Definition:
Removal of nitrogenous non gaseous wastes like ammonia,
urea, uric acid etc along with excess of water salts and pigments out of body
is called excretion.
The main of excretion is to maintain homeostasis.
Need:
Various types of wastes are produced by the living systems
during the metabolic processes. Some of the major wastes are:
- Nitrogenous wastes such as ammonia, urea, uric acid etc due to protein digestion
- CO2 from respiratory processes
- Water from respiratory processes
- Ions (Na+, K+ Cl- phosphates, sulphates etc) from excess intake or metabolism
- Pigments due to metabolism of haemoglobin from dead RBCs
It is important to remove these wastes completely or
partially.
The removal of nitrogenous wastes is stressed more since
these are toxic for the living systems.
Types of Excretion:
Nitrogenous wastes can be excreted as ammonia, urea or uric
acid. On this basis, there can be three methods of excretion.
- Ammonotelism:
- Nitrogenous waste excreted as ammonia
- Such animals known as ammonotelic.
- Eg. Aquatic animals such as bony fishes, aquatic amphibians, aquatic insects. Primitive organisms such as protozoans, coelenterates.
- Ammonia is formed as a digestive product of proteins in the liver.
- It is highly toxic due to its high pH.
- It is highly soluble in water so large amount of water is needed for its elimination leading to loss of 300-500ml water per gram ammonia.
- Thus there is a correlation between ammonotelism and aquatic life.
- Ammonia is excreted by diffusion across body surfaces or through gill surfaces as ammonium ions.
- No important role of kidneys.
- Ureotelism:
- Main nitrogenous waste is urea.
- Such animals known as ureotelic
- Eg. Land animals that can afford to lose large amounts of water such as terrestrial amphibians (frogs) OR animals that can maintain higher concentrations of urea. Also marine fishes, mammals (including humans).
- Urea is used in the latter category for maintaining osmolarity.
- In humans, 80-90% of total nitrogenous waste is ammonia.
- Ammonia formed during metabolism is converted into urea in liver by ammonia cycle.
- It is released in to blood, undergoes filtration and is excreted through kidneys.
- Some urea is retained in the kidney matrix to maintain blood osmolarity.
- Urea has lesser toxicity then ammonia and requires for excretion only 50ml water per gram urea.
- Thus ureotelism enables conservation of water and hence is associated with terrestrial mode of life.
Significance of ureotelism over
ammonotelism:
- Urea is 100,000 times less toxic than ammonia
- it can be retained for longer duration without harm
- needs less water for elimination
- Uricotelism:
- Elimination of nitrogenous wastes as uric acid.
- Such animals are known as uricotelic.
- Eg. Land reptiles (lizards and snakes), birds, land snails and insects.
- Uric acid is formed from ammonia and purines in liver. In insects it is formed in malpigian tubules.
- Uric acid eliminated as a pellet or paste with minimum water loss
Significance of uricotelism
Uric acid is least soluble form of
nitrogenous waste. Only 10ml of water is required for expulsion of 1 gram of
uric acid. It is also the least toxic hence can be retained in body for longer
duration without causing harm. So it is highly advantageous method of excretion
for animals that have limited access to water. It is highly suitable adaptation
for terrestrial mode of life.
S No.
|
Animal Group
|
Main form of nitrogenous waste
|
Excretory system
|
1.
|
Protozoa, Porifera & Coelentrata
|
Ammonia
|
Diffusion through cell membrane
|
2.
|
Platyhelminthes
|
Ammonia, fatty acids
|
Body surface, protonephridia (or flame cells)
|
3.
|
Nematoda
|
Ammonia
|
Body surface & Renett cells
|
4.
|
Annelida
|
Ammonia, urea in land forms
|
Nephridia
|
5.
|
Arachnida
|
Guanine
|
Coxal glanda, malpighian tubules
|
6.
|
Crustacea
|
Ammonia
|
Antennary or green glands
|
7.
|
Insecta
|
Uric acid in land, ammonia in aquatic forms
|
Malpighian tubules
|
8.
|
Mollusca
|
Uric acid in land, ammonia in aquatic forms
|
Kidneys (metanephric system)
|
9.
|
Echinodermata
|
Ammonia
|
Papulae, podia
|
10.
|
Vertebrata
|
Ammonia, urea, uric acid
|
Kidneys
|
HUMAN EXCRETORY SYSTEM
Comprises
of:
- A pair of kidneys
- A pair of ureters
- Urinary bladder
- Urethra
Figure 19.1
- Kidneys: Primary or major respiratory organs.
