Army Public School Gopalpur: Class 11 Science Subject - Biology

ARMY PUBLIC SCHOOL GOPALPUR
CLASS 11 SCIENCE SUBJECT - BIOLOGY

SESSION-2020-2021
Chapter -19 Excretory Products and their Elimination

NOTES
Excretion is the removal of nitrogenous waste products and other metabolites from the
animal body which is normally associated with the process of maintenance of osmotic
concentrations, i.e., osmoregulation within the body.
Both excretion and osmoregulation are important for the maintenance of homeostasis, i.e.,
for keeping the internal environment of the body constant that is necessary for normal life
processes.
Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by
animals. These substances get accumulated in the animal body either by metabolic
activities or by other means like excess ingestion.
Types of Nitrogenous Excretion

Depending upon the nature of excretory product, animals exhibit different processes of
nitrogenous excretion.
These are described as follows
(i) Ammonotelism Ammonia is the most toxic form of nitrogenous waste, it requires large
amount of water for its elimination. The organism that excrete ammonia are called
ammonotelic and this , process to eliminate ammonia is known as ammonotelism.
Examples of ammonotelic animals are Many bony fishes, aquatic amphibians and aquatic
insects. Ammonia, as it is readily soluble, is generally excreted by diffusion across body
surfaces or through gill surfaces (in fish) as ammonium ions.
Kidneys does not play any significant role in its removal.
(ii) Ureotelism The process of excreting urea is called ureotelism. Animals, which does not
live in high abundance of ‘water convert ammonia produced in the body into urea (in the
liver) and release into the blood, which is filtered and excreted out by the kidneys.
Examples of ureotelic animals are Mammals, many terrestrial amphibians and marine
fishes.
(iii) Uricotelism The process of excreting uric acid is called uricotelism. Uric acid, being the
least toxic nitrogenous waste can be removed with a minimum loss of water from the
animal body.
Thus, it is excreted in the form of pellet or paste (i.e., semi-solid form). Normally, the
animals which live in desert exhibit uricotelism.
Examples of uricotelic animals are Reptiles, birds, land snails and insects.
Note:
Some animals perform dual excretion, i.e., two modes of excretion. For example,
Earthworms excrete ammonia when sufficient water is available, while it excretes urea in
drier surroundings.
Other examples are lung fishes, Xenopus, crocodiles, etc.
Excretory Organs
Different animal groups have a variety of excretory structures (organs) to perform the
process of excretion. In most of the invertebrates, these structures are simple tubular form,
whereas, vertebrates have complex tubular organs called kidneys.
Some of these structures are mentioned below in the given table
Excretory Organs and Main Nitrogenous Wastes of Different Animal Croups
Human Excretory System
Human excretory system consists of a pair of kidneys, a pair of ureters, urinary bladder and
urethra, these are described below in detail
1. Kidneys
These are reddish brown, bean-shaped structures situated between the levels of last
thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. –
Kidneys are mesodermal in origin as they developed from mesodermal nephrostomes or
mesomeres (ciliated structures, functional in embryonic conditions).
Position of Kidneys
The kidneys are located below the diaphragm on the left and right sides. The right kidney is
lower and smaller than the left kidney because the liver takes up much space of the right
side.
Note:
Each kidney of an adult human measures. 10-12 cm in length, 5-7 cm in width, 2-3 cm in
thickness with an average weight of 120-170 gm (i.e., 150 gm in males and about 135 gm in
females).
Structure of Kidney
Structure of kidney can be studied well under two heads, i.e., external as well as internal
structure.
These are described below as
The outer surface of each kidney is convex and inner surface is concave, where it has a
notch called hilum, through, which the supply of blood occurs, i.e., renal artery and renal
vein, pass in and out of the kidneys along with the ureter and the nerve supply of kidney.
If we look from outside to inside, three layers cover the kidneys, i.e., renal fascia
(outermost), the adipose layer and then renal capsule (innermost layer). These coverings
protect the kidneys from external shocks and injuries.
The LS of a mammalian kidney seems to have of an outer cortex and inner medulla.
Inside the kidney, the ureter is expanded as a funnel-shaped cavity called pelvis. The free
end of pelvis has number of cup-like cavities called calyces (sing, calyx) major and minor.
Medulla projects into the calyces as conical processes, called renal pyramids or medullary
pyramids. The tip of pyramids are called renal papillae. The cortex spreads in between
medullary pyramids as renal columns called columns of Bertini.
Microscopic Structure
Each kidney is composed of numerous (nearly one million) complex tubular structure
called nephrons, which are the functional units of kidney.
Structure of Nephron Uriniferous Tubule
Each nephron consists of two parts, i.e., the Malpighian body or renal corpuscle and the
renal tubule.
i. Malpighian Body or Renal Corpuscle
Glomerulus along with Bowman’s capsule is called the Malpighian body or renal corpuscle
which filters out large solutes from the blood and delivers small solutes to the renal tubule
for modification.
* Glomerulus It is a tuft of capillaries formed by the afferent arteriole (a fine branch of
renal artery).
The afferent arteriole is short and wide that supplies blood to the glomerulus, while, the
efferent arteriole is narrow and long carrying blood away from the glomerulus.
Differences between Afferent Arteriole and Efferent Arteriole
ii. Bowman’s Capsule (Glomerular capsule) It is a double walled cup-like structure that
surrounds the glomerulus. The outer parietal wall which is composed of flattened
(squamous) cells and the inner visceral wall is composed of a special type of less flattened
cells, called podocytes.
iii. Renal Tubules
