Digestive System

 

Introduction

Learn all about the human digestive system. 

There are a total of 4 learning objectives, each learning objective contains information and questions to help prepare you for the upcoming lesson.

 

 

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1. Describe the functions and process of the digestive system

2 Major Functions of the Digestive system

= For Therapists =

It is essential for therapists to have a good knowledge of the process of digestion in order to understand how the body utilises nutrients for efficient and healthy body functioning.

Understanding the structure of the digestive system and its links with the parasympathetic nervous system can also help therapists to understand the link between digestive disorders and stress.

The alimentary tract is a long continuous muscular tube extending from?

Alimenta

Digestion is The breakdown and transformation of solid and liquid food into microscopic substances. These substances are then transported by the blood into different areas of the body.

There are four stages of digestion:

  1. Mouth: ingestion (the taking in of food or liquid into the body), chewing and swallowing; start of starch digestion
  2. Stomach: mixing and protein digestion
  3. Small intestine: carbohydrate and fat digestion; absorption
  4. Large intestine: waste and excretion.

  • mouth to anus
  • stomach to anus
  • small intestine to anus
  • large intestine to anus

Which of the following completes digestion?

1. Ingestion

The act of taking food into the alimentary canal through the mouth.

2. Digestion

Mechanical digestion – known as mastication. the breaking down of solid food into smaller pieces by the chewing action of the teeth. the churning action of the stomach assisted by peristalsis.

Chemical digestion – the breakdown of carbohydrates, proteins and fats into smaller ones by the action of digestive enzymes.

3. Absorption

The movement of soluble materials out through the walls of the small intestine. Nutrients are absorbed through the villi and pass out into the network of blood and lymph vessels.

4. Elimination/defecation

This is the expulsion of the semi-solid waste called faeces through the anal canal.

  • mouth
  • small intestine
  • stomach
  • large intestine

Peristalsis

Trypsin is an enzyme produced by what?

WHAT ARE CARBOHYDRATES?

Carbohydrates are classified as monosaccharides, disaccharides or polysaccharides. Monosaccharides include fructose in fruit.

Disaccharides include lactose in milk. Polysaccharides include starch and fibre in cereals, potatoes and other plant sources, and glycogen in meat. All carbohydrates are broken down to monosaccharides for absorption and all eventually become glucose to supply the body with energy.

Carbohydrates are broken down in the body by the following processes:

  • In the mouth, salivary amylase begins the breakdown of polysaccharides
  • In the small intestine, intestinal amylase breaks down polysaccharides to disaccharides
  • In the small intestine, maltase, lactase and sucrase convert disaccharides to monosaccharides ready for absorption.

WHAT ARE PROTEINS?

Proteins foods include dairy products, meat, fish and beans. They are made up of interlinked polypeptide chains and are the building material for the body. In order to be used by the body they must be broken down into their smaller components – amino acids.

There are approximately 20 amino acids classified by whether they are essential (those the body cannot make, that must therefore be supplied in the diet) and non-essential (those the body can make).

Proteins are broken down in the body by the following processes:

  • In the stomach, the enzyme pepsin begins the digestion of proteins in the stomach by breaking them down into large polypeptides.
  • In the small intestine, enzymes from the pancreas, including trypsin and chymotrypsin, break the large polypeptides into smaller chains.
  • Finally, still in the small intestine, enzymes from the intestine, including aminopeptidase, breaks up the small polypeptides into individual amino acids ready for absorption.

WHAT ARE FATS?

Fats are classified into saturated, monounsaturated or polyunsaturated categories. Saturated fats can be found in dairy products and meat. Monounsaturated fats can be found in olive oil and avocados. Polyunsaturated fats can be found in sunflower oil and oily fish. Some polyunsaturated fats cannot be made by the body and are therefore also classified as essential fats and must be consumed in the diet. In order to be used by the body, fats must be broken down to fatty acids and glycerol.

Fats are broken down in the body by the following processes:

  • In the small intestine, fat are emulsified by bile salts from the liver (i.e. turned into liquid form and carried in another liquid – bile)
  • In the small intestine, lipase from the pancreas breaks down emulsified fats into fatty acids and glycerol ready for absorption

  • duodenum
  • gall bladder
  • liver
  • pancreas

2. Describe the structure and functions of the mouth and stomach associated with digestion

What does salivary amylase commences?

What is Saliva?

