8. Excretrory System

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Excretory System

Various Metabolic activities are being taken place in ourbody.

As a result large amount of wastes are produced which have

to be eliminated from the body to maintain health. Removal of wastes and undigested food collectively called

faeces from the anus is called Defecation.

Separation and elimination of the metabolic nitrogenouswastes from the body is called Excretion.

The wastes eliminated are called excretory products and theorgans concerned with excretion are called excretory

organs. They are a pair of kidneys, a pair of ureters, a urinary

bladder and a urethra.


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Function

It maintains the constant environment necessaryfor normal functioning of tissues and cells.

During metabolism of Carbohydrates, fats andproteins a large amount of CO2 and H2O areproduced.

CO2 is eliminated from lungs and much H2O iseliminated through excretory organs but a certainlevel is maintained in body.

The process where excess water from the body iseliminated to keep H2O content stable is termed asOSMOREGULATION.


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Kidney

Mammals possess a pair of 2 dark red, bean shaped kidneyslocated in the abdomen, one on either side of the VertebralColumn in front of the last 2 pairs of ribs for protection

(floating ribs) Right kidney is slightly lower than the left one, this is

because the right side of abdomen is occupied by the liver.

Each kidney is 10 13 cm long and 2.5 4 cm wide.

It is covered by a layer of fibrous connective tissue calledRenal Capsule.

There is a layer of fat called Renal Fat around the capsule. The outer surface is convex and inner surface is concave.

The inner surface has a deep notch called HILUS through

which pass the ureter, renal artery, renal vein and nerve.


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Anatomy of Kidney Longitudinal section shows

1). a Peripheral dark reddish brown region called CORTEX

2). a Central Light red area called MEDULLA

The medulla is divided into 15-16 cone like areascalled MEDULLARY PYRAMIDS having their basetowards cortex.

Each pyramid terminates into a Renal Papilla. The cortex substance extends and forms columns of

Bertini between the pyramids.

The pyramids are connected with 7-13 MINORACALYCES.

The human kidney has 2-3 MAJOR CALYCES


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Nephrons:

- The major calyces open into a funnel shaped renalpelvis which opens into the ureter.

- There are about 1 million nephrons in each kidney.

- They are the functional and structural unit of

Kidney.

- Each nephron is a coiled tube about 3 cm long witha width of 20-60 um.

- It is has 2 main parts

1. Malpighian Body (renal corpuscle)

2. Renal Tubule


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The Malpighian body consists of

a cup shaped Bowmans Capsule

A network of blood capillaries called Glomerulus Glomerulus receives blood through Afferent Arteriole

and moves out through Efferent Arteriole.

The diameter of afferent arteriole is more than thediameter of efferent arteriole i.e. Lumen of efferentarteriole is narrow and Lumen of Afferent arteriole isbroad.

The Bowmans lead into a small neck which opens intothe renal tubule which is coiled in the cortex andcalled Proximal Convoluted Tubule (PCT). It leads to

Loop of Henle. This can be distinguished into descending limb, the

medulla region and ascending limb and The ascendinglimb opens into Distal Convoluted Tubule (DCT).


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DCT of many nephrons open into collecting

ducts which open into larger ducts that runthrough renal pyramids and open in the renalpelvis

The efferent arteriol breaks up into capillariesaround the tubules.

The capillaries surrounding loop of Henle are

called Vasa Recta. (Function: to retainreabsorbed ions and urea).

The walls of the bowmans capsule are a single

layer of squamous epithelium. The remaining parts of nephrons have highly

specialized cells in a single layer.


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Ureters These are the tubes 25-30 cm long

It carries urine from kidney to urinarybladder.

Each ureter leaves its kidney in the Hilusregion.

Their walls are composed of transitional

epithelium.


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Urinary Bladder Its pear shaped sac like structure in the pelvic

cavity. Its muscular walls are internally lived with

transitional epithelium.

The ureters open into it.

This opening is guarded by internal andexternal sphincters.

Sphincters is a specialized ring of muscle thatsurrounds an orifice.


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Urethra It is a tube which runs from neck of bladder

and opens outside the body. It carries urine from bladder to be removed

from the body.

In Males, it is approx. 20 cm long while in

females, it is about 4 cm in length.

The Male urethra carries both urine andsemen.


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Miscellaneous

Diuresis: Increased flow of urine is called Diuresis.- The substances that increase the urine flow

are caffeine and alcohol, are called diuretic

substances

Micturition:

- The process of urination is called micturition.Dysuria: Painful urination

Polyuria: Excess of urine formationOliguria: Scanty urine

Anuria: Absence of urine


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Urinary Bladder is absent in snakes, crocodile

and birds except rhea and ostrich. Dialysis: Separation of toxic substance from the

blood stream through a membrane by kidney

machine (artificial kidney).

Albuminaria: A condition in which there is

presence of albumin protein in urine. Glycosuria: Presence of Glucose in the urine.

Haematurea: Presence of blood in the urine.

Urochrome: A pigment that gives urine its typical

yellow colour.


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Excretion:

The process of excretion consists ofFormation of Urea and Formation of Urine

Formation of Urea:- Most of the urea is formed in the liver

through the process called Orinithine Cycle /

Krebs Henseleit Cycle.

- Urea is formed due to catabolism of

Proteins


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Formation of Urine It includes the process of

ULTRA-FILTERATION

SELECTIVE REABSORPTION

TUBULAR SECRETION.

The entire process occurs in the nephrons of

kidneys


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Ultra-filtration

Capillaries of glomerulus are thin. Diameter of Afferent arteriole is more than the

diameter of Efferent arteriole.

This causes high BP in the Glomerulus due to which aprocess of ultra-filtration i.e. filtration under pressuretakes place continuously.

H2O & many dissolved substances from blood arefiltered into the lumen of Bowmans Capsule.

This fluid is called GLOMERULAR FILTRATE.

It resembles PLASMA except it does not possessPROTEINS & FATS as they are too big to pass out of thepores of capillaries of glomerulus


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The rate of blood flow in the glomerulus is about

750 cc/min, out of which 120 cc ultrafiltrate isformed.

Thus, 180 lit of ultrafilterate is formed during 24

hrs.

The glomerular filtrate has a large amount of

H2O and dissolved substances like urea, uricacid, amino acids, glucose, sodium, potassium,

vitamins etc.


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Selective Reabsorption

Ultra-filtrate is almost isotonic to plasma since itresembles plasma in composition (except the

absence of proteins and fats)

As this passes through the PCT about 80% H2O, Na

ions, Cl ions, bi-carbonates, glucose and amino

acids are reabsorbed by the surrounding networkof capillaries.

Now the glomerulus filtrate is calledRENAL FLUID.


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A Small amount of H2O is absorbed by Descending

Limb of loop of Henle

Ascending Limb of loop of Henle is impermeable to

H2O but Na ions move out of renal fluid.

In DCT and Collecting tubule, water is furtherreabsorbed under the influence of ADH (Anti-

Diuretic Hormone) of pituitary gland.

Na is also absorbed in Collecting tubule under the

influence of hormone Aldosterone.

The filtrate is now called URINE.


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Tubular Secretion Substances like hippuric acid and ions like K+

and H+ are secreted by blood directly intorenal tubules.

Penicillin, other drugs and some toxic

substances are also secreted here.

Thus, out of 180 litres of glomerular filtrate

only 1.5 1.75 litre of urine is formed perday.


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Organs involved in Excretion Skin and Lungs in human are also involved in

elimination of some amount of Nitrogenous

wastes.

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8. Excretrory System

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