Digestive System

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Function of the Digestive system

The function of the digestive system is to break down the food we eat into something small enough to be absorbed into the blood & then transported to the cells of the body.

The end product of carbohydrate digestion is glucose & glycogen. The end product of fat digestion is fatty acids & glycerol. The end product of protein digestion is amino acids.

The key digestive enzymes produced by different parts of the digestive system are :-

Protease (breaks down protein)

Lipase (breaks down fat)

Amylase (breaks down carbohydrate)

Other key digestive substances are :-

Hydrochloric acid produced by the stomach

Bile produced by the liver & stored in the gall bladder

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Oral Cavity

This is where digestion starts.

Food/drink is chewed and then swallowed.

Saliva is produced by the salivary glands

in the oral cavity.  It contains salivary 

amylase which is an enzyme that starts 

to break down long carbohydrate molecules into shorter ones.

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A muscular tube going from the 

back of the throat (pharynx) to the stomach.

It’s function is to carry chewed  food to the stomach.

Peristaltic waves of muscular  contraction 

occur along it.

The lining of the oesophagus secretes mucus to help food pass along it.

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The stomach is a muscular sac that moves to  mix food and digestive chemicals /enzymes. 

It transfers food into the small intestine by waves of squeezing contractions called peristaltic waves. This action creates a mixture called chyme. 

The stomach contracts violently during vomiting. It absorbs, sugars, water & alcohol.

It produces Hydrochloric acid (also refered to as gastric acid) which activates digestive enzymes.

The lining of the stomach secretes mucus which protects it from being damaged by the hydrochloric acid produced there.

It secretes pepsin to digest protein.

It stores food .

The stomach has a sphincter (ring of muscle) at either end. The sphincter at the top of the stomach is the cardiac sphincter and the one at the bottom is the pyloric sphincter.

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Small Intestine

  • Approx. 7m long
  • It digests & absorbs fats & lipid and water.
  • It absorbs monosaccharides/electrolytes
  • Contains bowel flora that aid digestion
  • Secretes enzymes

Divided into 3 parts :-

  • The duodenum
  • The jejunum
  • The ileum

The duodenum produces large amounts of sodium bicarbonate to completely neutralize any gastric acid that has passed further down into the digestive tract from the stomach. The inner wall of the small intestine is covered with finger-like projections called villi which massively increase the surface area. The function of the small intestine is the digestion of food and the absorption of nutrients released during digestion.  

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The liver is just below the diaphragm and divided into the L&R lobes.

It produces bile which plays a key role in the digestion of fats.

The bile produced by the liver is stored in the gall bladder and then released into the small intestine after a meal.

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Gall Bladder

The gall bladder is a pear shaped sac attached to the 

back of the liver.  It empties into the duodenum 

(part of the small intestine) via the bile duct.

The main purpose of the gall bladder is to store and 

concentrate bile.

Bile is a liquid produced by the liver, which helps digest fats. It is 

passed from the liver through a series of channels, known as bile ducts, into the gall bladder.

The bile is stored in the gall bladder and over time it becomes more concentrated, which makes it better at digesting fats. The gall bladder is able to release bile into the digestive system when it is needed.

It empties bile into the duodenum by contracting.

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Is a gland behind the stomach between the duodenum & the spleen.

It delivers pancreatic juices to the small intestine.

The islets of Langerhans produce insulin.

The pancreas is an important part of the endocrine and digestive systems.

Insulin regulates blood sugar by causing conversion of glucose to glycogen for storage in the muscles & liver.

The pancreas also produces several enzymes which break down food :-

Lipase – breaks down fats

Amylase – breaks down starch

Trypsin – breaks down protein

Diabetes is caused by a lack of insulin 

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Large Intestine

Approx. 1.5m long &

Drapes around the small intestine

By the time food reaches the large 

intestine all nutrients have been absorbed in the small intestine.

The function of the large intestine is to re-absorb any water from the undigested food before expelling it 

from the body through defacation.

There are 4 parts to the large intestine :-

  • The ascending colon
  • The transverse colon
  • The descending colon
  • The sigmoid colon
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After excess water is absorbed back into the body in the large intestine, all that is left is undigested food.  This is stored in the rectum, which is the lower part of the large intestine.

The rectum moves the contents by muscular waves and forms remaining waste into faeces where it is stored priorto defecation.

When we are ready to have a bowel movement faeces is moved from the rectum to the anus. 

The rectum secretes mucus to help move the faces along.

This process is called egestion.

Faeces – is made up of indigestible things like cellulose, dead blood cells, bacteria, fatty acids and mucus.  It gets it’s colour from dead blood cells & bilirubin (a bile pigment)

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Digested food molecules are absorbed in the small intestine.

The villi in the small intestine massively increase the surface area to maximise the amount of absorption that can occur.  The villi are folded even further to form microvilli.

The walls of the microvilli are only one cell thick and each has its own capillary blood supply.

Digested food molecules pass through the walls of the micro villi into the bloodstream.

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