The Digestive System Supplies Nutreints for the Body 6

What is the function of the digestive system?

extracts nutrients from the food we eat and absorbs them into the body for use by the cells

What are the 6 activities carried out by the digestive system?

• ingestion of food and water

• mechanical digestion of food

• chemical digestion of food

• movement of food along the alimentary canal

• absorption of digested food and water into the blood and lymph

• elimination of material that is not absorbed

Define digestion.

the process in which carbohydrate, protein and fat molecules are broken down to products small enough to be absorbed into the blood and into the cells

Define mechanical digestion.

the physical breakdown of food particles into smaller pieces to increase the surface area

What does mechanical digestion involve?

• involves the following processes in the mouth, stomach and small intestine:

  • the teeth cut, tear and grind the food

  • churning action in the stomach breaks the food down further

  • the gall bladder releases bile into the small intestine and bile salts act as emulsifying agents, breaking fat down into smaller droplets

  • the increased surface area allows more effective chemical digestion, as the chemicals can access more of the food

What is the aim of mechanical digestion?

to break the food down into smaller pieces so that the total surface area increases, allowing a more effective chemical digestion, as the chemicals can access more of the food

What is chemical digestion?

uses enzymes to break large, complex molecules into small, simpler molecules where these smaller molecules are then small enough to be absorbed into the bloodstream

What do large, complex molecules break into?

• carbohydrates split into monosaccharides (e.g. glucose, fructose, galactose)

• proteins split into peptides and amino acids

• lipids split into fatty acids and glycerol

• nucleic acids split into nucleotides

What is the alimentary canal and its function?

• continuous tube that runs from the mouth to the anus

• together with associated organs (e.g. pancreas, gall bladder), the alimentary canal makes up the digestive system

• the lining of the alimentary canal is the surface through which nutrients are absorbed

Define ingestion.

intake of food, occurring at the mouth

Define mastication.

process in which food is chewed

Describe the saliva and chemical digestion in the mouth.

• as food is chewed, the tongue mixes it with saliva

• saliva contains mucus to lubricate the food and a digestive enzyme, salivary amylase, which begins the chemical digestion of starch into the disaccharide maltose

Define tongue.

a muscular organ in the mouth, covered with moist, pink tissue called mucosa

Define saliva.

a fluid that is secreted into the mouth cavity by 3 pairs of salivary glands

What does a full adult set of teeth in the lower jaw consist of?

• 4 incisors - chisel-shaped teeth used for biting/cutting

• 2 canines - one on each side of the incisors, conical teeth used for tearing

• 4 premolars - 2 on each side of jaw

• 6 molars - 3 on each side of jaw,

  • molars and premolars have broad crowns with rounded cusps, which fit into depressions of the surface of teeth on the other jaw, making them ideal for crushing and grinding food

Describe the mechanical digestion of the teeth.

• the action of the jaws and teeth begins mechanical digestion

• tiny bumps called papillae give the tongue its rough texture with thousands of taste buds covering its surface

  • taste buds - collections of nerve-like cells that connect to nerves running into the brain

• after chewing, the tongue shapes the food into a rounded lump called a bolous

• to swallow, the tongue moves upwards and backwards, pushing the bolous into the back of the mouth, the pharynx, which leads to the oesophagus

Describe the oesophagus and its function.

• a tube about 23-25 cm long that connects the pharynx to the stomach

• the wall of the oesophagus has a double layer of muscle

  • circular muscle has muscle fibers arranged in a circle

  • longitudinal muscle has fibers arranged along the length of the canal

• as the lump of food enters the pharynx and oesophagus, the circular muscle behind it contracts to narrow the tube

• the contraction of successive bands of circular muscle causes the constriction to move in a wave called peristalsis, pushing the food into front of it, assisted by the secreting of mucus that lubricates the inner lining

Describe the stomach and its function.

• the oesophagus opens into the stomach, a roughly J-shaped, enlarged section of the alimentary canal

• mechanical digestion in the stomach is achieved by waves of muscular contraction that move along the stomach wall

• it has an oblique muscle layer as well as a circular and longitudinal layer, enabling the stomach to contract in a variety of ways to churn the food and mix it with the gastric juices, until it is converted to a soupy liquid called chyme

What is gastric juice?

digestive juice containing HCl, mucus and digestive enzymes

What is the function of the mucosa lining the stomach?

• the mucosa is specialized for the secretion of gastric juices by gastric glands located in the narrow, tube-like structures of gastric pits

  • each of the gastric juice is secreted by a different type of cell in the gastric pits

• gastric juice is responsible for chemical digestion in the stomach, which is mainly the start of protein digestion

• stomach pH is around 2-3 due to HCl and cells lining the stomach are protected from the acid by a layer of mucus

Why is the acidic environment in the stomach essential?

