Excertory Product And Their Elimination
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EXCRETORY SYSTEM
Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+, K+, Cl–, phosphate,
sulphate, etc., either by metabolic activities or by other means like excess ingestion. These substances have to
be removed totally or partially.
Excretion :
Removal of mainly nitrogenous substances from the body which are end product of metabolic activity.
OR
The process which is concerned with removal of nitrogenous waste materials (e.g.. urea, uric acid, CO2, Ammonia,
salts, excess water etc.) is termed excretion.
Homeostasis : Maintenance of steady state (Walter Cannon).
Homeostatic mechanism are important for normal life as they maintain condition within a range in which, the
animals metabolic processes can occur.
Osmoregulation :
The regulation of solute movement and hence water movement (which follows solutes by osmosis) is called
osmoregulation. Maintenance of salt water concentration in steady state.
On the basis of osmoregulation, animals are either osmoconformers or osmoregulators.
(1) Osmoconformers :
These animals can not actively control the osmotic condition of their body fluids. Instead of this, they change or
adapt the osmolarity of body fluids according to the osmolarity of the surrounding medium.
Eg. All marine invertebrates and some fresh water invertebrates.
(2) Osmoregulators :
These animals maintain an internal osmolarity different from the surrounding medium in which they inhabit.
Osmoregulator animals must either eliminate excess water if they are in hypotonic medium or they should
continously take in water to compensate for water loss if they are in hypertonic medium.
Due to this the osmoregulator animals have to spend energy
Eg. Most vertebrates, except hag fish
Urea is produced in the liver by urea cycle or ornithine cycle.
ORNITHINE CYCLE :-
It is also termed as the Kreb-Henseleit cycle. In this cycle, 2 molecules of NH3, 1 molecule of CO2,
3 ATP molecules are consumed in urea formation.
Human excretory system consists of :
– A pairs of kidneys.
– A pair of ureters.
– A urinary bladder
– A urethra
LOCATION AND STRUCTURE OF KIDNEYS
Mammalian kidneys are bean shaped, reddish brown coloured with a tough fibrous connective tissue covering
called renal capsule.
Kidneys are located lateraly on either side of vertebral column levels between the last thoracic and third lumber
vertebra close to the dorsal inner wall of the abdominal cavity.
In humans right kidney is at slightly lower level than left kidney.
Dorsal surface of the kidney is attached to the dorsal abdominal wall, so only its ventral surface is covered by
peritoneum, therefore this type of kidney is called retro-peritoneal kidney or extra peritoneal kidney.
Each kidney measures 10-12 cm in length, 5-7 cm in width and 2-3 cm in thickness, weighing about
120-170 gm in an adult. Lateral surfaces of kidney are convex while medial surfaces are concave.
On the concave margins of the kidney longitudinal opening called Hilum (Hilus renalis) is present. Through
this, renal artery and nerve enter while renal vein and ureter leave the kidney.
The Hilum leads to a funnel shaped space called the renal pelvis.
The kidney tissue surrounding the pelvis is arranged in an outer renal cortex and inner renal medulla
The renal medulla forms conical pyramid shaped masses which project into the renal pelvis. These are called as
medullary pyramids or renal pyramids (8 to 12 in humans, while only one pyramid is present in kidney of
rabbit)
The cortex extends in between the medullary pyramids as renal columns called columns of Bertini.
Each kidney has nearly one million complex tubular structures called Nephrons which are the functional units.
These nephrons are arranged in a radiating fashion within the renal pyramids.
Urine produced by each nephron empties into collecting duct.
The collecting duct passes through a papilla into the renal calyx (Pleural - calyces).
The renal calyces drain urine in the central cavity of renal pelvis
POST RENAL URINARY TRACT
URETER
Urine passes from the pelvis into the ureter. Both the ureters open through separate oblique openings into the
urinary bladder. The obliquity of the openings prevent the backflow of urine.
URINARY BLADDER
Externally, the bladder is lined by detrusor muscle, it is involuntary in nature while internally the bladder is lined
by transitional epithelium or urothelium. This epithelium has great capacity to expand so that large volume
of urine can be stored. Opening of urinary bladder is controlled by sphincters made of circular muscles. In
human two sphincters are present. Inner = Internal sphincter (made up of involuntary muscle) Outer = External
sphincter (Voluntary muscle). These normally remain contracted and during micturition these relax to release
URETHRA
Urinary bladder opens into a membranous duct called Urethra.
The urethra leads to end of the penis in males and into the vulva in females. In males the urethra has three
parts, prostatic, membranous & penile urethra respectively. In Females both sphincters are present in membranous
urethra.
PASSAGE OF URINE
Nephron - Collecting duct - Duct of bellini - Papilla - Renal calyx - Renal pelvis - Ureters - Urinary bladder - Urethra
MICTURITION
The process of release of urine is called micturition and the neural mechanism causing it is called micturition
reflex. This reflex is initiated when interoceptors present in the wall of urinary bladder, get stimulated by the
tension created due to stretching of bladder wall as the bladder gradually fills with urine brought into it by the
ureters.
In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on
motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous
relaxation of urethral sphincter causing the release of urine.
STRUCTURE OF NEPHRON
l Nephron is the structural and functional unit of kidney. It is an epithelial tube which is about 3 cm long and
20-60 mm in diameter.
