Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid etc. from the tissues. It is 3 types:
1. Ammonotelism: Process of excretion of NH3.
Ammonotelic animals: Aquatic invertebrates, bony fishes, aquatic amphibians, tadpoles, aquatic insects etc.
NH3 is highly toxic. So excretion needs excess of water.
NH3 is highly toxic. So excretion needs excess of water.
2. Ureotelism: Process of excretion of urea.
In liver, NH3 is converted into less toxic urea (for conservation of water). This is called Ornithine cycle. For excretion urea requires only moderate quantity of water.
Ureotelic animals: Terrestrial & semi-aquatic amphibians (frogs, toads etc), cartilaginous fishes, aquatic or semi-aquatic reptiles (alligators, turtles) etc.
3. Uricotelism: Process of excretion of uric acid. It is insoluble in water. So water is not required for excretion.
Uricotelic animals: Birds, terrestrial reptiles, insects, land snails and some land crustaceans.
Some excretory organs in animals
Protonephridia (flame cells)
Flatworms, rotifers, some annelids & cephalochordate.
Antennal or green glands
HUMAN EXCRETORY SYSTEM
It includes a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
Structure of Kidney
- Reddish brown, bean-shaped structures situated between the levels of last thoracic and 3rd lumbar vertebra.
- 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness. Average weight: 120-170 gm.
- Enclosed in a tough, 3-layered fibrous capsule.
- On the concave side of kidney, there is an opening (hilum or hilus) through which blood vessels, nerves, lymphatic ducts and ureter enter the kidney.
- Hilum leads to funnel shaped cavity called renal pelvis with projections called calyces.
- Each kidney has outer cortex and inner medulla.
- Medulla has few conical projections called renal pyramids (medullary pyramids) projecting into the calyces.
- Cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
- Each kidney has nearly one million tubular nephrons.
- Nephrons are the structural & functional units of kidney.
- Each nephron has 2 parts: Glomerulus and Renal tubule.
o Glomerulus: A tuft of capillaries formed by afferent arteriole (a fine branch of renal artery). Blood from the glomerulus is carried away by an efferent arteriole.
o Renal tubule: It begins with a double walled cup-like Bowman’s capsule, which encloses the glomerulus.
- Glomerulus + Bowman’s capsule = Malpighian body.
- The tubule continues with proximal convoluted tubule (PCT), Henle’s loop and distal convoluted tubule (DCT).
- Henle’s loop is hairpin-shaped. It has descending and ascending limbs.
- The DCTs of many nephrons open into collecting duct. Collecting duct extends from cortex to inner parts of medulla. They converge and open into the renal pelvis through medullary pyramids in the calyces.
- Malpighian body (Renal corpuscle), PCT and DCT are situated in renal cortex. Loop of Henle dips into medulla.
- The efferent arteriole emerging from glomerulus forms a fine capillary network (peritubular capillaries) around the renal tubule. A minute vessel of this network runs parallel to the Henle’s loop forming a ‘U’ shaped vasa recta.
- Nephrons are 2 types:
o Cortical nephrons (85%): In this, the Henle’s loop is short and extends only very little into the medulla. Vasa recta is absent or highly reduced.
o Juxtamedullary nephrons (15%): In this, Henle’s loop is long and runs deep into medulla. Vasa recta present.
URINE FORMATION (PHYSIOLOGY OF KIDNEY)
3 processes: Glomerular filtration, reabsorption & secretion.
1. Glomerular filtration (ultra filtration)
- The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e. endothelium of glomerular blood vessels, epithelium of Bowman’s capsule and a basement membrane between these 2 layers.
- The epithelial cells (podocytes) of the Bowman’s capsule are arranged in an intricate manner so as to leave some minute spaces called filtration slits (slit pores).
- Almost all constituents of the blood plasma except the proteins pass onto the lumen of the Bowman’s capsule.
- About 1100-1200 ml of blood is filtered by the kidneys per minute which constitute 1/5th of the blood pumped out by each ventricle of the heart in a minute.
- The amount of the filtrate formed per minute is called Glomerular filtration rate (GFR).
- Normal GFR = 125 ml/minute, i.e., 180 litres/day.
- Even though 180 litres of glomerular filtrate is produced daily, about 99% of this is reabsorbed by the renal tubules.
So normal volume of urine released is 1.5 litres.
- Substances like glucose, amino acids, Na+, etc in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport.
- Passive reabsorption of water occurs in the initial segments of the nephron.
- PCT reabsorbs most of the nutrients, and 70-80% of electrolytes & water. Simple cuboidal brush border epithelium of PCT increases surface area for reabsorption.
- In loop of Henle, minimum reabsorption takes place. It maintains high osmolarity of medullary interstitial fluid.
The descending limb is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate.
The ascending limb is impermeable to water but allows transport of electrolytes. So, the filtrate gets diluted.
- In DCT, conditional reabsorption of Na+ & water takes place.
- Collecting duct extends from cortex to inner parts of medulla. It reabsorbs large amount of water to concentrate urine. It also allows passage of small amounts of urea into medullary interstitium to keep up the osmolarity.
