DIGESTIVE SYSTEM AND DIGESTION IN HUMANS
Organs that are involved with feeding in humans constitute the digestive system
Table of Contents
Digestive System and Associated Glands
Human digestive system starts at the mouth and ends at the anus. This is the alimentary canal. Digestion takes place inside the lumen of the alimentary canal. The epithelial wall that faces the lumen has mucus glands (goblet cells). These secrete mucus that lubricate food and prevent the wall from being digested by digestive enzymes. Present at specific regions are glands that secrete digestive enzymes. The liver and pancreas are organs that are closely associated with the alimentary canal. Their secretions get into the lumen and assist in digestions
Digestive system consists of:
- Mouth
- Oesophagus
- Stomach
Small intestines
– consist of duodenum, the first part next to the stomach, ileum – the last part that ends up in a vestigial caecum and appendix which are non¬functional
Large intestines
Consist of: colon and rectum that ends in the anus
Ingestion, Digestion and Absorption
- Feeding in humans involves the following processes:
- Ingestion: This is the introduction of the food into the mouth
- Digestion: This is the mechanical and chemical breakdown of the food into simpler, soluble and absorbable units
- Absorption: Taking into blood the digested products
- Assimilation: Use of food in body cells
- Mechanical breakdown of the food takes place with the help of the teeth
- Chemical digestion involves enzymes
Digestion in the Mouth
In the mouth, both mechanical and chemical digestion takes place Food is mixed with saliva and is broken into smaller particles by the action of teeth Saliva contains the enzyme amylase
It also contains water and mucus which lubricate and soften food in order to make swallowing easy Saliva is slightly alkaline and thus provides a suitable pH for amylase to act on cooked starch, changing it to maltose The food is then swallowed in the form of semisolid balls known as boluses
Each bolus moves down the oesophagus by a process known as peristalsis Circular and longitudinal muscles along the wall of the alimentary canal contract and relax pushing the food along
Digestion in the Stomach
In the stomach, the food is mixed with gastric juice secreted by gastric glands in the stomach wall Gastric juice contains pepsin, rennin and hydrochloric acid The acid provides a low pH of 1.5-2.0 suitable for the action of pepsin
Pepsin breaks down protein into peptides Rennin coagulates the milk protein casein The stomach wall has strong circular and longitudinal muscles whose contraction mixes the food with digestive juices in the stomach
Digestion in the Duodenum
In the duodenum the food is mixed with bile and pancreatic juice Bile contains bile salts and bile pigments The salts emulsify fats, thus providing a large surface area for action of lipase Pancreatic juice contains three enzymes:
Trypsin which breaks down proteins into peptides and amino acids, Amylase which breaks down starch into maltose, and Lipase which breaks down lipids into fatty acids and glycerol These enzymes act best in an alkaline medium which is provided for by the bile
Digestion in ileum
Epithelial cells in ileum secrete intestinal juice, also known as succus entericus This contains enzymes which complete the digestion of protein into amino acids, carbohydrates into monosaccharides and lipids into fatty acids and glycerol
This is the diffusion of the products of digestion into the blood of the animal It takes place mainly in the small intestines though alcohol and some glucose are absorbed in the stomach
The ileum is adapted for absorption in the following ways:
It is highly coiled The coiling ensures that food moves along slowly to allow time for its digestion and absorption It is long to provide a large surface area for absorption The epithelium has many finger-like projections called villi (singular villus) They greatly increase the surface area for absorption
Villi have microvilli that further increase the surface area for absorption The wall of villi has thin epithelial lining to facilitate fast diffusion of products of digestion. Has numerous blood vessels for transport of the end products of digestion Has lacteal vessels; for absorption of fatty acids and glycerol and transport of lipids
Absorption of Glucose and Amino Acids
Glucose and other monosaccharaides as well as amino acids are absorbed through the villi epithelium and directly into the blood capillaries First they are carried to the liver through the hepatic portal vein, then taken to all organs via circulatory system
Absorption of Fatty Acids and Glycerol
Fatty acids and glycerol diffuse through the epithelial cells of villi and into the lacteal When inside the villi epithelial cells, the fatty acids combine with glycerol to make tiny fat droplets which give the lacteal a milky appearance The lacteals join the main lymph vessel that empties its contents into the bloodstream in the thoracic region Once inside the blood, the lipid droplets are hydrolysed to fatty acids and glycerol
Absorption of Vitamins and Mineral Salts
- Vitamins and mineral salts are absorbed into the blood capillaries in’ the villi
Water is mainly absorbed in the colon
- As a result the undigested food is in a semi-solid form (faeces) when it reaches the rectum
Egestion: This is removal of undigested or indigestible material from the body
Faeces are temporarily stored in the rectum then voided through the anus
Opening of the anus is controlled by sphincter muscles
Assimilation: This is the incorporation of the food into the cells where it is used for various chemical processes
Carbohydrates
It used to provide energy for the body Excess glucose is converted to glycogen and stored in the liver and muscles Some of the excess carbohydrates are also converted into fat in the liver and stored in the adipose tissue’ (fat storage tissue), in the mesenteries and in the connective tissue under the skin, around the heart and other internal organs
Proteins
Amino acids are used to build new cells and repair worn out ones They are also used for the synthesis of protein compounds Excess amino acids are de-aminated in the liver Urea is formed from the nitrogen part The remaining carbohydrate portion is used for energy or it is converted to glycogen or fat and stored
Lipids
Fats are primarily stored in the fat storage tissues When carbohydrates intake is low in the body, fats are oxidised to provide energy They are also used as structural materials e.g. phospholipids in cell membrane They act as cushion, protecting delicate organs like the heart. Stored fats under the skin act as heat insulators
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