Intake of raw materials and elimination of wastes is essential for the normal functioning of the body. The body takes oxygen, water and food from the environment and excretes carbon dioxide, urine and feces.
Intake of oxygen
The atmosphere contains about 21% oxygen by weight. Oxygen is essential for life because it is needed for almost all chemical reactions that take place in the body. Oxygen is required for the release of energy from nutrients.
The respiratory system is responsible for the intake of oxygen and excretion of carbon dioxide. The upper respiratory system transports air between the nose and the lungs. Air moves through a system of passages including the pharynx, larynx, trachea, two bronchi and bronchial passages. The lungs contain millions of tiny air sacs known as the alveoli. The alveoli are surrounded by networks of capillaries and act as the site where gaseous exchange takes place.
The atmosphere also contains about 80% nitrogen by weight. This nitrogen is breathed in and out but is not used by the body directly. The nitrogen required by the body is obtained through food.
Intake of nutrients
Proper diet is essential for a healthy body. It provides nutrients i.e. substances which are absorbed after digestion and promote body functions. Nutrients include water, carbohydrates, proteins, fats, vitamins and minerals. Nutrients perform the following functions:
- Provide energy
- Act as raw materials for the synthesis of larger and complex molecules
- Help in cell growth and repair
- Maintain water balance in the body
Carbohydrates and fats are the main sources of energy. If these are in short supply, proteins are broken down to release energy.
Digestion is the process by which food is broken down into simpler molecules for absorption and use by the body. The digestive system consists of the alimentary canal and accessory glands.
The alimentary canal begins at the mouth and continues through the pharynx, esophagus, stomach, intestines, rectum and anus.
Some glands (salivary glands, pancreas and liver) are situated outside the alimentary canal. These secrete enzymes into the alimentary canal through ducts. There are also many tiny glands located in the walls of the alimentary canal. Digestive enzymes secreted by the glands help in the chemical breakdown of the food. Some glands also secrete mucus which provides lubrication.
Metabolism can be defined as the sum total of all chemical reactions taking place in the body. Anabolism refers to chemical reactions by which larger and complex molecules are synthesized. Catabolism refers to breakdown of complex molecules to release energy, raw materials for anabolism and wastes which are excreted out of the body.