We may feel like we're getting older, but many of our most important body parts are constantly regenerating themselves -- some regularly, others over a period of years.

And what that means is certain parts of our body are a good deal younger than we are.

Studies into cell renewal are still ongoing and more is being learned all the time, but here's what we know so far:

Hair

Hair grows at a rate of about half an inch per month, and goes through three distinct stages, which can last a total of six years in women and three years in men.

Most of this time is taken up by growth, and at any one time 90 percent of the hair on our head are growing. Over the final six months the hair starts to loosen, and then finally it is pushed out by a new follicle.

In this way, your hair is never more than six years old at most.

Heart

The heart is one area in which studies into cell regeneration have produced new findings.

It used to be assumed that the heart wasn't capable of regenerating itself, but now it's known that heart tissue is indeed replaced.

How long this takes is still the subject of study, but it's known to be less than 20 years.

Added to that, the heart can repair some of the damage caused by a heart attack.

Lungs

We all put our lungs under a great deal of strain, even if we're not smoking. Breathing in the pollutants from everyday life leads to cells on the outermost levels of our lungs replacing themselves every few days.

Deeper tissue, meanwhile, remains in place for longer, but is still regenerated over the course of a year, although that doesn't mean we can't do ourselves permanent damage with things such as cigarettes.

Stomach

The stomach acid which helps us digest food is strong stuff, meaning that our stomachs also need to be strong to cope with it. For this reason, our stomachs are lined with mucus which replaces itself at least every five days.

But smoking can slow this and lead to stomach ulcers as the lack of mucus will allow acid to attack the stomach wall.

Intestines

Just as our stomachs have to work hard, so too do our intestines, which have the job of absorbing much of the nutrients from what passes through our system. To help with this, our small intestines contain tiny projections which increase the area available for nutrients to be absorbed, and the tips of them are replaced by our bodies every couple of days.

Skin

Nowhere is the body's regenerative power more obvious than on its biggest organ -- the skin -- which has an ability to heal all manner of scratches, scrapes, cuts and burns.

The outer later of the skin, the epidermis, is already dead, and hardens to protect the living layers underneath, but it is still replaced in a two-week cycle thanks to everything we expose it to.

Bones

Anyone who has ever broken a bone will know that it can take a long time for it to knit back together, but it will get there eventually.

As with other types of cells, our bones are continually being replaced in order to keep them strong and young, but the process takes about 10 years in total.

Liver

The liver carries out a surprising number of vital functions for our body, as well as the detox function we all know about. It combats infections and controls levels of things such as fats, amino acids and glucose.

Because of this, liver cells are replaced every six months, but this doesn't mean we can't do the liver damage.

Things such as heavy drinking will cause cirrhosis, which is permanent scarring.

Tongue

Our taste buds are groups of cells on the surface of the tongue which send messages to the brain about what we're eating and drinking. We have about 10,000 taste buds, each made up of about 50 cells.

These cells intentionally destroy themselves so they can be replaced by new ones every 10 days or so, and in this way keep our tongues sensitive to the different things we eat and drink.

Blood

As we're able to give a pint of blood every 12 weeks or so, you could deduce that this is how long it takes blood to replace itself.

Indeed, the life-span of our red blood cells, which carry oxygen around the body and remove waste, is three or four months, before finally being destroyed in the spleen.