Excretion//Excretory wastes//elimination of excretory wastes//excretory products and their elimination

 

Excretory Products and Their Elimination

 

Removal of nitrogenous waste like ammonia, urea, uric acid etc. along with excess of water and salt and pigment called Excretion or Removal of metabolic waste from body is called Excretion.

These wastes are called excretory substances.  For elimination of these wastes animals have specialized organs are Excretory Organs.

Depending upon the nature of excretory products, there are three mode of elimination of nitrogenous wastes from living body. These are as follows-

A.   Amoinotelism

B.   Ureotelism

C.   Urecotelism

 

A.   Ammonotelism:

The organisms which are found in water or have eccess amount of water, eleminates their nitrogenous wastes in the form of ammonia. Animals which reove their excretory waste in the form of ammonia, called ammonotelic animals. The process which they remove wastes from the body called, ammonotelism.

Eg: Bony fishes, aquatic amphibians and aquatic insect.

B.   Ureotelism:

Due to toxic nature of ammonia it can’t be stored in body for a short period of time. Hence more toxic substance are converted  into less toxic substance, known as urea.

The organisms are excretes urea as a nitrogenous wastes are called Ureotelic animals and the process to eliminate urea is called Ureotelism.

Mammals, and many terrestrial amphibians and marine fishes are the example of ureotelic animals.

 

C.   Uricotelism:

The animals, which faces the high deficiency of water, excretes their nitrogenous wastes in the form of uric acid. These are the urecotelic animals, process to eliminate uric acid is called uricotelism.

Reptiles, birds, land snails, desert vertebrates and insects are the example of  Ureotelic organisms.

 

Excretory Organs:

Organs, which are involved in the process of excretion, are called excretory organs. Different animal group have a variety of excretory organs to perform the process of excretion. Some to these excretory organs are-

Animal groups	Excretory organs	Main Nitrogenous waste
Lower Invertebrates	Body surface	Ammonia
Platyhelminthes	Protonephridia or Flame cells	Ammonia
Aschelminthes	Renette cells	Ammonia
Annelida	Metanephridia	Ammonia and Urea
Arthropoda	Malpighian tubules	Uric acid and Ammonia
Mollusca	Renal gland or organ of Bojanus and Keber’s organ	Ammonia and Uric acid
Echinodermata	Tube feet	Ammonia
Chordata	Neural gland and kidney	Ammonia, Urea and Uric acid

Excretory System of Human

Human excretory system consists of a pair of   Kindeys, a pair of ureter, a urinary bladder and urethra.

Human excretory system

1.     Kidneys:

These are reddish brown, bean-shaped structures situated between the levels of last thoracic and third lumber vertebra. Kidenyes are mesodermal in origin.

Right kidney is slightly lower than left kidney due to presence of liver.

kidneyes of an adult human measures, 10-12cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 gm.

Structure of kidney:

a.     External structure:

The outer surface of each kidney is convex and inner concave surface has a notch called Hilum (The  supply of blood occurs through hilum).

Kidneys covered by three layers i.e., renal facia (outermost), the adipose layer and then renal capsule (innermost layer). These coverings protect the kidneys from external shocks and injuries.

b.      Internal structure:

Inside the kidney, there two zones, an outer cortex and inner medulla. Inside the kidney funnel shaped cavity present called pelvis.

The free end of pelvis has number of cup like cavities are called major calyces and minor calyces.

Medullary projects into the calyces as conical processes known as Renal pyramids or Medullary pyramids.

Renal pyramids are 8 to 12 in number in humans.The tips of pyramid are called Renal papillae. The cortex spreads in between medullary pyramids as renal columns called Columns of Bertini.

L.S. of Kidney

Each kidney is composed of numerous tubular structure called Nephrons or Uriniferous tubule.

Nephrons are the functional units of kidney. Each kidney has a millions of nephron.

 

Structure of Nephron:

Each nephron consist two parts –

(i)                Malpighian Body or  Renal Corpuscle

(ii)             Renal  tubules

(i)                Malpighian Body or  Renal Corpuscle:

Glomerulus with Bowman’s capsule is called the Malpighian Body or  Renal Corpuscle which filter the large solutes from the blood.

Glomerulus -is thin walled capillaries formed by the branching of afferent arteriole.

Afferent arteriole is short and wide that supply blood to glomerulus and efferent arteriole is narrow and long carry blood away from glomerulus.

Bowman’s capsule- is double walled cup like structure which surrounds the glomerulus.

(ii)             Renal  tubules:

The remaining part of nephron attached to Bowman’s capsule is renal tubules.

This part differentiated into following region-

(1)   Proximal Convoluted Tubule (PCT):

Behind the Bowman’s capsule, it makes few coils and present in the cortex region.

(2)   Henle’s Loop:

It is U- shaped and quit narrow and having a descending limb that ends into the medulla and an ascending limb that extend back from the medulla into cortex.

(3)   Distal convoluted tubule (DCT):

Ascending limb continues as another highly coiled tubular region known as DCT and situated in the cortex as PCT.

(4)   Collecting ducts:

The last region of nephron is Collecting duct.

Several collecting ducts join to form the larger ducts of Bellini in the medulla region.

The efferent arteriole emerging from the glomerules forms a fine capillary network around the renal tubule called the peritubular capillaries.

A minute vessel of this network runs parallel to the Henle’s loop forming U-shaped vasa recta.

Diagrammatic representation of Nephron

Types of Nephrons:

Based on the location of in the kidney, nephrons are two types-

(A). Cortical Nephrons:

 In majority of nephrons, the loop of Henle is too short and extends only small part into the medulla i.e.,lie in the renal cortex of kidney. Such nephrons are called Cortical nephrons.

They make up about 80% of all nephrons.

(B). Juxtamedullary Nephrons:

In some nephrons, the loop of henle is very long and runs deep into the medulla. These nephrons are called Juxtamedullary nephrons.

They make up about 20% of all the nephrons.

Function of Kidney:

·        Regulation of water and electrolyte balance.

·        Regulation of arterial pressure

·        Excretion of metabolic waste and foreign chemicals.

·        Secretion of hormones like rennin.

2. Ureter:

The pelvis of each kidney is continued as ureter. Ureter is a long and muscular tube. Both sides ureter open into the urinary bladder. Their function is to conduct water.

3. Urinary Bladder:

It is thin walled, pear shaped, white transparent sac present in the pelvic cavity.

Its function is stores the urine (0.5-1 liters urine)

4. Urethra: 

It is a membranous tube, which conduct urine to the exterior.

 

Role of other organs in Excretion:

Other than kidneys, there are some accessory excretory organs also that help in the removal of excretory wastes. These are as fallows-

1.     Lungs:

Lungs remove the CO₂ and some water as vapour in the expired air. About 18 liter of Co₂   per hour and 400 ml of water per day are eliminated by human  lungs.

2.     Liver:

Liver secretes bilirubin and biliverdin pigments and pass out into the alimentary canal also excretes cholesterol, steroid hormones, vitamins and dugs.

3.    Skin:

The sweat gland and sebaceous glands in the skin can excrete some substances like NaCl, lactic acid, waxes, sterols, fatty acid through their secretion.