THE HUMAN DIGESTIVE SYSTEM
The digestive system of man consists of the following parts:
- The alimentary canal
- Accessory organs
- Liver
- Pancreas
Parts of alimentary canal
- Mouth
- Oesophagus
- Stomach
- Small intestine
- Duodenum
- Ileum
- Large intestine
- Colon
- rectum
- Appendix
- Anus
Digestion is the process of breaking large food particles into small parts which are easy to swallow, absorbable and utilizable.
The process of digestion has the following phases:
- Ingestion
- Digestion
- Absorption
- Utilization
- Egestion
Digestion in the mouth
Food is first ingested, and then digestion starts. Two types of digestion take place in the mouth.
Mechanical digestion – the mechanical digestion in the mouth is by chewing or MASTICATION.
Importance of chewing
- Reduces large food particles to smaller particles which are easy to swallow.
- Mixes the food with SALIVA which contains the digestive enzymes and MUCIN (substance which makes food slippery).
- Increases the surface area of the food for better enzyme activity.
Role of saliva in digestion
- Contains an enzyme called PTYALIN or SALIVARY AMYLASE which digests cooked starch.
- Moistens the food.
- Contains MUCIN which makes the food slippery for easy swallowing.
- Dissolves the chemicals taken in the mouth. The chemicals to reach the taste buds of the tongue for tasting.
- Makes the food alkaline.
Role of the tongue in digestion.
- Tasting the food
- Mixes the food with saliva
- Rolls the food into small balls called BOLI for easy swallowing
- Pushes the food into the PHARYNX for swallowing. Pharynx is a cavity behind the mouth.
- Chemical digestion
The enzymes involved in digestion are SALIVARY AMAYLASE. Salivary amylase helps in the hydrolysis of cooked starch into maltose. It acts in alkaline conditions.
Only cooked starch starts its digestion in the mouth.
Digestion in the stomach.
A stomach is a muscular bag which performs the following functions:
- Temporarily stored food
- Churns and mixes the food – mechanical digestion
- Carries out chemical digestion of protein
- Produces gastric juice
- Absorbs water and alcohol
- Produces mucus
Once food reaches the stomach
- The digestion of cooked starch stops since the stomach contents are acidic.
- Digestion of protein starts. The wall of stomach has gastric glands which produce GASTRIC JUICE.
Composition of GASTRIC JUICE.
- Hydrochloric acid
- Renin
- Pepsin
- Mucus
ROLES OF GASTRIC JUICE
The components of the gastric juice perform the following roles:
A) Roles of hydrochloric acid
- Kills the micro-organisms that come along with the food/water.
- Converts the inactive form of pepsin (i.e. the precursor) into the active form.
- Converts the inactive forms of the renin into the active form.
- Breaks down complex sugars into simple sugars.
B) Roles of renin
Renin converts the insoluble forms of milk protein into soluble form of milk protein.
C) Roles of pepsin
Pepsin speeds up the hydrolysis of proteins into peptides.
Protein + water ————-> Peptides
pepsin
D) Roles of mucus
Mucus prevents:
Corrosion of the stomach by HCl.
Digestion of the stomach wall by pepsin. The food is well mixed by churning into a semi solid acidic food mixture called CHYME.
The chyme can be stored in the stomach for variable times depending on the nutrient type.
- Water can be stored for just a few minutes.
- Carbohydrates can be stored for up to 4 hours.
- Proteins and lipids can be stored for 4 to 6 hours. The chime moves into the duodenum by the periodic opening and closing of the pyloric sphincter.
Digestion in the Duodenum
The duodenum is a 30cms long portion of the small intestine. It is connected to the stomach by the pyloric sphincter. Two ducts open into the DUODENUM.
Duodenum and its accessory organs
The bile duct- bile duct transports bile from the gall bladder into the duodenum. Bile is produced in the liver and stored in the gall bladder.
Composition of bile
Bile is greenish yellow juice. It consists of:
- Water
- Greenish yellow substance
- Bile salts
- Mucin
- Bi carbonates
Bile is periodically released into the duodenum every time food entersin.
Role of bile
- Bile salts react with the fat soluble vitamins to make them water soluble.
- Bile salts carry out EMULSIFICATION. Emulsification increases the surface area upon which the enzymes act.
- Bile salts convert the inactive form of enzymes form into active form.
- The sodium bicarbonate salts neutralize the HCL and changes the food mixture into Alkaline chyme which makes it suitable medium of enzyme activity in the DUODENUM.
The pancreatic duct
The pancreatic duct connects the pancreas to the duodenum. It transports the pancreatic juice from the pancreas to the duodenum. Pancreatic juice is made in the pancreas.
Composition of pancreatic juice
- Pancreatic amylase
- Pancreatic lipase
- Trypsin
- Bicarbonate salts
Role of pancreatic juice
- The bicarbonate makes the duodenum contents alkaline
- Pancreatic amylase speeds up the hydrolysis of starch into maltose
- Pancreatic lipase hydrolyses lipids into fatty acids and glycerol.
- Trypsin speeds up the hydrolysis of proteins into peptides.
Trypsin has a precursor called trypsinogen which is converted into active form by an enzyme called ENTEROKINASE.
Digestion of protein continues in the duodenum. Digestion of starchy cooked starch starts in the duodenum. Digestion of lipids starts and ends in the duodenum. The food mixture in the duodenum becomes watery and alkaline mixture called CHYLE.
The chyle moved into the ileum by peristalsis.
Role of mucin/mucus
- Protects against self digestion
- Makes food slippery
- Prevents corrosion of alimentary canal
Digestion in the ileum
The ileum is a 3 meter long portion of the small intestine. It connects the duodenum and the large intestine. The intestinal wall produces an intestinal juice called succus/ entericus. The ileum is a center for digestion and absorption.
Composition of succus/entericus
- Erepsin
- Maltase
- Sucrose
- Lactose
- Water
- Bicarbonate salts
Role of succus/entirecus
Intestinal juice- has the above enzymes which performs the following roles.
- Erepsin completes the hydrolysis of proteins into amino acids.
- Maltose completes digestion of starch into glucose
- Sucrose hydrolyses sucrose into glucose and fructose.
- Lactose hydrolyses lactose into glucose and galactose.
End products of digestion
- Fatty acids and glycerol
- Glucose
- Fructose
- Galactose
- Amino acids
Fate of the end products of digestion
- Fatty acid and glycerol.
The fatty acid and glycerol are combined to make fats and oils. The fats and oils may be converted to carbohydrate for energy production.
- Glucose, fructose and galactose
Glucose is directly used for energy production. Fructose and galactose are converted into glucose for energy production. If in excess, glucose is converted to fats and stored under the skin.
- Amino acids
The amino acids are combined to form proteins. Proteins are used for body building and repair. If in excess the amino acids are deaminated by the liver. The amino parts is converted into urea by the liver and removed as a metabolic waste product in urine through kidneys. The carbonyl part is converted into fats and stored.
Adaptations of the ileum for its function
- It is long to ensure a complete digestion of the foods and a complete absorption of the end products of digestion.
- The internal surface has finger like projections called villi which increases the surface area for absorption.
- The inner layer is made up of a single layer of cell to reduce the distance through the end products pass into the blood.
- Each villus is provided with a dense network of blood capillaries and lymphatic vessels called lacteal. The vessel increases the rate of absorption by immediately transporting the product away from the site of absorption. The blood capillaries join to form the hepatic digestion to the liver.
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