Competitive Exams: Excretion

There are mainly 4 organs that help the body to keep a constant body environment: The lungs, the liver, the skin and the kidneys.


The lungs are responsible to exchange of gases in the body. They exchange carbon dioxide with oxygen from the air. Also, the lungs must provide the oxygen with a temperature of around 37 degress Celsius so that chemical reactions involving oxygen can take place.

The Liver

The liver is a major organ in the human body that makes a large amount of chemical reactions that produce heat (chemical reactions that produce heat are called exothermic).

Therefore, the liver produces all the necessary heat for the body to keep its internal temperature around 37oC.


The skin is responsible for transferring excess heat from inside the body to the outside environment. For that reason it is one of the organs that does homeostasis.

It also protects the body from germs.


The kidneys are responsible for osmoregulation, i.e.. To control the amount of water in the body, by filtering blood from salts, water and waste products (urea).

Blood is involved and so the kidneys are also part of homeostasis, because blood transports heat and helps to keep the body at a constant temperature.

The Excretory System

The excretory system is the system responsible for the disposal of waste material produced by the body--Urine. The major organs in the excretory system are the kidneys. The body can survive with just one kidney, but with none, the person must use the kidney machine (explained in the following pages) or else he or she dies. The function of the kidneys is to filter blood from urea (waste produced by chemical reactions in the body) excess water, and excess salts. This process is called ultra-filtration and it is done by nephrons (explained further in the following pages)

The Kindey

The diagram below shows the kidneys, the bladder and blood vessels connected to it.

Renal Vein: The vein that transports blood OUT OF the kidneys. Blood in the renal vein is deoxidized or reduced (without oxygen) and filtered by kidneys, thus it is clean.

Renal Artery: The artery that transports blood INTO the kidneys. Blood in the renal artery is full of oxygen but also full of waste (urea and salts) thus it has to be filtered.

Ureters: Carry urine (urea, excess water, excess salts) into the bladder.

Bladder: The structure, which stores urine before it is excreted out of the body.

Ring of Muscle: A ring of muscle that is kept closed before one goes to the toilet to excrete the urine. They control the passage of urine out of the body.

Urethra: The last structure from which urine passes before going out of the body.

The nephron is the structure, half inside a pyramid and the other half inside the cortex, where blood is filtered (ultra-filtered) from urea, excess water and salts.

The structure of the nephron is shown above:

Blood in the renal artery is oxygenated and with urea.

Glomerulus: A network of blood capillaries.

Selective re-absorption: Not everything is re-absorbed at once, but every tubule re-absorbs a particular nutrient.

The renal artery is wider than the blood vessel through which it moves out. This increases pressure in the glomerulus. The pressure causes some constituents of blood to leak out of the capillary tube.

The filtrate contains glucose, urea, water and salts. Proteins and Erythrocytes (red blood cells) are too large and they don't pass through the capillary walls.

This filtration takes place on a microscopic scale. It is known as ULTRAFILTRATION. This takes place in the Bowman's capsule. The First Coiled Tubule: Here, all the glucose that passed from the capillary walls to the nephron is re-absorbed. In a diabetic person, not all glucose is reabsorbed and it is found in Urine. Since each part of the nephron re-absorbs the useful nutrients one at a time, it is called a selective re-absorption.

Loop of Henle: Here some water is re-absorbed. The amount of water re-absorbed depends on the concentration of blood. If it is concentrated (has little water), a lot of water will be re-absorbed. If it is not that concentrated it will re-absorb less water. The amount of water re-absorbed also depends on a chemical called ADH (Anti-diuretic hormone). ADH is produce by the pituitary gland in the brain and causes thirst; hence, more water will be re-absorbed by the loop of henle.

When there is a lot of ADH, urine is full of waste and with relatively few water.

When ADH is not found in the blood, urine is in large amounts, very dilute (full of water) and with few waste.

Second Coiled Tubule: Here some salts (Na +, Cl-) are re-absorbed.

Collecting Duct: Here, urea, water and salts pass down the ureter into the bladder which stores urine. Urine is a mixture of urea, water and salts.

The Skin

The skin is the organ responsible for: Protection, Sensitivity, and Temperature Control (Homeostasis).

As a Protective Organ: The skin acts as a barrier against foreign bodies (germs). In some animals, it has the same colour as its surroundings (camouflage), other animals are covered in spines or produce an oil to make it water proof.

As a Sense Organ: The skin contains many receptors or sense organs (heat receptors, cold receptors, pressure receptors, pain receptors, touch receptors) and these make the skin sensitive.

As the Organ which Controls Temperature: Warm blooded animals are called Endothermic or homoeothermic (warmblooded).

This means that they have a constant body temperature. Some animals have blubber (thick fat layer) under their skin to keep warm in very cold weather (e. g. Penguins, polar bears)

Ectothermic or poikilothermic (cold-blooded) animals have their internal temperature controlled by their surroundings. In fact, some reptiles (cold-blooded animals) stay long hours in the sun to heat up their bodies.