Saliva is a liquid secreted by three pairs of salivary glands

  • The parotid gland (situated below the ear)
  • The submandibular gland
  • The sublingual gland (both situated below the tongue).

It contains water, mucus and the enzyme salivary amylase.

Saliva has three functions:

  • to lubricate the food with mucus, making it easier to swallow
  • to start digestion: it contains the enzyme salivary amylase, which acts on cooked starch turning it into shorter polysaccharides
  • to keep the mouth and teeth clean.

  • protein digestion
  • fat digestion
  • carbohydrate digestion
  • breakdown of roughage

Tongue

The tongue has three digestive functions:

  • Taste: the tongue is covered with thousands of taste buds. They help us enjoy what we eat and drink and act as the first line of defence, warning us when food, drink or foreign matter are off or inedible.
  • Chewing: the tongue aids chewing by moving food around the mouth, pushing it between the teeth and covering it with saliva, which contains enzymes that start the digestive process. The food is turned into a partially digested mass known as a bolus.
  • Swallowing: when the food is ready to travel to the stomach, the tongue pushes it to the back of the mouth.

WHAT IS THE TONGUE?

Structure: the tongue is a muscular organ, covered with a membrane. It is held in place by attachments to the mandible (lower jaw) and the hyoid bone. Tiny projections known as papillae cover the top, increasing its surface area and producing a rough texture. Sensory nerve endings in the papillae form what we commonly know as taste buds.

Epiglottis

Epiglottis

A small flap of cartilage which forms part of the larynx (the windpipe) moves upwards and forwards, blocking the entrance to the larynx. This stops the food from ‘going down the wrong way’ and prevents choking. 

Via the action of swallowing and through the portion of the gastrointestinal tract known as the oesophagus. The tongue pushes the bolus to the back of the mouth, towards the pharynx, a muscular tube behind the mouth. The food passes into the pharynx and down to the oesophagus. 

Oesophagus

Function: to carry chewed food from the pharynx to the stomach. Food moves along it by a muscular contraction known as peristalsis. The muscle fibres contract and relax which acts like a wave on the tube, pushing the bolus forwards. The lining of the oesophagus secretes mucus to ease and lubricate the passage of food.

Stomach

The stomach is a J-shaped, elastic organ which expands and contracts depending on what is in it. Food enters it from the oesophagus via the cardiac sphincter, a valve that stops back flow of the stomach’s contents, and leaves it through the pyloric sphincter into the duodenum, the first part of the small intestine.

The wall of the stomach is a combination of layers of muscle fibre with an inner mucous membrane. The latter has lots of folds, called rugae. When the stomach is full they stretch out, enabling expansion, then they contract when it empties.

Functions:

  • Digests proteins through the action of enzymes
  • Churns food with gastric juices
  • Helps to lubricate the food by producing mucus (from the mucous membrane)
  • Absorbs alcohol
  • Kills bacteria by producing hydrochloric acid.
  • Storage of food prior to it passing to the small intestine

What are the main constituents of gastric juice?

Gastric Juices contain:

  • Hydrochloric acid: neutralises bacteria and activates pepsin
  • Rennin: enzyme that curdles milk protein (only in infants)
  • Pepsin: enzyme that acts on proteins turning them into polypeptides.
  • Mucus: protective coating, lubricant. Protects from HCL and pepsin part of gastric mucosa

  • Mucus
  • Gastric amylase
  • Pepsinogen
  • Hydrochloric acid
  • Pepsin
  • Gastrin

You can choose more than one answer

3. Describe the structure and functions of the small intestine, large intestine and accessory organs associated with digestion

Which are the three sections of the small intestine from beginning to end?

Small Intestine

90% of absorption takes place in the small intestine whereas only 10% takes place in the stomach and large intestine.

The walls of the small intestine have several layers, including a muscular layer, a layer containing blood vessels, lymph vessels and nerves and an inner mucous membrane.

Completion of the chemical digestion of food and the subsequent absorption of nutrients takes place in the small intestine.

Nutrients are absorbed through the villi into the blood and lymph vessels. Hardly any food is absorbed elsewhere in the digestive system.

  • ascending, transverse and descending
  • jejunum, ileum and duodenum
  • duodenum, jejunum and ileum
  • duodenum, ileum and jejunum

Intestinal Juice

Carbohydrates are classified as monosaccharides, disaccharides or polysaccharides. Monosaccharides include fructose in fruit. 

