The human digestive system. Image credit

The human digestive system. Image credit

ZIMSEC O Level Combined Science Notes: Alimentary Systems

  • Unlike plants, animals are just consumers and cannot make their own food.
  • They rely on plans as producers.
  • Most of the food eaten is made up of large insoluble molecules which need to be changed into small, soluble molecules that can be easily absorbed by the body.
  • Digestion is the process by which large food particles are converted into smaller ones.
  • Alimentary canal-the whole passage along which food passes through the body from mouth to anus during digestion.
  • The alimentary canal is the whole system of organs and structures that digests food.
  • The alimentary canal in  mammals is made up of different organs.
  • Each organ has its own function but all the organs act together and complement each other during the digestive process.
The human digestive system and some of the organs's functions. Image credit

The human digestive system and some of the organs’s functions. Image credit

  • Food is taken into the mouth.
  • Each mammal has its own way of eating food.
  • Ingestion-is the process of taking in food.
  • Digestion of food begins as soon as the food enters the mouth.
  • Different food molecules are digested at different parts of the digestive system.
  • Juices are produced in their respective organs e.g. pancreas, salivary glands and liver.
  • These juices assist with the digestive process.
  • By the time the food reaches the large intestines digestions would have been completed.
  • In the small intestine the food which has been digested is taken from the alimentary system into the bloodstream.
  • Absorption-is the process by which soluble food molecules pass from the small intestines into the bloodstream.
  • The absorbed food molecules are used for growth and repairing damaged structures.
  • Assimilation-is the process where absorbed food molecules are used for growth and repairing damaged body structures. Food molecules are re-arranged into new molecules for example to form bone, muscle, skin and other structures of the body.
  • Water is absorbed in the large intestines.
  • Not all the food eaten can be used and  some cannot even be digested at all.
  • These leftovers or waste from the alimentary system are passed out as waste through the anus.
  • This process is referred to as ingestion.
  • Different kinds of mammals have different alimentary systems.
  • The digestive system of a wow is different from say that of a lion.

The table below shows different organs and their functions in the human digestive system.

TongueMixes food with saliva and takes part in the swallowing process.
Salivary glandsProduce saliva, which moistens food and contains
TeethMasticate (chew food into small pieces) and mix food with saliva
Oesophagus/GulletTransports food from the mouth to the stomach
Stomachholds food for about an hour. Produces gastric juice which contains acid and enzyme to begin protein digestion.
LiverProduces bile, stores vitamins and mineral and regulates blood sugar.
Gall bladderStores bile. Bile emulsifies fats and oils and neutralizes stomach acids.
PancreaseProduces many enzymes for the digestion of proteins and starch and to begin the digestion of fats and oils.
Bile ductCarries bile to the duodenum.
Duodenumthe first part of the small intestine immediately beyond the stomach.
Small intestines/ileumGlands in the ileum wall produce enzymes which continue the digestion of proteins, starch, sugars, fats and oils. End products are absorbed into the blood stream through the villi.
Colon/Large intestinesAbsorbs water and salts
CeacumIn herbivores, the cecum stores food material where bacteria are able to break down the cellulose. Serves no useful purpose in humans.
RectumHolds undigested remains (faeces) before egestion.
AppendixNo useful function in humans.
AnusIndigestible matter (faeces) are passed out of the body (egestion)

To access more topics go to the Combined Science Notes page.

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