Human Digestive System

The Human Digestive System

Ingestion:The tongue mixes food with saliva to form “bolus”.

Saliva contains:• Mucin (a glycoprotein) which is slippery, helping food

to pass down tube and protecting oral cavity.• Buffers to neutralise acids (preventing tooth decay)• Antibacterial agents• Amylase, an enzyme to breakdown starch and

glycogen.

Digestion in Humans

Food passes down the top of the oesophagus by swallowing and by gravity

The rest of the movement through the gut system is via a process called peristalsis

Peristalsis

Food is passed through the gut system by muscles contractions behind the bolus.

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The Stomach

The food passes through the cardiac sphincter valve at the top of the stomach. This valve is like a drawstring purse.

The stomach contains:• Gastric juice (Hydrochloric acid (pH 2) • Pepsin (enzyme for protein breakdown)

Processes in the Stomach

The acid chemically breaks down food to form “chyme” and also kills microbes.Mucus is secreted to form a thick lining to protect stomach.

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Food takes about 3-4 hours to completely leave the stomach after a meal.

Pyloric Sphincter

The pyloric sphincter is a valve which frequently opens as pressure from the chyme increases.

This means the small intestine works on a small but continuous supply of chyme from the stomach.

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Duodenum

From the stomach, the chyme enters the duodenum (first part of small intestine).

Its function is to digest foods.

The Liver and the Pancreas

The liver and the pancreas supply bile and pancreatic juices to the duodenum through a common duct.

Protein digestion is completed by the pancreatic juice (which contains the enzymes; trypsin, chymotrypsin and carboxypeptidase).

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Other enzymes are supplied by the pancreas and the intestinal glands for carbohydrate breakdown (e.g. Amylase and maltase). Cellulose is undigested.

Other Secretions

Fat is emulsified by bile salts from the gall bladder (which is supplied by the liver).

Lipase from the pancreas breaks down fat molecules into fatty acids and glycerol.

Bile and intestinal glands secrete alkaline liquids which neutralise the stomach acids.

Absorption in Humans

The Ileum is the second and largest part of the small intestine. Its function is to absorb materials.

Efficient absorption of food molecules occurs in the ileum because of its very large surface area from finger-like protrusions on the wall of the ileum called Villi.

Villi

Glucose and amino acids are absorbed into the blood by active transport, which is one reason why the gut uses more energy after a meal.

These molecules pass into the blood capillaries within each villus

Transportation by Villi

Blood is taken from villi by hepatic portal vein to the liver where the food molecules are:• Altered for storage• Detoxified• Further broken down for cellular use

Fatty acids pass into villi and along lacteal vessels within villi to connect with vessels of the lymph system.

Egestion in Humans

Indigestible food (not absorbed) passes into the large intestine (colon) as faeces.

In the colon, faeces are formed and water, enzymes and minerals are reabsorbed into the blood vessels.

Faeces is stored in the rectum and egested through anus via anal sphincter.