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Excretion

By vedant-patel Dec 03, 2013 1472 Words
Excretion
Removal of nitrogenous wastes from the body
Nitrogenous wastes
Ammonia
oMost toxic
oNeeds more water for getting excreted
oDiffuses across general body surfaces
oExamples of organisms excreting ammonia: fishes, aquatic amphibians and aquatic insects oSuch organisms are called ammonotelic.
Urea
oLess toxic
oRequires less water for excretion
oTerrestrial adaptation for conservation of water
oAmmonia Urea
oExamples of organisms excreting urea: terrestrial amphibians and marine fishes oSuch organisms are called ureotelic.
Uric acid
oLeast toxic
oEliminated with the least loss of water, as pellets or paste oExamples of organisms excreting uric acid: birds, reptiles, land snails and insects oSuch organisms are called uricotelic.
Excretory Organs in Animal Kingdom
Protonephridia (flame cells): Excretory structures in platyhelminthes and some cephalochordates like Amphioxus •Nephridia: In earthworms and other annelids
Malpighian tubules:In insects, including cockroach
Antennal glands(Green Glands): In crustaceans (prawns)
Vertebrates and some molluscs have kidneys to perform excretion Human Excretory System
Consists of: Kidneys (2), Ureters (2), Urinary bladder (1), Urethra (1) •Kidneys
oLocation: Between levels of the last thoracic and the third lumbar vertebra oMeasurement: 10−12 cm (length) × 5−7 cm (width) × 2−3 cm (thickness) oWeight: 120−170 g
oHilum: A notch present towards the centre of the inner concave surface of the kidney oThrough the hilum, the ureter, blood vessels and nerves enter the kidney. oOn the inside of the hilum, the renal pelvis is present. Renal pelvis has projections called calyces. o2 zones in the kidney: Cortex (outer)

Medulla (inner)
oMedulla divides into medullary pyramids (cone-shaped). These medullary pyramids project into the calyces. oCortex is present in between the medullary pyramids as renal columns called the columns of Bertini.

oEach kidney consists of about 1 million nephrons. These are the structural and functional units of the kidneys. [[Page Break]]
Nephrons

A Nephron has two parts:
oGlomerulus
oRenal tubule
Glomerulus
oTuft of capillaries formed by the afferent arteriole
Renal Tubule: Has many parts
(Bowman’s capsule Proximal Convoluted Tubule (PCT) Hairpin-shaped Loop of Henle Distal Convoluted Tubule (DCT) Collecting duct Medullary pyramids → Renal pelvis.) •Glomerulus + Bowman’s Capsule = Malpighian body (Renal Corpuscle) •Nephrons are of 2 types:

oCortical nephrons: Here, the loop of Henle is short and confined to the cortex only. Vasa rectae are absent. These nephrons are more common (85%). oJuxta medullary nephrons: Here, the loop of Henle is long and extended to the medulla. Vasa rectae are present. These nephrons are less common (15%). •Malpighian corpuscle, PCT and DCT are present in the corticle region only. •Capillary network in a nephron:

Peritubular capillaries: Emerging from the glomerulus, the efferent arteriole forms a capillary network around the renal tubule called the peritubular capillaries. Urine Formation Involves 3 basic steps:

Glomerular filtration
Re-absorption
Secretion
Glomerular filtration
Kidney filters 1100−1200 mL blood/min
Filtration of blood occurs as it passes through three layers. oEndothelium of glomerular blood vessels
oEpithelium of Bowman’s capsule
oBasement membrane between these two layers
Epithelial cells of Bowman’s capsule are called podocytes. Arrangement of these podocytes leaves extremely minute spaces called filtration slits or slit pores. •Ultrafiltration occurs through these slits pores, i.e., all plasma components (except proteins) get filtered into the lumen of the Bowman’s capsule. •Glomerular Filtration Rate (GFR): 125 mL/min or 180 L/day •The glomerular filtration rate is regulated by the juxta glomerular apparatus which releases the hormone renin. Re-absorption

GFR = 180 L/day; Urine released = 1.5 L/day. Thus, 99% of filtrate is re-absorbed by the renal tubules. •Re-absorption occurs through the epithelial cells of the various segments of the nephrons. •Active Re-absorption: Glucose, Na+, amino acids

Passive Re-absorption: Nitrogenous waste, water
Secretion
Tubular cells secrete substances such as H+, K+ and ammonia into the filtrate. •Importance: Maintenance of ionic and acid-base balance of body fluids Function of the Tubules
Proximal Convoluted Tubule
oSpecialised for re-absorption as it is lined by a simple cuboidal brush border epithelium which increases the surface area for absorption oRe-absorbs all essential nutrients, electrolytes and water oSecretes H+, NH4+, K+ ions and to maintain pH

