Excretion: Substances to Be Eliminated

EXCRETION SYSTEM IN HUMAN BEINGS:-

The excretory system consists of

• A pair of Kidneys

• A pair of ureters

• The urinary bladder

• The urethra

Kidneys:-

Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.

The purpose of making

urine is to filter out waste products from the blood.

Just as CO2 is removed from the blood in the lungs,

nitrogenous waste such as urea or uric acid are

removed from blood in the kidneys.

FORMATION OF URINE:-

• Each kidney contains many filtration units called as Nephrons.

• Nephrons are made up of a cluster of thin walled capillaries called glomerulus which is associated with a cup like structure called as Bowman’s capsule and the long tube which terminated through this capsule

• The renal artery brings oxygenated blood to the kidneys along with the nitrogenous wastes like urea and uric acid and many other substances

• The blood gets filtered through the glomerulus and this filtrate enters the tubular part of nephron

• As this filtrate moves down the tubular part, glucose, amino acids, salts and excess of water gets selectively reabsorbed by the blood vessels surrounding these tubules

• The amount of water reabsorbed depends upon how much excess of water is there in the body and how much nitrogenous wastes need to be excreted out so the fluid now flowing in the tubular part is urine which gets collected in collecting ducts of nephrons

• These collecting ducts together leave the kidney at a common point by forming the ureter

• Each ureter drains the urine in the urinary bladder where it is stored until the pressure of the expanded bladder leads to an urge to pass it out through urethra

• This bladder is a muscular structure which is under nervous control.

• 180 litres of filtrate is formed daily but only 2 litres is excreted out as urine so the rest is reabsorbed in the body

Artificial kidney (Hemodialysis):-

• Kidneys are vital organs for survival. Several factors like infections, injury or restricted blood flow to kidneys reduce the activity of kidneys. This leads to accumulation of poisonous wastes in the body, which can even lead to death.

   

• In case of kidney failure, an artificial kidney can be used. An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis.

   

• Artificial kidneys contain a number of tubes with a semi-permeable lining, suspended in a tank filled with dialysing fluid. This fluid has the same osmotic pressure as blood, except that it is devoid of nitrogenous wastes.

   

• The patient’s blood is passed through these tubes. During this passage, the waste products from the blood pass into dialysing fluid by diffusion. The purified blood is pumped back into the patient.

   

• This is similar to the function of the kidney, but it is different since there is no reabsorption involved. Normally, in a healthy adult, the initial filtrate in the kidneys is about 180 L daily. However, the volume actually excreted is only a litre or two a day, because the remaining filtrate is reabsorbed in the kidney tubules.

EXCRETION IN PLANTS:-

• They can get rid of excess water by transpiration. For other wastes, plants use the fact that many of their tissues consist of dead cells, and that they can even lose some parts such as leaves.

• Many plant waste products are stored in cellular vacuoles.

• Waste products may be stored in leaves that fall off.

• Other waste products are stored as resins and gums, especially in old xylem. Plants also excrete some waste substances into the soil around them.