- Reddish brown
- Bean shaped
- Large size: length 10-12cm, width 5-7 cm, thickness 2-3 cm. weight 120-170 gm.
- Present in upper part of abdominal cavity, close to dorsal inner wall of abdominal cavity.
- Located between levels of last thoracic and second lumber vertebra, one on each side of vertebral column.
Figure 19.2
The parts of kidney include
- Hilum or the notch at the centre of the inner concave surface of kidney. Ureter, blood vessels and nerves enter kidney through hilum.
- Renal Pelvis: broad funnel shaped space present inner to hilum. It has projections known as Calyces (singular calyx).
- Capsule or the outer tough layer of the kidney.
There internal organization of
the kidney comprises into two parts:
a) Outer
Cortex
b) Inner
Medulla
Medulla is divided in to two conical masses or medullary pyramids that project in to
calyces.
In between the medullary pyramids, portions of cortex form
renal columns known as Columns of
Bertini.
Nephrons: Functional
units of kidney.
The kidney is made of almost 1 million complex tubular
structures called nephrons.
Function:
To form urine
- Ureters: A pair of long narrow tubular structures, each arising from hilum of kidney.
Function: By peristalsis conduct urine from kidneys
to urinary bladder.
- Urinary Bladder: Large thin walled pear shaped sac present in the pelvis region of abdomen.
·
The upper broader part of the bladder is known
as body of bladder and it stores urine.
·
The lower narrow par known as neck of bladder is
the site of origin of urethra. It is guarded by two sphincters.
- Urethra: muscular and tubular structure extending from neck of bladder to outside. It is responsible for the discharge of urine.
Structure
of Nephron
Structural and functional units of kidneys
Figure 19.3
The nephron can be divided in to two parts:
1.
Glomerulus
2.
Renal Tubules
The glomerulus is a cluster of capillaries formed by the
afferent arteriole, which is a smaller branch of renal artery. Blood from
glomerulus is carried away by efferent arteriole.
The renal tubule comprises of:
·
Bowman’s
capsule: a double walled cup like structure, enclosing the glomerulus.
Glomerulus along with Bowman’s capsule is known as Malpigian Body or Renal
Corpuscle.
·
Proximal
Convoluted Tubule (PCT): The Bowman’s capsule proceeds in to the PCT.
·
Henle’s Loop: The PCT next forms the Henle’s loop which has a
descending limb and an ascending limb.
·
Distal
Convoluted Tubule (DCT): The ascending limb of Henle’s loop forms a highly
coiled tubular region called DCT.
The Malpighian body, along with PCT and DCT are in the
cortex of kidney, whereas the Henle’s loop is in the medulla.
On the basis of length of Henle’s loop, the nephrons can be
of two types:
- Cortical nephrons: In about 85% of nephrons, the Henle’s loop is short and extends very little in the medulla.
- Juxta-Medullary Nephron: In about 15% of the nephrons the loop of Henle is longer and runs deep in to the medulla.
Renal Blood Supply
Each kidney receives blood supple from the renal artery
branching from abdominal aorta. Within the kidney, the renal artery divides in
to many afferent arterioles. One afferent arteriole enters a Bowman’s
capsule and divides in to a tuft of capillaries known as glomerulus.
From glomerulus the blood is drained by efferent
arterioles. The efferent arterioles break up in to peritubular network of
capillaries. The peritubular capillaries join and form the renal vein.
Abdominal aorta→Renal Artery→Afferent
artery→glomerulus→efferent artery→peritubular network→renal vein
The efferent arteriole also forms U shaped blood capillaries
close to loop of Henle, known as Vasa recta. Vasa rectae help in counter current mechanism of urine concentration.
URINE FORMATION (UROPOEISIS)
All the body cells produce
nitrogenous wastes that are carried by blood to the kidney. Inside the kidney
they are converted to urine by three processes:
- Glomerular Filtration or Ultrafiltration
- Reabsorption
- Secretion
- Glomerular Filtration or Ultrafiltration:
- Filtration of blood carried out by glomerulus.
- It is completely passive and non selective process.
- 1100 – 1200 ml of blood filtered by kidneys per minute. This is 1/5 of the total volume of blood pumped by each ventricle in a minute.
- The blood pressure in capillaries of glomerulus is twice that in other capillaries since the efferent arteriole is much narrower than the afferent arteriole.
