The Digestive System
Food can be used by the body only after it has been broken down into small molecules. The process by which food is changed into a form the body can use is digestion. Digestion takes place in a continous tubelike passageway that extends from the mouth to the anus. This passageway is known as the alimentary canal, or digestive tract. The alimentary canal and other organs associated with digestion make up the digestive system.
The digestive system serves two major functions. The first, of course, is digestion – the breaking down of food into molecules the body can use. The nutrient molecules must then get to the cells where they are needed, Therefore, the second major function of the digestive system is absorption. Absorption is the movement of the nutrient molecules into the blood vessels or other vessels. The blood carries these nutrients to the cells, which use the nutrients for energy, growth, and repair.
Digestion takes two forms – mechanical and chemical. Mechanical digestion is the physical tearing and grinding of food into smaller pieces. Mechanical digestion thus increases the amount of surface area of food exposed to the action of digestive enzymes. These enzymes help bring about the second form of digestion. Chemical digestion changes food particles into molecules the body can use.
Mechanical and chemical digestion both start in the mouth, or oral cavity. Food is bitten, cut, and torn by the incisors, the sharp teeth at the front of the mouth, and the teeth next to them, the canines. Strong muscles of the jaws and tongue move the food into position for chewing. Food is then crushed and ground by the broad, flat surfaces of the premolars, flat surfaces of the premolars and the molars at the rear of the mouth.
While in the mouth, food is moistened by saliva, a mixture of mucus and digestive enzyme called ptyalin, or salivary amylase. Saliva is produced by the three pairs of salivary glands. The largest of these are the parotid glands located in the cheek region. The sublingual glands are in the floor of the mouth under the tip of the tongue, and the submaxillary glands are along the lower jaw.
Saliva lubricates food so that it moves smoothly through the digestive tract. Saliva may also kill some bacteria in the mouth. Ptylian starts the breakdown of starches to glucose. However, because food remains in the mouth for such a short time, ptylian acts on less than 5 percent of the starches. The food and saliva eventually form a moist, soft ball called a bolus.
When you swallow, your tongue presses against the hard palate, the bony plate in the roof of the mouth. The pressure forces the bolus to muscle tissue called the soft palate.
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