EXCRETION
Excretion: Removal
of metabolic wastes from a cell or an organism is called excretion. The main
excretory products are:
Volatile waste products
like CO2 and water,
Nonvolatile waste products
like ammonia, urea, uric acid and excess of ions such as phosphates, sulphates,
Cl--, Na+ and K+
Nitrogenous waste
products like ammonia, urea, uric acid and amino acid.
Nitrogenous wastes are produced in the following two ways;
1. Deamination
of excess amino acids absorbed from food (exogenous source)
2. Breakdown
of body’s own proteins and nucleic acids(endogenous source)
Need to Deaminate Aminoacids:
Body can store carbohydrates and fats for future use but
cannot store proteins and amino acids. Thus the excess of aminoacids is
deaminated .The remaining organic acid is either used as energy source or
changed to carbohydrate or fats and stored. The accumulation of –NH2
group in body is toxic. It is either used in amination in the synthesis of new
amino acid or excreted immediately.
Human excretory system
Structure of human kidney:
The organs concerned with the formation, storage, and
elimination of urine constitute urinary system
in human. It consists of a pair of kidney, a pair of ureters, a urinary bladder
and urethra.
Kidneys are located in the abdominal cavity, one on either
side of vertebral column just beneath the diaphragm. They are attached to the
dorsal abdominal wall by peritoneum called mesorchium.
Left kidney is placed slightly higher than the right kidney.
External structure of
kidney: Kidneys are chocolate brown, bean-shaped structures about 11.25 cm
long, 5-7.5 cm broad and 2.5 cm thick. The outer margin is convex, while inner
margin is concave having a notch, called hilum,
through which ureters and renal vein come out of kidney and renal artery enters
it. Kidney is enclosed in a tough renal
capsule formed of white fibrous tissue.
Internal structure of
kidney (L.S): A longitudinal section of kidney shows two distinct regions:
(i) Outer dark red layer-renal cortex containing Malpighian corpuscles, and both distal and
proximal convoluted tubules. The cortical region close to medulla is called
Juxtamedullary region.
(ii) Inner pale red zone-the medulla, contains loops of
Henle and collecting tubules.
Medulla is formed of about a dozen of triangular masses called medullary/renal pyramids. The pyramids have a striated appearance. The base or wide margin of each pyramid
faces towards cortex and its narrow apex
or papilla opens into small tube
like structure called calyx minor,
which joins with other calyces to form major
calyx. This in turn joins with
other major calyces to form renal pelvis
that give rise to ureters.The cortex between two adjacent pyramids forms the so
called renal columns or columns of Bertini.
Structure of nephron:
Uriniferous tubule or nephron is
the structural and functional unit of kidney and each kidney contains about
1.25 million nephrons held together by connective tissue. Each nephron has
following parts:
(i) Renal corpuscle
(also called Malpighian corpuscle
after Marcello Malpighi) is composed
of two parts; (a) Bowman’s capsule
is the blind end of nephron, which is like a double-walled cup whose outer wall
is formed of flattened epithelial cells and inner wall of specialized podocyte cells. These cells have
numerous feet like processes called pedicels.
(b) Glomerulus is
the bunch of blood capillaries invested in Bowman’s capsule. The endothelial
cells of glomerular capillaries also have minute pores , called fenestrae.. Blood enters glomerulus
through afferent arteriole and
leaves through efferent arteriole.
The visceral layer of Bowman’s capsule and the endothelial lining of glomerular
capillaries collectively form filtering
membrane to carry out ultra-filtration.
(ii) Renal tubule
is the remaining part of nephron after Bowman’s capsule which is a complicated
long tubule divisible into following three parts.
(a) Proximal
convoluted tubule (PCT): it is the initial convoluted part that lies in
cortex .PCT is lined with a layer of columnar epithelial cells with microvilli.
The major function of PCT is to reabsorb large quantities of sodium, water,
glucose, amino acids, and some other constituents of the tubular filtrate.
(b) Loop of Henle
is the middle U shaped part. It consists of descending limb and an ascending
limb. Each limb has a thick region towards the cortex and a thin region
towards medulla
(c) Distal convoluted
tubule (DCT): It is also highly coiled tubule. Its short terminal part is
straight and is called connecting tubule. It opens in to collecting duct.
(d) Collecting ducts are
larger ducts, each receive the connecting tubules of several nephrons. These
open into larger ducts of Bellini and
then open into Calyces which lead
into renal pelvis.
Two types of nephrons can be identified in kidneys. They are
cortical nephrons and juxtamedullary nephrons.
Cortical nephron does
not go deep into the medulla, and their glomerulus is in the cortex. Their
loop of Henle is short and extends only for a short distance in the medulla.
Juxtamedullary
nephron is going deeper into the medulla, and their glomerulus lies in the
border of cortex and medulla. They have a very long loop of Henle. cortical
nephrons are the most abundant nephrons in the kidney.
