EXCRETION

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EXCRETION

 

The biological process of removal of harmful The biological process of removal of harmful nitrogenous wastes from the body is called exretion.

 

 

Introduction

During cellular respiration, various metabolic reactions occur in the body leading to formation of various waste products such as carbon dioxide, urea, etc. These waste products are harmful if they are allowed to accumulate in the body. Therefore removal of these waste products is must. The process of removal of these metabolic wastes from the body is known as excretion. The process of maintaining the right amount of water and ionic balance is called osmoregulation.

Singnificance of excretion
(i) The unwanted by-products of the metabolic of the metabolic activities are removed.
(ii) Many toxic chemicals, which damage the cells and affect metabolic activities, are removed.
(iii) The ionic concentration of body fluids is maintained by excretion and osmoregulation.
(iii) The water content and pH of body fluids is regulated by it.

 

EXCRETION IN ANIMALS

In animals principal wastes produced by various metabolic activities are nitrogenous substances like ammonia, urea and uric acid along with respiratory wastes (CO2) and others.

EXCRETION IN HUMAN (Human Excretory System)

In human beings excretion mainly occurs through a urinary system.  Urinary or excretory system consists of  (1) The kidneys
(2) The ureters
(3) The urinary bladder
(4) The urethra

(1) Kidneys
External Structure
Colour  – Dark red
Shape  – Bean shaped
Weight  – 125 – 170 gms.
Size  – 10 cm length, 5 cm breadth, 3 cm thickness.
Position  – Located laterally either sides of
vertebral column.

Kidneys are the main organs of urinary system. Each kidney is bean shaped, lateral border is convex and its medial border is concave in the middle and convex at each. In the centre of the medial concave border there is present a notch known as the hilium, which contains the renal blood vessels and nerves and the renal pelvis, which is the funnel-shaped upper end of the ureter. Urine produced by the kidneys is temporarily stored in the urinary bladder and passed out through urethra.

    • Functions of the Kidney
      1. It maintains water equilibrium, pH equilibrium, ionic equilibrium of the blood and osmotic equilibrium.
      2. It helps to excrete out waste product urea in the dissolved form from the blood.
      3. It excretes poisonous substances like drugs, toxins etc., from the body.
      4. It regulates blood pressure by controlling the fluid balance in the body.
      5. Many ions derived from food are excreted in the urine.They include sodium, potassiun magnesium,. calcium, chloride, phosphate, sulphate and oxalate ions. (These movements of ion are important in helping to maintain the acid-base balance of the body and keeping the pH of arterial blood at 7.40.)

 

Internal Structure
The internal structure of kidneys can be divided into two parts. Its outer part is called cortex and inner part is called medulla.

 Internal Structure of kidney

NEPHRON

The nephron is the structural and functional unit of the kidney. Each kidney of man is formed of about one million nephrons. Each nephron has a length of about 3cm. It is differentiated into 4 regions.

 

(a) Bowman’s capsule
(b) Proximal convoluted tubules (PCT)
(c) Loop of henle
(d) Distal convolulated tubule (DCT)

(a) Bowman’s capsule
It is a large double walled cup. It lies in the renal cortex. It contains a tuft of capillaties called glomerulus and the outer wall is continuous with the rest of the nephron. The space between the two walls of the Bowman’s capsule is continuous with the lumen of the next part of the nephron. The bowman’s capsule and the glomerulus together constitutes the renal corpuscle or malpighian body.

(b) PCT
It starts from the back of the Bowman’s capsule and it is bighly convoluted. It lies in the renal cortex. The wall consists of a single layer of columnar cells bearing a lot of microvilli on the surface.

(c) Loop of Henle
It is a V shaped segment of the nephron located in the renal medulla. It consists of two straight parallel limbs : a descending limb which is a continuation of the PCT and enters into the renal medulla and an ascending limb which re-enters the renal cortex and joins the DCT.

(d) DCT
It is greatly twisted like the pct and lies in the renal cortex. The terminal relatively short part of the DCT is called the collecting tubule. It
open into the collecting duct. The collecting ducts receive the collecting tubules of several nephrons.

Structure of Nephron

(2) Ureter 
They are a pair of whitish narrow distensible muscular tubes of about 30 cm length. Each ureter arises from hilus part of the kidney. It moves downwardly and opens obliquely into urinary bladder. Ureters carry urine from kidneys to the urinary bladder.

(3) Urinary
Bladder  It is a median pear shaped distensible sac that occurs in the pelvic part of abdomen. It stores urine brought by the two ureters. The storage capacity is 300-800 ml.

(4) Urethra 
It is a tube that takes urine from urinary bladder to outside. The opening of urinary bladder into ‘urethra is guarded by a ring of muscles or sphincter. Urethra is 4 cm long in females and about 20 cm long in males. Its opening is separate in females but is common with reproductive tract in males.

