Homeostasis (homeo means steady, stasis means state) is a phenomenon in which the body regulates its functions to keep the internal conditions as stable as possible. Homeostasis is necessary because the body cells need to have suitable conditions around them for proper functioning. These conditions include, the presence of proper concentration of chemicals, proper temperature, and a suitable pH (degree of salinity or acidity), etc. inside its cells. But these conditions inside our body as well as inside other organisms keep fluctuating within a narrow range. Tolerance to any change from this range differs in different organisms. Organisms adopt a variety of measures to cope with such changes.

Example 1: Drinking water and keeping a steady water balance

In all kinds of weather, your blood and other body fluids must maintain a particular percentage of water. If the volume of water in the body tends to rise, the excess is passed out in urine and, if it tends to fall short, more water is withheld inside the blood to the extent required. 

In hot summers you feel thirsty at regular intervals. You drink lots of water or even cold drinks, yet you do not urinate much. The urine passed out is more concentrated. This is because during hot weather you lose more water through perspiration but your body needs to maintain its requisite amount of water and so the water is withheld within, by passing out only little and concentrated urine. In cold winters you do not feel much thirsty. You do not drink large quantities of water. But, may be, you take more of hot drinks only to keep warm. During
such days you urinate more frequently and the urine passed out is more dilute.

Example 2: Eating sugar and keeping steady sugar level in blood

Suppose you have been consuming too much sugar in food, beverages and sweets. Presuming you are otherwise normal, your body will handle the excess sugar (more than the normal percentage in the blood) by storing it in the form of glycogen in the liver. 

At some other time, when you are fasting or doing much physical work, your blood sugar is used up rapidly. At that time, the liver converts the stored glycogen back into its usable form, that is glucose, to fill the gap and restore the normal blood sugar level.

Example 3: Maintaining normal steady state of blood alkalinity

Sometimes you eat too much salt (sodium chloride) in your food. But your blood normally maintains only the particular level of alkalinity (pH 7.34-7.43) which is only slightly alkaline. Any extra salt consumed is passed out through urine as it cannot be stored in the body.

If at some other time you have been eating too little salt, or you are losing much of it through sweating, your kidneys will hold back the required quantity through sodium-potassium balance.

Example 4: Managing the number of red blood cells

A normal human adult possesses about 5 million red blood corpuscles (RBCs) per cubic millimetre of blood. Whenever a plain-dweller visits a hill station at high altitude without any breakjourney in between, he is likely to feel exhausted for a couple of days. Later, the person becomes normal.

At high altitudes the atmospheric pressure is lower and the amount of oxygen carried by this normal number of RBCs is insufficient. Within a day or two, the body adds more RBCs into the blood to pick up the normal required quantity of oxygen.

When the same person returns to the plains at a lower altitude the higher RBC level that was acquired at the hills now begins to take up oxygen in excess, which is harmful. The body readjusts the red blood cells which get reduced in number to become stable at the original level.

Example 5: Warming and cooling of the body (maintaining steady body temperature)

During hot summers you wear light clothes. You perspire a lot, you sit under a fan or under a tree and feel comfortable. Your body is trying to cool against the higher temperature outside.

Then, there is the reverse side, that is, cold winter. Inspite of wearing thick warm clothes you still feel cold. In mid-daytime, you go out in the open sunshine to warm yourself. At night, you cover yourself with a thick blanket. You are doing all this to maintain warmth inside steadily your body.

In both the above situations, you are trying to regulate your internal body temperature. This is called thermoregulation.