HEALTHY LIVING
Our skeleton is made of more than 200 bones. Calcium and other minerals make the bone strong but slightly flexible. Bone is a living tissue with a blood supply. It is constantly being dissolved and laid down, and it can repair itself if a bone is broken. Exercise and a balanced diet are important for a healthy skeleton.
The skeleton has three main functions:
- to support the body
- to protect some of the vital organs of the body
- to help the body move.
Support
The skeleton supports the body. For example, without a backbone we would not be able to stay upright.
Protection
Here are some examples of what the skeleton protects:
- the skull protects the brain
- the ribcage protects the heart and lungs
- the backbone protects the spinal cord.
Movement
Some bones in the skeleton are joined rigidly together and cannot move against each other. Bones in the skull are joined like this. Other bones are joined to each other by flexible joints. Muscles are needed to move bones attached by joints
Muscles work by getting shorter. We say that they contract, and the process is called contraction.
Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.
Muscles can only pull and cannot push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. The problem is solved by having muscles in pairs, called antagonistic muscles.
Biceps and triceps
The elbow joint lets our forearm move up or down. It is controlled by two muscles, the biceps on the front of the upper arm, and the triceps on the back of the upper arm:
- when the biceps muscle contracts, the forearm moves up
- when the triceps muscle contracts, the forearm moves down.
This solves the problem. To lift the forearm, the biceps contracts and the triceps relaxes. To lower the forearm again, the triceps contracts and the biceps relaxes.
The human respiratory system contains the organs that allow us to get the oxygen we need and to remove the waste carbon dioxide we don't need. It contains these parts:
- lungs
- tubes leading from the lungs to the mouth and nose
- various structures in the chest that allow air to move in and out of the lungs.
Ventilation
Movements of the ribs, rib muscles and diaphragm allow air into and out of the lungs. Take care - this is called breathing or ventilation, not respiration. When we breathe in, we inhale. When we breathe out, we exhale.
Air passes between the lungs and the outside of the body through the trachea or windpipe. The trachea divides into two bronchi, with one bronchus for each lung. Each bronchus divides further in the lungs into smaller tubes called bronchioles. At the end of each bronchiole, there is a group of tiny air sacs called alveoli.
We need to get oxygen into the blood from the air, and we need to remove waste carbon dioxide from the blood into the air. Moving gases like this is called gas exchange. The alveoli are adapted to make gas exchange in lungs happen easily and efficiently.
Here are some features of the alveoli that allow this:
- they give the lungs a really big surface area
- they have moist, thin walls (just one cell thick)
- they have a lot of tiny blood vessels called capillaries.
The gases move by diffusion from where they have a high concentration to where they have a low concentration:
- oxygen diffuses into the blood, from the air in the alveoli
- carbon dioxide diffuses from the blood, into the air in the alveoli.
Aerobic respiration
The glucose and oxygen react together in the cells to produce carbon dioxide and water. The reaction is called aerobic respiration because oxygen from the air is needed for it to work.
Here is the word equation for aerobic respiration:
glucose + oxygen = carbon dioxide + water (+ energy)
(Energy is released in the reaction. We show it in brackets in the equation because energy is not a substance.)
Carbon dioxide from cells to the air
The carbon dioxide produced during respiration diffuses out of the cells and into the blood plasma. The blood carries it to the lungs. It then diffuses across the walls of the alveoli and into the air, ready to be exhaled.
Interactive Animations - joints, muscles, breathing
A lesson on Breathing and Respiration