Henle’s Loop
oMinimum re-absorption occurs here
oHelps in maintaining high osmolarity of the medullary fluid oDescending loop of Henle: Permeable to water and impermeable to electrolytes; Concentrates the filtrate oAscending loop of Henle: Impermeable to water and permeable to electrolytes; Dilutes the filtrate •DCT

oConditional re-absorption of Na+, water, HCO3−
oSelectively secretes H+, K+, NH3
oMaintains pH and Na−K balance in blood
Collecting Duct
oConcentrates the urine by absorbing large amounts of water oAllows passage of urea into the medullary interstitial fluid to maintain osmolarity oSecretes H+ and K+ ions; hence, maintains pH and ionic balance

o
o
oVasa Recta: A minute vessel of the capillary network present in a nephron runs parallel to Henle’s loop to form the U-shaped Vasa Recta. Micturition
Passing of urine through the opening in the urinary bladder •Urine is stored in urinary bladder. As the bladder gets filled with urine, it gets stretched. •Stretch receptors on the walls of the bladder send signals to CNS. •CNS sends counter signals to initiate contraction of the smooth muscles of the bladder, and relax the urethral sphincter to cause urine to be released (micturition). •Neural mechanism: Micturition reflex

Urine: 1−1.5 L/day; pH 6.0; Light yellow in colour
On an average, 25−30 gm of urea is excreted everyday.
Presence of glucose and ketone bodies in urine indicates diabetes mellitus. •Concentration of urine is achieved by the presence of a concentration gradient in the medullary interstitium. •Such concentration gradient is maintained by a specialised mechanism called the counter-current mechanism. •The flow of filtrate in the two limbs of Henle’s loop is in the opposite direction. Flow of blood in vasa recta is also in the opposite direction. Both these flows form a counter-current. •Close proximity of Henle’s loop and vasa recta as well as the counter-current in them maintains an osmolarity that increases towards the medullary interstitium. •NaCl and urea play a role in the maintenance of the gradient. •NaCl: Transported by the ascending limb of Henle’s loop and provided to the descending limb of vasa recta •Urea: Enters the thin segment of the ascending limb of Henle’s loop •NaCl is transported back to the interstitium by the ascending portion of vasa recta and urea is transported back to the interstitium by the collecting tubule.

Regulation of Kidney Function
The functioning of kidney is efficiently regulated by hormonal feedback mechanism. It involves − Hypothalamus, Juxtaglomerular apparatus (JGA), and heart. •Hypothalamus
oAntidiuretic hormone or ADH plays an important role in regulation. oExcessive loss of fluids from the body activates the osmoreceptors and stimulates hypothalamus to release hormone ADH, which helps in water re-absorption and prevents diuresis. oA further increase in body fluid volume can turn off the osmoreceptors. This inhibits the ADH release, which in turn completes the feedback. oADH constricts the blood vessels and causes blood pressure to increase, which in turn increases glomerular blood flow and therefore GFR. •Juxtaglomerular Apparatus (Renin-Angiotensin Mechanism)

oWhen GFR falls, JG cells release renin.
Renin plays an important role in the production of angiotensin II.
oAngiotensin II − Constricts blood vessels to increase blood pressure and hence GFR It also activates adrenal cortex to release aldosterone that causes re-absorption of Na+ and water from tubule. This also results in an increase in blood pressure and hence GFR. •Heart

oAs the blood flow to the atria increases, atrial Natriuretic Factor (ANF) is released. oANF causes vasodilation and decreases blood pressure.
oIt checks renin-angiotensin mechanism.
Role of other Organs in Excretion
Lungs
Remove CO2 (18L/day) and water
Liver
Excretes bile pigments (such as bilirubin, biliverdin), vitamins, cholesterols, and drugs along with digestive wastes •Skin
Sweat and sebaceous glands eliminate substances such as sterols, hydrocarbons, wax sebum, etc. [[Page Break]]
Disorders of Excretory System
Uremia
oAccumulation of urea in blood
oMay lead to kidney failure
oUrea is removed by haemodialysis. In acute cases, kidney transplant may be required. •Renal Calculi
oStones or insoluble mass of crystallised salts are formed within the kidney. •Glomerulonephritis
oInflammation of glomeruli of kidney
Treatments for Disorders of Excretory System
For Uremia - Haemodialysis
oBlood drained from the convenient artery is pumped into dialysing unit. Anticoagulant (heparin) is added. oDialysing unit − Cellophane tube surrounded by dialysing fluid oDialysing fluid − Same as the plasma composition except the nitrogenous waste oMolecules move according to concentration gradient. Nitrogenous wastes are absent in dialysing fluid. Therefore, they are transported to the fluid, thereby cleansing the blood. •For Renal Calculi - Kidney Transplantation

oA functioning kidney from the donor is used for transplantation. oDonor − close relative to minimize the immune rejection

o

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