6. LIFE PROCESSES
Excretion is the process of removal of harmful metabolic nitrogenous wastes from the body.
Unicellular organisms remove these wastes by simple diffusion
from body surface into surrounding water.
Complex multicellular organisms use specialised organs for
excretion.
Excretion in Human Beings
Human excretory system includes
a pair of kidneys,
a pair of ureters, a urinary bladder & a urethra.
Kidneys are located in the abdomen, one on either side of the backbone.
The purpose of making urine is to filter out waste
products from the blood.
In the kidneys, nitrogenous wastes such as urea or
uric acid are removed from blood.
Each kidney has large numbers of filtration units called nephrons
packed close together.
At the end of each nephron, a cup-shaped structure
called Bowman’s capsule is seen. It encloses a cluster of very
thin-walled blood capillaries called glomerulus.
In glomerulus, blood is filtered and the Bowman’s
capsule collects the filtrate.
Glucose, amino acids, salts & major amount of water in
the initial filtrate are selectively reabsorbed as the urine flows along
the tube.
In a healthy adult, the initial filtrate is about 180
L daily. However, only 1-2 litre/day is excreted out because the remaining
filtrate is reabsorbed in the kidney tubules.
Water is reabsorbed based on amount of excess water in
the body and amount of dissolved waste is to be excreted.
The urine from each kidney enters a long tube (ureter),
which then passes to urinary bladder and stored in it.
As the bladder expands, pressure increases that leads to
the urge to urinate through the urethra.
The bladder is muscular, so it is under nervous control. As
a result, we can control the urge to urinate.
Artificial
kidney (Hemodialysis)
- Kidney failure leads
to accumulation of poisonous wastes in the body. In this case, an artificial
kidney can be used. It is a device to remove nitrogenous wastes from the blood
through dialysis.
- Artificial
kidneys contain many semi-permeable tubes suspended in a tank filled with dialysing
fluid (it has same osmotic pressure of blood, but no nitrogenous wastes).
- When patient’s
blood is passed through these tubes, the waste products from blood diffuses
into dialysing fluid.
- The purified
blood is pumped back into the patient.
- This is similar
to the function of the kidney, but there is no reabsorption involved.
Organ donation
Any people can
donate their organ or tissue regardless of age or gender.
Transplantation
is required when recipient’s organ has been damaged or failed by disease or
injury.
In organ
transplantation, the organ is surgically removed from one person (organ donor)
and transplanted to another person (recipient).
Most organ and
tissue donations occur just after the death or brain death of the donor. But
some organs (kidney, part of a liver, lung, etc.) and tissues can be donated
while the donor is alive.
Excretion in Plants
Oxygen is formed as a waste product of photosynthesis.
O2, CO2 & excess water are removed by transpiration.
Many other waste products are stored in vacuoles.
Waste products may be stored in leaves that fall off.
Some waste products are stored as resins and gums,
especially in old xylem.
Plants also excrete some waste substances into the soil around them.
Select Your Next Topic 👇