Disaccharides include lactose in milk. Polysaccharides include starch and fibre in cereals, potatoes and other plant sources

1. Peristaltic movements mix food with intestinal and pancreatic juices as well as bile. The movements push the food against the villi. Intestinal juices are composed of enzymes:

  • maltase, sucrase and lactase which split disaccharides into monosaccharides
  • enterokinase which activates trypsin in pancreatic juice
  • peptidases which split polypeptides into amino acids 

2. A number of hormones in the small intestine help digestion by stimulating the production of pancreatic or intestinal juices and regulating acidity levels, for example cholecystokinin (CCK) 

3. Absorption: digested food is absorbed by either active transport or diffusion

  • Most nutrients, including amino acids and sugars, are absorbed by active transport through the walls of the villi where they enter the bloodstream and are carried to the liver in the hepatic portal vein.
  • Fats, fatty acids and glycerol diffuse into the lacteals (lymphatic capillaries). They are called lacteals because the fat passes into them in suspension, causing the lymph to look milky.

To protect the digestive system from infection. It is the only section of the digestive system with a direct link to the protective lymphatic system.

In which part of the large intestine is faeces stored before defecation?

Large Intestine

Functions:

  • Absorption of nutrients, vitamins, salt or water left in digestive waste.
  • Secretion of mucus to help passage of faeces.
  • Defecation: a ‘mass movement’ pushes waste along the transverse colon, often stimulated by food arriving in the stomach. It is a reflex but humans have control of it. If the reflex is ignored, more water will be absorbed from the faeces which may cause constipation.

  • rectum
  • caecum
  • descending colon
  • appendix

Accessory Organs

The secondary group of organs help the digestive system, but not all are actively involved in digestion. They contribute to  the break down of food into smaller usable components for digestion.

  • Liver: produce bile
  • Gall Bladder: store bile
  • Pancreas: produce pancreatic juices
  • Tongue and
  • Teeth

Which of the following is responsible for producing bile?

Liver

The largest gland in the body, the liver sits at the top of the abdomen, just below the diaphragm and just above and to the right of the medial line. It is vital because it performs many essential functions.

Functions:

The liver is vital for cleansing and storage as well as production.

It produces:

  • glycogen (a compound that stores energy)
  • iron, from the breakdown of red blood cells and food 
  • heat (the liver is the body’s radiator, producing more heat than any other organ as a result of its various functions) 
  • vitamin A (from carotene, found in green-leafed vegetables and carrots)
  • plasma proteins: albumin, globulin, prothrombin, fibrinogen
  • uric acid and urea, from breakdown of red blood cells and deamination of amino acids. 
  • bile - 

    emulsifying fats, stimulating peristalsis and 

    creating alkaline conditions in the small intestine.

It converts:

  • stored (saturated) fat into other fat products (like cholesterol)
  • glycogen to glucose, when energy is needed
  • glucose back to glycogen, in presence of insulin
  • metabolises protein.

  • gall bladder
  • liver
  • duodenum
  • pancreas

Gall bladder

Gall Bladder is a pear-shaped sac attached by the cystic and bile ducts to the posterior of the liver. Whenever there is excess bile secreted by the liver which can’t be used immediately for digestion, the bile passes first along the bile duct then along the cystic duct to the gall bladder where it will be stored until needed.

Functions:

  • reservoir for bile (from liver)
  • secretes mucus to add to bile
  • absorbs water from bile, making it more concentrated
  • contracts in order to empty bile into duodenum.

Pancreas

Pancreas is a gland situated behind the stomach, between the duodenum and the spleen. It delivers pancreatic juices to the duodenum through the pancreatic duct. 

Functions:

The pancreas works with both the digestive and the endocrine systems.

It produces enzymes to break down food, the hormone insulin which regulates the blood sugar level after eating by causing the conversion of glucose to glycogen for storage in the liver and muscles, and the hormone glucagon which converts glycogen back to glucose.

Pancreatic juices contain:

  • lipase (fat digestion)
  • amylase (starch digestion)
  • trypsin (protein digestion).

Video of Digestive system

4. Describe diseases and disorders of the digestive system.

Anorexia Nervosa

Anorexia is a loss of appetite. Anorexia nervosa is a psychological condition which often affects teenage girls and young women. The sufferers have a fear of gaining weight or being fat and refuse to eat very much or stop eating altogether. It can be severely debilitating and sometimes fatal.

Appendicitis

Acute inflammation of the appendix, usually treated by removal of the organ.

