:
The process of release of urine. On average 1-1.5 litres of urine and 25-30 gm of urea is excreted out per day. Urine is slightly acid (pH 6.0).
Roles of other organ in secretion:
Lungs, sweat gland and sebaceous gland also helps in the elimination of waste products.
The process of release of urine. On average 1-1.5 litres of urine and 25-30 gm of urea is excreted out per day. Urine is slightly acid (pH 6.0).
Roles of other organ in secretion:
Lungs, sweat gland and sebaceous gland also helps in the elimination of waste products.
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On 2014-09-27 20:04:59.553390 by Anonymous
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On 2014-08-25 14:01:03.175410 by virender verma
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On 2014-08-25 14:00:32.535800 by virender verma
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On 2014-06-08 07:15:41.150090 by raj
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On 2013-09-22 19:36:23.816380 by
About the diseases in kidney
On 2013-09-22 19:36:01.007880 by Anonymous
About the diseases in kidney
On 2013-08-04 06:41:13.405560 by Anonymous
Please Upload a File With Notes. So that we can download it.!