Juxtaglomerular Apparatus
The juxtaglomerular apparatus is a specialized
structure formed by the distal convoluted tubule and the glomerular afferent
arteriole. It is located near the vascular pole of the glomerulus and its main
function is to regulate blood pressure and the filtration rate of the
glomerulus. The macula densa is a
collection of specialized epithelial cells in the distal convoluted tubule that
detect sodium concentration of the fluid in the tubule. In response to elevated
sodium, the macula densa cells trigger contraction of the afferent arteriole,
reducing flow of blood to the glomerulus and the glomerular filtration rate.
The juxtaglomerular cells, derived from smooth muscle cells, of the afferent
arteriole secrete renin when blood
pressure in the arteriole falls. Renin increases blood pressure via the renin-angiotensin-aldosterone
system.
Blood vessels of kidneys
Afferent arteriole, a
branch of renal artery forms glomerulus in the cavity of Bowman’s
capsule. An efferent arteriole
leaves the Bowman’s capsule and forms a network of peritubular capillaries around the tubule.
Vasa recta are a series of straight capillaries arise
from the peritubular capillaries. They lie parallel to the loop
of Henle. Peritubular capillaries and vasa recta join to form a renal venule which join together to
form renal vein that opens into inferior vena cava.
The functions performed by nephron are:
(i)
Formation of urine
(ii)
Regulation of fluid balance or maintenance of osmotic
pressure.
(iii)
Regulates electrolyte concentration by selective
tubular reabsorption.
(iv)
Maintains acid base balance.
Formation of urea (Ornithine-arginine cycle)
Formation of urea from ammonia takes place in liver. The
cyclic reactions during urea formation begin with an amino acid, ornithine and urea is produced from Arginine. Hence the reaction is called
as Ornithine-arginine cycle.
Reactions of the urea cycle
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Step
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Reactants
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Products
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Catalyzed by
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Location
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1
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mitochondria
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2
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citrulline + Pi
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Ornithine transcarbamoylase
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mitochondria
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3
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AS Synthetase
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cytosol
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4
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cytosol
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5
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Arg + H2O
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cytosol
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Urine formation (uropoieses):
Following
three steps are involved in urine formation.
(i) Ultrafiltration or
Glomerular filtration: The process
of filtering of all constituents of blood plasma except for colloids like
proteins and lipids into the Bowman’s capsule from the glomerulus is called
glomerular filtration.
Ultrafiltration makes almost all the substances dissolved in
plasma like urea, salts, glucose, creatinine, etc along with water except blood
cells and colloids and certain plasma proteins to filter out into the cavity of
Bowman’s capsule. The filtrate is called nephric
or glomerular filtrate. Glomerular
filtration rate in a normal adult is about 120 ml per minute and average volume
filtered from the plasma into Bowman’s capsule is about 190 litres per
day.
(ii) Tubular or
Selective reabsorption: Ultrafiltration not only filters urea but also
other useful substances like glucose, amino acids, inorganic salts, PO4
ions and vitamin C, etc., into nephric filtrate. Their removal from plasma
along with urea will be harmful to the body.They need to be reabsorbed along
with a major portion of water.
The process of reabsorbing only useful substances from the
nephric filtrate into peritubular blood capillaries is called selective reabsorption.
It is achieved by diffusion,
facilitated diffusion, active transport, cotransport and osmosis.
K+, Ca2+
and Mg2+ are reabsorbed
by diffusion.
K+, Cl-
, HCO3-- ,amino
acids, glucose, fructose etc. are reabsorbed by facilitated diffusion. They are also cotransported with Na+.
Na+ is reabsorbed by active
transport. Water is absorbed by
osmosis.
Reabsorption in PCT
PCT is
specialized for reabsorption. Entire glucose, amino acids and most of the
inorganic ions like Na+, K+
and Cl- and
water are reabsorbed here.
Reabsorption in loop of Henle
Lower thin segment of
Descending limb is permeable to water, hence water is reabsorbed.
Ascending limb is
impermeable to water. Its lower thin segment is permeable to Na+, K+ and
Cl- ions.
Its thick segment
actively transports NaCl from filtrate to interstitial fluid. This results
in high osmolarity of inner medulla of kidney. Due to loss of NaCl, the
filtrate become hypotonic to blood plasma.
Reabsorption in DCT, collecting tubule and collecting duct:
In the distal convoluted tubule, collecting tubule and
collecting duct , Na+ is reabsorbed actively from filtrate by the influence
of Aldosterone. water is reabsorbed
in the terminal part of DCT and the collecting tubule under the action of ADH.
(iii) Tubular
secretion: Those products of body which are not at all required by body and
are rather injurious to health are secreted out by the tubular epithelium in
the tubular filtrate. Creatinine, uric
acid, potassium and hydrogen ions are secreted by the distal convoluted tubules
and collecting ducts. The tubular secretion results in shift of K+
and H+ ions from the epithelial cells to renal filtrate in exchange
of Na+ ions present in renal filtrate. This shift causes the renal
loss of H+ ions from blood and greatly helps in acid-base balance as well as
electrolyte balance of the body.