- At a pressure of 60mm of Hg the blood undergoes ultrafiltration through three layers:
ü
Glomerular capillary endotheliu m with numerous
pores known as fenestrae
ü
The epithelium of the Bowman’s capsule which is
formed of two layers of cells: the outer or the parietal layer and the
inner or visceral layer. The visceral layer is formed of special cells known
as podocytes or foot cells because of their feet like structures. The
feet like structures are known as pedicels. They have millions of minute
pores known as slit pores that allow the ultrafiltrate to pass through.
ü
The basement membrane between these two layers.
·
The above three membranes together form the
filtering or dialyzing membrane that acts as an ultrafilter and is responsible
for ultrafiltration.
·
The difference between the pressure at which the
blood enters the glomerulus and the pressure that resist it, is known as
glomerular filtration pressure (GFP).
·
The GFP causes filtration of:
ü
Water (about 70 litres a day)
ü
Small soluble molecules such as glucose, amino
acids, vitamin C, Na+
ü
Harmful substances such as urea, uric acid,
creatinine, ammonium salts, pigments, K+ etc.
·
Blood corpuscles, proteins, fats etc. are not
filtered out.
·
The filtrate obtained after glomerular
filtration is known as glomerular or capsular filtrate, nephric filtrate,
ultrafiltrate, or primary urine. It is isotonic to blood plasma.
Ultrafiltrate = Blood – cells – proteins
·
The amount of filtrate formed by the kidneys per
minute is called glomerular filtration rate (GFR). GFR in a healthy individual
is approximately 125ml/min or 180 litres/day.
Autoregulation of Ultrafiltration: The kidneys have built-in
mechanism of regulation of GFR. The GFR is regulated by 3 modes:
- GFP: GFR is directly proportional to GFP
- Juxtaglomerular Apparatus (JGA): it is a special sensitive region formed by cellular modifications in distal convoluted tubule and efferent arteriole at the site of their contact.
The cells of JGA secrete an
enzyme Renin that regulates blood pressure, renal blood flow, and rate
of Ultrafiltration. A fall in GFR activates the JGA cells to release more
rennin causing increased blood flow and raising GFR.
- Nervous Control: Sympathetic nerve fibres of ANS stimulate vasoconstriction of renal arteries which decreases renal blood flow and GFR.
Selective Reabsorption:
The primary urine or ultrafiltrate produced after Ultrafiltration is subjected
to reabsorption. The volume of ultrafiltrate produced in the body is 180 l/day,
while the amount of urine produced is 1.5 litres. Thus approximately 99% of the
ultrafiltrate is reabsorbed by selective reabsorption.
The ultrafiltrate passes through
renal tubules by ciliary action. As it passes, the following substances are
reabsorbed:
- 99% of water
- whole of glucose
- amino acids
- most of Na+ and Cl-
- some urea and uric acid
Two mechanisms are involved in selective reabsorption:
- Back diffusion or passive reabsorption: It is a passive process and occurs along the concentration gradient. It is not energy dependent and is therefore a slow process.
- Water and nitrogenous wastes are reabsorbed by this mechanism.
- Urea is reabsorbed mainly in PCT.
- Quantity of water reabsorbed depends on the body needs and environmental temperature.
- Active reabsorption: It is an energy requiring mechanism that occurs against the concentration gradient. Rapid process.
Glucose, amino acids, ions etc
are reabsorbed by this mechanism.
Tubular Secretion:
Highly selective process.
·
Glandular cells of nephrons present in PCT and
DCT, extract certain molecules from the peritubular capillaries and return them
to ultrafiltrate.
·
This process involves active transport.
·
The substances secreted include uric acid,
creatinine, K+, H+, ammonia etc.
·
Tubular secretion helps in maintenance of ionic
and acid balance of body fluids.
FUNCTIONS OF THE TUBULES
Proximal Convoluted Tubules:
- PCT is lined by simple cuboidal brush border epithelium that increases the surface area for absorption.
- Almost all the essential nutrients and 70-80% of water and electrolytes are reabsorbed.
- Tubular secretion is also performed here. PCT secretes ions, and ammonia in to primary urine. This helps in maintaining ionic and pH balance of the filtrate.
- This helps in maintaining ionic and pH balance of the filtrate.
Henle’s Loop:
- Minimal reabsorption
- It plays significant role in maintaining high osmolarity of medullary interstitial fluid.
- Comprises of two regions:
ü
Descending Limb: Permeable to water but almost
impermeable to electrolytes. Concentrates filtrate.
ü
Ascending Limb: impermeable to water but allows
both active and passive transport of electrolytes.
Thus as concentrated fluid passes
upward it gets diluted due to passage of electrolytes in to the medullary
fluid.
Distal convoluted tubule:
·
Involves conditional reabsorption of Na+ and
water.