Just below the glomerulus, the tubule has a very short neck.
Attached to each Bowman’s capsule is a long, thin tubule with three distinct regions.
These regions are described as follows
(a) Proximal Convoluted Tubule (PCT) Behind the neck, it makes few coils and is restricted
to the cortical region of the kidney.
(b) Henle’s Loop It is quite narrower and U-shaped (or hair pin-shaped) having a
descending limb that ends into the medulla and an ascending limb that extends back from
the medulla into the cortex.
Differences between Descending Limb and Ascending Limb of Henle’s Loop
Note:
* Peritubular Capillary Network (PTCN), is formed when a minute vessel of peritubular
capillaries runs parallel to the loop of Henle forming a U-shaped vasa recta.
* All these capillaries join to form renal venules, which join to form a renal vein that opens
into the inferior vena cava.
Types of Nephrons
Based on the location in the kidney, nephrons are of following two types
1. Cortical Nephrons
In majority of nephrons, the loop of Henle is too short and extends only very little into the
medulla i.e., lie in the renal cortex. Such two nephrons are called cortical nephrons.
Juxtamedullary Nephrons
In some of the nephrons, the loop of Henle is very long and runs deep into the medulla.
These nephrons are called juxtamedullary nephrons.
The cortical nephron forms about 80% of the total nephron count while rest 20% are the
juxtamedullary nephron.
Functions of Kidney
Following functions are served by kidney
(i) Regulation of water and electrolyte balance.
(ii) Regulation of arterial pressure.
(iii) Excretion of metabolic waste and foreign chemicals.
(iv) Secretion of hormones like renin.
2. Ureters
The pelvis of each kidney is continued as a ureter and emerges out at hilus. Ureter is a long
and muscular tube. Ureters of both sides extend posteriorly and open into the urinary
bladder.
3. Urinary Bladder
It is a thin-walled, pear-shaped, white transparent sac present in the pelvic cavity. It
temporarily stores the urine.
4. Urethra
It is a membranous tube, which conduct urine to the exterior. The urethral sphincters keep
the urethra closed except during voiding of urine.
The formation of urine is the result of the following processes
1. Glomerular Filtration
The first step of urine formation is the filtration of blood, which is carried out by the
glomerulus. That’s why this step is called glomerular filtration.
Kidneys filter about 1100-1200 mL of blood per minute, which constitute roughly l/5th of
the blood pumped out by each ventricle of the heart in a minute.
The glomerular capillary bloodpressure causes filtration of blood through three layers, i.e.,
(i) the endothelium of glomerular blood vessels.
(ii) the epithelium of Bowmans capsule.
(iii) a basement membrane (present between the above mentioned two layers).
The podocytes (epithelial cells of Bowman’s capsule) are arranged in such a manner so, as
to leave some minute spaces called filtration slits or slit pores.
On account of the high pressure in the glomerular capillaries, the substances are filtered
through these pores into the lumen of the Bowman’s capsule (but the RBC, WBC and
plasma proteins having high molecular weight are unable to pass out).
That’s why this process of filtration through glomerular capillaries in the Bowman’s
capsule is known as ultra filtration and the filtrate is called glomerular filtrate or primary
urine.
It is hypotonic to urine that is actually excreted. Basic function of nephron is to clear out
the plasma from unwanted substrates and also maintain the osmotic concentration of the
blood plasma. Thus, the fluid coming out is known as urine, whose formation occurs inside
the kidney.
Glomerular Filtration Rate The amount of the filtrate formed by the kidneys per minute is
called Glomerular Filtration Rate (GFR). In a healthy person it was found approximately
125 mL/min, i.e., 180 L/day.
GFR is regulated by one of the efficient mechanism carried out by Juxtaglomerular
Apparatus (JGA).
JGA is a special sensitive region formed by cellular modifications in the distal convoluted
tubule and the afferent arteriole at the location of their contact.
This apparatus includes
(i) granular juxtaglomerular cells in the afferent arteriole.
(ii) macula densa cells of DCT.
(iii) agranular lacis cells situated in between the above two.
A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular
blood flow and thereby, the GFR back to I normal.
2. Selective Reabsorption
This is the second step in the formation of urine from filtrate. The urine released is 1.5 L as
compared to the volume of the filtrate formed per day (180 L). It suggests that as much as
99% of the material in the filtrate is reabsorbed by the renal tubules. Thus, the process is
called reabsorption.
Depending upon the types of molecules being reabsorbed, movements into and out of
epithelial cells in different segments of nephron occur either by passive transport or active
transport.
These are described as follows
(i) Water and urea, are reabsorbed by passive transport (i.e., water is reabsorbed by
osmosis and urea by simple diffusion).
(ii) Glucose and amino acids are reabsorbed by active transport.
(iii) The reabsorption of Na+, occurs both by passive and active transport.
3. Tubular Secretion
It is also an important step in urine formation. Certain chemicals in the blood that are not
removed by filtration from the glomerular capillaries are removed by this process of
tubular secretion. It helps in the maintenance of ionic and acid-base balance of body fluids
by removing chemicals like foreign bodies, ions (K+, H+, NH–) and molecules (medicines),
etc., that are toxic at elevated levels.
Difference between the Tubular Reabsorption and Tubular Secretion
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Army Public School Gopalpur: Class 11 Science Subject - Biology

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ARMY PUBLIC SCHOOL GOPALPUR

CLASS 11 SCIENCE SUBJECT - BIOLOGY

Chapter -19 Excretory Products and their Elimination

Types of Nitrogenous Excretion

ALL THE BEST

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