Excretion

Excretion is the process by which living organisms remove harmful and waste substances, such as urea, uric acid, and ammonia, from their bodies. These substances are produced as by-products of metabolic activities. If not eliminated, they can accumulate in the body and cause severe harm, even leading to death. Therefore, excretion is essential for the survival and health of all living organisms.

Methods of Excretion:

1. Unicellular Organisms: In simple organisms like amoebas, waste materials are directly eliminated through the cell surface via diffusion.
2. Multicellular Organisms: In more complex organisms, excretion involves specialised systems.

For example:

• Humans use organs like the kidneys to filter waste (urea) from the blood and expel it as urine.
• The skin removes waste through sweat, and the lungs excrete carbon dioxide produced during respiration.

Plants can get rid of excess water through transpiration. For other wastes, they use the fact that many of their tissues consist of dead cells and 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.
• Some plants shed leaves in a specific season. Waste substances stored in the leaves of plants are shed along with the leaves. Excretion is a characteristic of living things.

A Tree Shedding Leaves

1. Aim: To observe the excretion of water in plants through a process called transpiration.

2. Requirement: transparent plastic bag and plant with leaves.

3. Procedure

• Select a healthy leaf from the plant.
• Tie a transparent plastic bag over the leaf, ensuring it is sealed.
• Leave the setup undisturbed for 6 to 7 hours.

4. Observation: After that time, droplets of water are seen collecting inside the bag.

5. Conclusion: The droplets indicate that plants excrete water in the form of vapour, confirming excretion as a characteristic of living things.

Excretion on the leaves of a plant

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 urinating 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 is removed from blood in the kidneys.

Each kidney contains many filtration units called Nephrons. It comprises a cluster of thin-walled capillaries called a glomerulus, associated with a cup-like structure called Bowman’s capsule and the long tube that terminates through this capsule.

• The renal artery brings oxygenated blood to the kidneys, along with nitrogenous wastes like urea and uric acid.
• The blood gets filtered through the glomerulus, and this filtrate enters the tubular part of the nephron.
• The blood vessels surrounding these tubules selectively reabsorb glucose, amino acids, salts, and excess water as this filtrate moves down the tubular part.
• The amount of reabsorbed water depends on the amount of excess water in the body and the amount of nitrogenous waste that needs to be excreted.
• The fluid now flowing in the tubular part is urine, collected in the collecting ducts of nephrons. These collecting ducts leave the kidney at a common point by forming the ureter.
• Each ureter drains the urine into the urinary bladder, where it is stored until the pressure of the expanded bladder leads to an urge to pass it out through the urethra. This bladder is a muscular structure that is under nervous control.
• 180 litres of filtrate is formed daily, but only 2 liters is excreted out as urine, so the rest is reabsorbed in the body

Nephron

Kidneys are vital organs for survival. Several factors, like infections, injury, or restricted blood flow to the kidneys, reduce kidney activity. This leads to the accumulation of poisonous wastes in the body, possibly leading to death.

• An artificial kidney, a device that removes nitrogenous waste products from the blood through dialysis, can be used in case of kidney failure.
• Artificial kidneys contain several tubes with a semi-permeable lining suspended in a tank filled with dialysing fluid. This fluid has the same osmotic pressure as blood, except it lacks nitrogenous wastes.
• The patient’s blood is passed through these tubes. During this passage, the waste products from the blood pass into a dialysing fluid by diffusion. The purified blood is pumped back into the patient.
• This is similar to the kidney's function but is different since no reabsorption is involved. Normally, in a healthy adult, the initial filtrate in the kidneys is about 180 L daily. However, the volume excreted is only a litre or two a day because the remaining filtrate is reabsorbed in the kidney tubules.