1.0 Introduction to Excretion
Every factory produces waste along with its main products. Our body is no different. As our cells perform various chemical activities (metabolism), they produce toxic by-products. If these are not removed, they can poison the body. The process of removing these metabolic wastes is called Excretion.
Excretion: The biological process by which an organism rids itself of metabolic nitrogenous waste products and other toxic substances from the body.
Major Excretory Organs in Humans
While the kidneys are the primary organs of excretion, other organs also help in removing waste:
- 1. Kidneys: Remove nitrogenous wastes like urea and excess water in the form of Urine.
- 2. Sweat Glands (Skin): Eliminate excess water, salts, and small amounts of urea through Sweat.
- 3. Lungs: Excrete gaseous waste like Carbon Dioxide and water vapor.
- 4. Liver: Breaks down excess amino acids into urea and removes bile pigments.
Do not confuse these two! Egestion is the removal of undigested food (faeces) through the anus. Excretion is the removal of chemical wastes produced inside the cells during metabolism.
In one day, your blood passes through your kidneys about 400 times! This constant filtering ensures your internal environment remains clean and balanced.
2.0 The Human Urinary System
The urinary system is the main excretory system in humans. It consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. Its primary job is to filter blood and create urine as a waste by-product.
Components and Their Functions:
- 1. Kidneys: Two bean-shaped organs located on either side of the backbone. They filter the blood to remove urea and excess salts.
- 2. Ureters: Two narrow muscular tubes that carry urine from the kidneys down to the urinary bladder.
- 3. Urinary Bladder: A muscular sac that stores urine temporarily until it is full.
- 4. Urethra: A duct through which urine is finally discharged out of the body.
The Structural Unit: Nephron
Each kidney contains millions of microscopic filtering units called Nephrons. A nephron is composed of:
- Bowman’s Capsule: A cup-shaped structure that surrounds a cluster of capillaries.
- Glomerulus: A knot of capillaries where the actual filtration of blood takes place.
- Renal Tubule: A long tube where useful substances (like glucose and water) are reabsorbed back into the blood.
The right kidney is usually placed slightly lower than the left kidney. This is because the large Liver occupies a lot of space on the right side of the abdominal cavity!
A healthy adult's bladder can hold about 400-600 ml of urine. When it's about half full, nerves send a signal to your brain telling you it’s time to find a restroom!
3.0 Urine Composition and Common Disorders
Urine is the final fluid produced after the kidneys have filtered the blood and reabsorbed all necessary nutrients. Its composition can tell a doctor a lot about a person's health.
What is in Urine?
A healthy human typically excretes about 1 to 1.5 litres of urine per day. Its typical composition is:
- Water: Approximately 95%
- Urea: Approximately 2.5% (The main nitrogenous waste)
- Other substances: 2.5% (Includes salts, uric acid, and creatinine)
Note: The amber color of urine is due to a pigment called Urochrome.
Common Excretory Disorders
- 1. Urinary Tract Infection (UTI): Caused by bacteria entering the urinary tract, leading to a burning sensation and frequent urination.
- 2. Kidney Stones: Hard deposits of minerals (like calcium oxalate) that form inside the kidneys. They can cause severe pain when passing through the ureter.
- 3. Kidney Failure: When kidneys lose the ability to filter waste from the blood. This requires Dialysis or a transplant.
Dialysis (Artificial Kidney): A medical procedure to filter a person's blood using a machine when their own kidneys are no longer functioning.
The kidney doesn't just excrete waste; it also maintains the Water Balance in the body. This process is called Osmoregulation. On a hot day, you sweat more and urinate less to conserve water!
If one kidney is removed or stops working, the other kidney can actually grow slightly larger to handle the work of both! Humans can live a perfectly normal life with just one healthy kidney.