 

 

PHYSIOLOGY OF URINE FORMATION (EXCRETION)

Main function of nephron is to form urine. There are three main processes involved in the urine formation :

1. Glomerular ultrafiltration

  • It is the filteration of body fluids and solutes from the blood, out of the glomerular capillaries  into the Bowman’s capsule due to the pressure in the glomerulus.
  • All substances from the blood are filtered out except the large protein molecules. This fluid in the glomerular capsule is called glomerular filtrate.
  • It consists of water, urea, salts, glucose and other plasma solutes.
  • Blood coming out of the efferent arteriole is therefore thick.
  • About 180 litres of glomerular filterate is ormed by both kidneys in a day but urine excreted is, about 1-2 litres a day. This shows that most of glomerular filterate is reabsorbed.

 

2. Tubular Reabsorption

  • Glomerular filterate contains a lot of useful materials like glucose, salts such as that of sodium and water.
  • These substances are reabsorbed from the renal tubule at various levels and in varied proportions.
  • Glucose is reabsorbed completely from the proximal convoluted tubules.
  • More than 85% of water is reabsorbed from the proximal, distal and even in collecting tubules.
  • Sodium chloride is reabsorbed in the proximal and distal tubules.
  • Potassium is completely reabsorbed from the proximal tubule.
  • Phosphate is reabsorbed in the proximal tubule, etc, Other substances reabsorbed are uric acid, sulphates, vitamin C,amino acids etc.

 

3. Tubular Secretion

  • This occurs mainly in the distal convoluted tubule and the collecting duct of the nephron.
  • It is an active, vital process performed by the cells of the cuboidal epithelium lining the tubules which excrete additional wastes from the blood stream into the filtrate by active transport.
  • In this process substances like potassium, hydrogen, creatinine and certain drugs like phenol,  penicillin etc., are directly excreted by the tubular cells from the blood.
  • The fluid which now flows through the last parts of the tubule is urine which consists of water, urea, uric acid, mineral ions like sodium, potassium, chloride, phosphates etc.

 

Composition of urine 

It is a transparent fluid produced by the excretory system. Normal urine is a yellow fluid and slightly (pH = 6) Urine contains –
(i) Water – 96%
(ii) organic substanace – 25%  (urea, uric acid, creatine, creatinine, vitamines)
(iii) Inorganic substances – 1.5%  (Na, Ca, Phasphate, Sulphate)

 

Artificial Kidney or Haemodialys

is Kidney is a very important organ which is essential for maintaining internal homeostasis as it is engaged in elimination of the nitrogenous and other metabolic by-products. Even if one kidney is damaged, the second kidney can carry on the function of excretion completely. However, if both the kidneys are damaged, a new compatible kidney has to be grafted. Till that period, waste products are removed with the help of haemodialysis (blood dialysis), injury or infection or artificial kidney.  Artificial kidney is a physico-chemical device to remove excretory products from blood in case of temporary disfunction (due to toxins, injury or infection) or near failure of kidneys. It is based on the principle of dialysis or separation of smaller solutes or ions from larger particles with the help of an ultrafilter. The artificial kidney or dialysis machine consists of a number of cellophane tubes embedded in a dialysate or dialysing fluid. The dialysing fluid has the same osmotic concentration as that of blood. However, it contains more of glucose. Nitrogenous waste products, phosphates and sulphates are excluded.

Aritificial kidney/Haemodialysis

Blood from an artery, or a vein fitted to a pumping mechanism, is mixed with heparin, cooled at 0° C and passed into cellophane tubes of artificial kidney. Nitrogenous waste products, sulphate and phosphate of blood pass into dialysing fluid. Purified blood is warmed and mixed with antiheparin. It is passed back into vein. The whole process takes 3-4 hours.

· Uses of Aritificial kidney/Haemodialysis

1. Toxins
Haemodialysis helps in removing toxins from the body before they are able to damage the body permanently.

2. Uraemia 
Patients suffering from kidney infections and uraemia (excess of urea in blood) are provided relief for some time.

3. Renal Failure
In case of near permanent damage to kidney, haemodialysis provides time to the patient to find a kidney donor.

4. Normal Life
In between two dialysis, a patient can lead a near normal life.

5. Clean Procedure 
Haemodialysis is a clean procedure where chances of infection are minimum.

 

Excretion in plants 
Plants produce a number of waste products during their life processes.

  • The main waste products produced by plants are carbon dioxide, water vapour and oxygen.
  • Plants get rid of excess water by transpiration.
  • The gaseous wastes of respiration and photosynthesis in plants (carbon dioxide, water vapour and oxygen) are removed through the ‘stomata’ in leaves and ‘lenticels’ in stems and released to the air.
  • Many plant waste products are stored in cellular vacuoles. Wastes products may be stored in leaves that fall off, other waste products are stored as resins and gums.
  • Plants excrete some waste substances into the soil around them.
  • Some of the plant wastes which are useful to humans are – Natural rubber, gum, resins and essential oils like sandalwood oil, eucalyptus oil, clove oil and lavender oil.

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