Bulimia Nervosa

Bulimia is an insatiable hunger during binging episodes coupled with compensatory evacuation methods such as self-induced vomiting and excessive use of laxatives. Bulimia nervosa is a psychological condition which often affects teenage girls and young women, and increasingly young men.

Cirrhosis of the Liver

Chronic damage to an organ causing hardening. Several types of cirrhosis exist but the most common is cirrhosis of the liver, which is frequently caused by excessive alcohol consumption.

Coeliac’s Disease

Is a common bowel condition that is caused by intolerance to a protein called gluten.

Constipation

Infrequent or uncomfortable bowel movements, causing hard faeces to block the rectum. Caused by lack of fibre in the diet, lack of fluids and lack of exercise. Sometimes caused by stress.

Gall Stones

Stones formed from residues of bile pigments, cholesterol and calcium salts, found in the gall bladder.

Heartburn

Burning sensation in oesophagus or throat, caused by back flow and regurgitation of acidic stomach contents.

Hernia

A rupture, in which an organ pushes through the surface of the structures which normally hold it in.

Irritable Bowel Syndrome (IBS)

No exact cause is yet known for irritable bowel syndrome (sometimes referred to as IBS), though stress and low-fibre, high fat diets are said to contribute. Symptoms include stomach and bowel pain and alternate bouts of diarrhoea and constipation. 

Jaundice

Excessive levels of bile pigments in the blood cause skin to turn yellow. Caused by malfunctioning gall bladder or obstructed flow of bile.

Stress

The most common effect of stress on the digestive system is ulcers. Anxiety and lack of relaxation cause overproduction of gastric juices and if they have nothing to work on they will start to attack the lining of the stomach or other structures. In short, the stomach starts digesting itself!

Ulcer

Erosion in the walls of the digestive system, often caused by too much acid.

Revision

Which of the following completes digestion?

  • stomach
  • large intestine
  • small intestine
  • mouth

Which of the following is responsible for the chemical reactions of digestion?

  • absorption
  • homeostasis
  • peristalsis
  • enzymes

What are the main constituents of gastric juice?

  • Pepsinogen
  • Hydrochloric acid
  • Gastrin
  • Mucus
  • Pepsin
  • Gastric amylase

 You can choose more than one answer 

Trypsin is an enzyme produced by what?

  • gall bladder
  • liver
  • duodenum
  • pancreas

What does the digestion of carbohydrates and fats occur?

  • mouth
  • stomach
  • liver
  • small intestine

Where does peristalsis occur?

  • in all sections of the alimentary canal
  • only in the mouth
  • only in the small intestine
  • only in the stomach

Which of the following effects does the parasympathetic nervous system have on the digestive system?

  • An increase in excretion of pancreatic juices
  • An increase in secretion of pancreatic juices
  • Inhibition of the secretion of digestive juices
  • Inhibition of micturition

What is the digestive disorder which presents as a burning sensation behind the sternum called?

  • hernia
  • hepatitis
  • heartburn
  • haemorrhoids

Fill in the blank 1

The  of the digestive system has a direct link to the lymphatic system

What is the most common effect of stress on the digestive system?

  • Appendicitis
  • Bulimia Nervosa
  • Constipation
  • Ulcer

Fill in the blank 2

The enzyme  act to digest fat in the duodenum.

Match the correct definition of each term

  • Anorexia
    The sufferers have a fear of gaining weight or being fat and refuse to eat very much or stop eating altogether
  • Cirrhosis
    Chronic damage to an organ causing hardening. The liver is the most common
  • Jaundice
    Excessive levels of bile pigments in the blood cause skin to turn yellow
  • Irritable Bowel Syndrome (IBS)
    Stomach and bowel pain and alternate bouts of diarrhoea and constipation

Which of the following is produced in the stomach?

  • bile
  • maltase
  • pancreatic juice
  • pepsin

The alimentary tract is a long continuous muscular tube extending from?

  • stomach to anus
  • mouth to anus
  • small intestine to anus
  • large intestine to anus

Where does protein digestion occurs?

  • stomach
  • small intestine
  • mouth
  • pancreas

What is Pancreas situated?

  • behind the small intestine
  • next to the bladder
  • behind the stomach
  • posterior of the liver

What does salivary amylase commences?

  • carbohydrate digestion
  • breakdown of roughage
  • fat digestion
  • protein digestion

Where is Hydrochloric acid produced?

  • mouth
  • stomach
  • liver
  • small intestine