·
Reabsorption of HCO3- ions.
·
Selective secretion of H and K ions; and ammonia
to maintain pH and sodium potassium balance in blood.
Collecting Duct:
·
Extends from cortex of kidney to medulla.
·
Reabsorption of large amounts of water to
produce concentrated urine
·
Allows passage of small amounts of urea in to
medullary interstium to maintain osmolarity.
·
Maintains pH and ionic balance by selective
secretion of K and H ions.
Mechanism of concentration of
filtrate:
A counter current mechanism is
used for production of concentrated urine.
2 parts involved:
·
Henle’s loop
·
Vasa recta
Flow of urine in ascending and
descending limbs of Henle’s loop is in opposite direction. This forms a counter
current.
Flow of blood in two limbs of vasa
recta is also in opposite direction; forming a counter current.
The region between cortex and
inner medulla medullary interstitium develops an osmolarity
gradient from 300 in cortex to 1200 in inner medulla. Two factors are responsible
for this gradient:
·
Counter current mechanism
·
Proximity of Henle’s loop and vasa recta
The two compounds responsible for
the osmotic gradient are:
·
Urea and
·
NaCl
These two mechanisms help in
maintenance of concentration gradient of medullary interstitium. This
interstitial gradient is helpful in easy passage of water from collecting
tubule. Thus primary urine is concentrated to almost 4 times to form urine.
Regulation of Kidney
Function:
Feedback mechanism involving:
·
Hypothalamus (Brain)
·
JGA
·
Heart
Hypothalamus: Our body
contains osmoreceptors that are activated by changes in blood volume, body
fluid volume and ionic concentration.
During situation of fluid loss
these receptors are activated.
They stimulate Hypothalamus
The hypothalamus causes pituitary
gland to release ADH (antidiuretic hormone) or vasopressin from its neurohypophysis.
ADH effects kidney function by two methods:
1.
It causes water reabsorption from the latter part of
renal tubule.
This leads to rise in body fluid
volume. And the osmoreceptors are switched off; ADH release is thus suppressed.
This is how feedback mechanism is
completed.
2.
It causes constriction of blood vessels; leading to
rise in blood pressure.
Higher blood pressure leads to
rise in glomerular blood flow and thus increase in GFR
JGA: Complex regulatory
role: Renin Angiostenin mechanism
·
Fall in glomerular blood flow
·
Fall in glomerular blood pressure
·
Fall in GFR
·
JGA cell activated to release Renin
·
Renin converts angiotensinogen in blood to
angiotensin I and then to angiotensin II.
·
Angiotensin II is a powerful vasoconstrictor.
It increases blood flow, blood pressure and thus GFR.
·
Angiotensin II also causes release of
aldosterone from adrenal cortex
·
Aldosterone causes reabsorption of water and Na+
ions from DCT
Renin
Angiostenin mechanism
HEART: rise in blood flow
to atria of heart leads to release of ANF (atrial Natriuretic Factor). This
causes vasodilation, thus reducing blood pressure and GFR.
This mechanism acts as a check on
Renin Angiostenin mechanism.
Micturition
Process of release of urine
Control by neural mechanisms.
Neural mechanisms causing micturition are called micturition reflex.
Process
·
Urine formed in nephrons carried to urinary
bladder by ureter
·
Stored in urinary bladder till a voluntary
signal given by CNS – Micturition Reflex (figure below)
·
The signal is initiated by stretching of urinary
bladder as it gets filled with urine
·
The effector are smooth muscles of bladder
·
The smooth muscles contract along with
simultaneous relaxation of urinary sphincter
·
This causes release of urine
Characteristics of human urine:
·
1 to 1.5 litres /day (with 25-30 gms urea)
·
Light yellow
·
Watery
·
pH slightly acidic (6.0)
·
Various conditions effect urine characteristics
and so urine is used for clinical diagnosis of many metabolic disorders. E.g.
in diabetes mellitus urine shows Glucose (Glycosuria) and ketone bodies
(ketonuria)
Accessory organs of
excretion
1.
Lung: CO2 (upto 18 litres/ day)
and water.
2.
Liver: Acts as an excretory organ. Toxins,
drugs and alcohol are broken down in the liver for excretion. Bile pigmnets;
bilirubin, bilverdin also pass out with digestive wastes and urine.
3.
Skin: sweat and sebaceous glands in skin
help in excretion
Sweat contains watery fluid with
NaCl, urea, lactic acid etc which are eliminated from the body.
Sebaceous glands excrete sterols,
hydrocarbons and waxes along with sebum.
Disorders of excretory
system