3. Tubular Secretion
- Cells of PCT & DCT maintain ionic (Na-K balance) and acid-base balance (pH) of body fluids by selective secretion of H+, K+ & NH3 into the filtrate and absorption of HCO3- from it.
- Collecting duct maintain pH and ionic balance of blood by the secretion of H+ and K+ ions.
Mechanism of the concentration of the filtrate
- Henle’s loop & vasa recta help to concentrate the urine.
- The flow of filtrate in the 2 limbs of Henle’s loop and the flow of blood through the 2 limbs of vasa recta are in opposite directions (i.e. in a counter current pattern).
- The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help to maintain an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL-1 in the cortex to about 1200 mOsmolL-1 in the inner medulla.
- This gradient is mainly caused by NaCl and urea.
- NaCl is transported by ascending limb of Henle’s loop that is exchanged with descending limb of vasa recta. NaCl is returned to interstitium by ascending limb of vasa recta.
- Similarly, small amount of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.
- The above described transport of substances facilitated by Henle’s loop and vasa recta is called Counter current mechanism. This helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from collecting tubule thereby concentrating the filtrate (urine).
- Human kidneys produce urine four times concentrated than the initial filtrate formed.
REGULATION OF THE KIDNEY FUNCTION
- It is done by hormonal feedback mechanisms involving the hypothalamus, JGA and the heart.
- Changes in blood volume, body fluid volume and ionic concentration activate Osmoreceptors in the body.
1. Regulation by ADH (vasopressin)
- When body fluid level decreases, the osmoreceptors stimulate the hypothalamus to release antidiuretic hormone (ADH). ADH prevents diuresis by facilitating water reabsorption from DCT and collecting duct.
- An increase in fluid volume switches off the osmoreceptors and suppresses the ADH release to complete the feedback.
- ADH constricts blood vessels resulting in an increase of BP. This increases the glomerular blood flow and GFR.
2. Regulation by JGA (Renin-Angiotensin mechanism)
- There is a special sensitive region called juxta glomerular apparatus (JGA) formed by cellular modification of DCT and the afferent arteriole at the location of their contact. JGA regulates the GFR.
- A fall in glomerular blood flow/glomerular blood pressure/GFR activates the JG cells to release renin.
- Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II (a vasoconstrictor). Angiotensin II increases glomerular blood pressure and thereby GFR. It also activates adrenal cortex to release Aldosterone.
- Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR.
3. Regulation by ANF
- ANF check on the renin- angiotensin mechanism.
- An increase in blood flow to the atria of the heart causes the release of Atrial Natriuretic Factor (ANF).
- ANF causes vasodilation (dilation of blood vessels) and thereby decreases the blood pressure.
- The gradual filling of urinary bladder causes its stretching. As a result, the stretch receptors on its wall send impulses to CNS. The CNS passes on motor messages. It causes the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing micturition (release of urine). The neural mechanism causing micturition is called the micturition reflex.
- An adult human excretes 1 to 1.5 litres of urine (25-30 gm urea) per day.
- Urine is a light yellow coloured watery fluid, slightly acidic (pH-6.0) and has a characteristic odour. Various conditions can affect the characteristics of urine.
- Analysis of urine helps in clinical diagnosis of many metabolic disorders and malfunctioning of the kidney. E.g. Glycosuria (presence of glucose) and Ketonuria (ketone bodies) in urine indicates diabetes mellitus.
Role of Lungs, liver & skin in Excretion
¨ Lungs: Remove CO2 (18 litres/day) and water.
¨ Liver: Secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of them pass out along with digestive wastes.
¨ Skin (Sweat & sebaceous glands): Sweat contains water, NaCl, small amounts of urea, lactic acid, etc. Primary function of sweat is to give a cooling effect on body surface. Sebaceous glands eliminate substances like sterols, hydrocarbons and waxes through sebum. Sebum provides a protective oily covering for the skin.
¨ Saliva eliminates small amounts of nitrogenous wastes.
DISORDERS OF EXCRETORY SYSTEM
· Uremia: Accumulation of urea in blood which may lead to kidney failure.
· Renal calculi: Stone or insoluble mass of crystallized salts (oxalates, etc.) formed within the kidney.
· Glomerulonephritits: Inflammation of glomeruli.
- In patients with uremia, urea is removed by hemodialysis.
- The dialyzing unit contains a coiled cellophane tube surrounded by dialyzing fluid having same composition of plasma except the nitrogenous wastes.
- Blood drained from a convenient artery is pumped into dialyzing unit after adding an anticoagulant like heparin.
- The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient.
- As nitrogenous wastes are absent in dialyzing fluid, these substances freely move out, thereby clearing the blood.
- The cleared blood is pumped back to the body through a vein after adding anti-heparin to it.
- It is the ultimate method in the correction of acute renal failures. A functioning kidney is taken from a donor.
- It is better to receive kidney from a close relative to minimize chances of rejection by immune system of host.