1) Bowman's capsule : At the proximal or closed end the nephron is expanded and curved inwardly to form a
double walled cup shaped Bowman's capsule. Within the Bowman's capsule a network or tuft of capillaries
is present, it is called Glomerulus. It is formed by the afferent arteriole (a fine branch of Renal artery). Blood
from the glomerulus is carried away by an efferent arteriole.
Malpighian body : Glomerulus and its surrounding Bowman's capsule together forms Malpighian body or
Renal corpuscle. It is responsible for first step of urine formation (Filtration).
l The outer wall of Bowmans capsule is composed of flattened squamous cells.
The inner, invaginated wall that lines the concavity of Bowmans capsule is composed of a special type of cells
called Podocytes. Which are arranged in an intricate manner so as to leave some minute spaces called
filtration slits or slit pores.
These cells are actually simple squamous cells and bear finger like projections which are coiled around the
capillaries of glomerulus.
The Bowmans capsule is followed by a short neck part lined by ciliated cuboidal epithelium.
2. Proximal convoluted tubule (PCT) : The epithelial cells of this region are specialised for transport of salts
and other substances from the lumen to the interstitial fluid. It is lined by simple cuboidal brush border epithelium.
The membranes of these cells facing the tubule lumen has numerous microvilli (finger like projections or Brush
Borders) which increase the surface area. Near its basolateral surface, the mitochodria are concentrated, to
allow reabsorption of salts by active transport.
3. Loop of Henle : It starts after the proximal convoluted tubule, It ends before the distal convoluted tubule. This
hairpin like loop has a descending limb, followed by an ascending limb.
(a) Descending limb :
Its upper part - constitutes thick segment
- has the same diameter as PCT
- is also lined by simple cuboidal epithelium
Its lower part - constitutes thin segment
- is lined by flat squamous cells
(b) Ascending limb :
Its upper part - constitutes thick segment
- has the same diameter as DCT
- is also lined by simple cuboidal epithelium
Its lower part - constitutes thin segment
- is lined by flat squamous cells
4. Distal convoluted tubule (DCT) : The ascending limb of Henle's loop merges into distal convoluted
tubule. This is lined by cuboidal epithelial cells.
The DCT of different nephrons open into a straight tube called collecting duct.
Collecting ducts (present in medullary pyramids) are long tubules which traverse through the medulla in the
pyramids. In the papilla of the medullary pyramid, several adjacent collecting ducts converge to open into a
common short and thick duct of Bellini (present in papilla of medulla).
All ducts of Bellini then open at the tip of the papillae into the pelvis.
Renal cortex : The malpighian corpuscle, PCT & DCT of the nephrons are located here.
Renal medulla : Loop of Henle, collecting duct and ducts of Bellini are found in this region.
The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule
called peritubular capillaries. A minute vessel of this network runs parellel to the Henle's loop forming a "U"
shaped Vasa recta.
MECHANISM OF URINE FORMATION
The mechanism of urine formation involves three steps or processes :
(I) Ultrafiltration or Glomerular filtration
(II) Selective tubular reabsorption
(III) Tubular secretion
(I) ULTRAFILTRATION OR GLOMERULAR FILTRATION :
The first step in urine formation is the filtration of blood, which is carried out by the glomerulus.
l This process occurs in the Malpighian corpuscle of the nephron.
l The glomerular capsular membrane (Filtration membrane) through which filtration of blood occur consists
of three layers.
(i) The endothelium of glomerulur blood vessels.
(ii) The epithelium of Bowmans capsule.
(iii) A basement membrane between these two layers.
l The epithelial cells of Bowman capsule called podocytes are arranged in an intricate manner so as to leave
some minute spaces called as filtration slits or slit pores.
l The blood is filtered so finely through these membranes that almost all the constituents of the plasma except
the proteins pass onto the lumen of the Bowmans capsule. Therefore it is considered as a process of ultra
filtration.
l The plasma fluid that filters out from glomerular capillaries is called as glomerular filtrate. It is protein less plasma.
l About 20% of plasma fluid filters out into Bowmans capsule.
l The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). GFR
in a healthy individual is approximately 125 ml/min ie. 180 litres per day.
l On an average 1100-1200 ml of blood is filtered by kidneys per minute (Renal blood flow) which constitute
roughly 20-25% of the blood pumped by each ventricle of the heart in a minute (cardiac output) and of this
blood about 650 ml is the blood plasma (55%). This 650 ml is called Renal plasma flow (RPF). About 20%
of the blood plasma filtered by all nephrons of both kidney in a minute. It is 125 ml which called glomerular
filtration rate (GFR).
The effective filtration pressure that causes ultrafiltration is determined by three pressures : (1) glomerular hydrostatic
pressure, (2) colloid osmotic pressure of blood and (3) capsular hydrostatic pressure.
Glomerular hydrostatic pressure is the blood pressure in glomerular capillaries, due to the difference is
diameter of afferent and efferent arteriole.
It is the main driving force to cause filtration. (it is 60 to 75 mm Hg)
Colloid osmotic pressure is the osmotic pressure created in the blood of glomerular capillaries due to plasma
proteins. It resists the filtration of fluid from the capillaries. (it is 30 to 32 mm Hg)
Capsular hydrostatic pressure is the pressure caused by fluid (filtrate) that reaches into Bowman's capsule
which resists filtration. (It is about 10 to 18 mm Hg)
Net filtration pressure
NFP = GHP – [BCOP + CHP]
= (75 or 60) – (32 + 18) mm of Hg
= 10 to 25 mm Hg.
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