Regulation of urine formation:
8. Briefly describe the endocrine
regulation of urine formation.
Ans. Different
hormones play important roles in endocrine regulation of urine formation.
(i)
Antidiuretic hormone (ADH) or vasopressin
released from the posterior pituitary increases permeability of distal convoluted
tubule and collecting tubule to water and hence concentrated urine is formed.
(ii)
Angiotensin
is formed from angiotensinogen in liver under influence of renin produced by Juxtaglomerular
apparatus (JGA) in kidneys. Angiotensinogen II working as a potent
vasoconstricting hormone. It (a)constrict efferent arterioles to increase
glomerular blood pressure (b) stimulate PCT to absorb more NaCl and water,
and (c) stimulate adrenal gland to
stimulate
Aldosterone.
(iii)
Aldosterone
promotes water and NaCl absorption from DCT.
(iv)
Atrial
Natriuretic Factor(ANF), a peptide hormone secreted by atrial of heart
inhibit the release renin from JGA.
Besides urine formation kidney also helps in osmoregulation.
Osmoregulation is the control of
electrolyte and organic solute concentrations in the body fluids, and the
maintenance of water balance and fluid volume. Hober (1902) coined the term ‘Osmoregulation’
for the collective activity of a variety of mechanisms used by organisms to
control water movement and water volumes. Kidneys regulate the concentration of
blood by regulating the amount of water in urine. If the water intake is less, hypertonic
urine is excreted to conserve water, while there is more water intake then the
urine become hypotonic in order to remove excess water.
Counter-current mechanism:
Mammals including human beings produce concentrated urine.
The urine is concentrated in Henle’s
loop and vasa recta. In the two
limbs of Henle’s loop, filtrate moves in
opposite directions. This forms a counter current. Similarly, the flow of the
blood through limbs of vasa recta is in a counter current pattern. This system
of flowing liquid in two limb in opposite direction is termed as Counter-current mechanism. It helps in
maintaining high solute concentration in renal medulla.
Henle’s loop: During
the passage of fluid through the ascending limb, sodium ions leave the tubular
fluid and re-enter the interstitial ( pertitubular) fluid. The increased
concentration of electrolytes in interstitial fluid increases its osmolarity
and draws out water from descending limb by osmosis.
Descending limb is permeable to both water and passive
diffusion of NaCl. So water drawn in the interstitial fluid quickly enters vasa
recta to increase blood volume. Henle’s loop is largely responsible for
concentrating urine. It is found that greater the ability of the animal to
excrete hypertonic urine, longer are the Henle’s loops in its kidneys.
vasa rectae: Blood
flows slowly through vasa rectae and allows sufficient time for the diffusion
of water and solutes from filtrate into blood.
The counter current mechanism in vasa rectae: (1)prevents
loss of Na+ and Cl- ions from renal medulla and (ii)
helps to maintain concentration gradient in renal medulla, thus assist loop of
Henle in concentrating urine.
Q.Why is the urine excreted during summer months hypertonic?
Ans: During
summer much water is lost as sweat and to conserve water and maintain osmotic
concentration of body fluid more of it is reabsorbed in nephron thus making
urine to be hypertonic.
10. Mammals can excrete hypotonic or hypertonic urine
according to the need of the body. Give reason.
Ans. Mammals have
renal tubules that can increase or decrease the reabsorption of water and other
materials from glomerular filtrate as per body’s need and hence can excrete
hypotonic or hypertonic urine according to the need of the body.
11. Why do terrestrial animals excrete hypertonic urine?
Ans. The
terrestrial animals excrete hypertonic urine in order to conserve water since
they do not get plenty of water. In human it is done by means of
counter-current mechanism in nephrons.
Role of skin and lungs in excretion:
Skin and lungs are accessory excretory organs. Skin has
sweat glands and sebaceous glands which play an excretory role to some extent.
Sweat from sweat glands is an aqueous fluid containing in solution sodium chloride,
lactic acid, urea, amino acids and glucose. Similarly, sebum from sebaceous
glands eliminates sterols, fatty acids and some other hydrocarbons.
Lungs mainly excrete out volatile wastes of respiration like
carbondioxide and water in the form of vapour.
Liver plays an important role in the excretion in mammals as
it converts ammonia to urea by the ornithine
cycle.
Micturition: The
process of release of urine is called micturition and the neural mechanisms
causing it are called micturition
reflex.
Diuretic substances:
Substance like tea, coffee,and alcohol increase urine output. These are called
Diuretic substances.
Diabetes mellitus vs diabetes insipidus:
Functions of Kidneys
1. Excretion
of nitrogenous wastes.
2. Maintenance
of osmotic pressure
3. Regulation
of electrolytes and salt concentration.
4. Osmoregulation
and homeostasis.
5. Regulation
of pH.
6. Regulation
of blood pressure.
7. Secretion
of Renin.
8. Production
of erythropoietin (help in formation of RBC).
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