• allows the enzyme pepsinogen to be converted to pepsin, which is able to break proteins down into shorter peptides and break down nucleic acids

• kills bacteria that enters the stomach

Why are nutrients not absorbed into the blood stream through the stomach?

nutrients are not absorbed into the blood stream through the stomach because the internal surface is covered by a thick layer of mucus (some alcohol/drugs are)

What is the pyloric sphincter and its function?

a thickening of the circular muscle, regulating the flow of material from the stomach to the duodenum after 2-8 hours

What is the small intestine and its function?

• longest part of the alimentary canal, around 6-7m in length

• receives materials pushed through the pyloric sphincter from the stomach

What are the 3 different sections of the small intestine?

1. duodenum - 1st and shortest section at 25cm

   • extends from the bottom end of the stomach in a curve around the pancreas

   • most of the chemical digestion occurs here before the chyme moves further along the small intestine

2. jejunum - middle section

   • its lining allows effective absorption of carbohydrates and proteins

3. ileum - final part

   • where vitamin B12, bile salts and any remaining products of digestion are absorbed

What does digestion continue in the small intestine under the influence of?

• pancreatic enzymes - secreted by the pancreas via pancreatic duct

• bile - produced by the liver, but stored in the gall bladder and secreted into the small intestine via the bile duct

• intestinal enzymes - secreted by glands in the lining of the small intestine

What are pancreatic enzymes and how do they help the small intestine in digestion?

• enters the duodenum through the common bile duct

• helps to neutralize the acid that has come with the material from the stomach and contains many of the enzymes involved in the digestion food

• the enzymes include:

  • pancreatic amylase, which breaks down starch into the disaccharide maltose

  • trypsin/pancreatic protease, which splits proteins into peptides

  • pancreatic lipases - enzymes that break down fats into fatty acids and glycerol

  • ribonuclease and deoxyribonuclease - enzymes that digest RNA & DNA

What is bile and how does it help the small intestine in digestion?

• produced by the liver, but stored in the gall bladder

• bile is secreted into the small intestine through the common bile duct

• does not contain any digestive enzymes but bile salts emulsify fats into tiny droplets which increases their surface area on which the lipases can act to bring about the chemical digestion of fat

What are intestinal enzymes and how do they help the small intestine in digestion?

• contains many enzymes that complete the digestion of carbohydrates, proteins and lipids

• enzyme examples:

  • peptides to break down peptides into amino acids

  • sucrase, lactase and maltose to break down sucrose, lactose and maltose into the monosaccharides glucose, fructose and galactose

  • lipases to break down lipids into fatty acids and glycerol

What process also occurs in the small intestine, continuing the mechanical digestion?

segmentation - a process which uses the contraction of circular muscles to push the chyme into segments, mixing it with digestive enzymes

How are nutrients absorbed in the small intestine?

• the products of digestion (simple sugars, amino acids, fatty acids and glycerol) along with substances (vitamins, minerals, water) are absorbed through the walls of the small intestine into the blood

• nutrients are absorbed through the internal surface of the small intestine, so effecient absorption requires a large surface area

How is a large surface area achieved by the small intestine?

• small intestine is very long, about 6-7m

• inner lining (mucosa), has folds that extend into the interior of the small intestine

• mucosa has small, finger-like projections called villi that extend from the folded surface

• the cells covering the outside of the villi have tiny microscopic projections from their external surfaces (microvilli)

Describe the structure of villi

• ideally suited to its function of nutrient absorption

• each villus is about 1mm long

• villi in the jejunum are longer than those in the duodenum and ileum

• each villus is covered by a single layer of cells

• lacteal - lymph capillary inside the villus, which is surrounded by a network of blood capillaries

What causes the constant movement of the vill?

• muscular movement of the intestinal wall causes constant movement of the villi which constantly brings different parts of the intestinal content with villi to enhance absorption

  • these contents are constantly changing as new material is emptied into the small intestine from the stomach

Describe the absorption of nutrients through the villi.

• some absorption occurs through simple diffusion, as there is a higher concentration of nutrient materials in the interior of the small intestine than in the cells lining the villi

• some absorption occurs through active transport, which involves the cells of the villi using energy to take in nutrients against the concentration gradient

• from the walls of the villi, simple sugars, amino acids, waster and water-soluble vitamins are absorbed into the blood capillaries

• fatty acids and glycerol recombine in the cells of the villi to form fats and along with the fat-soluble vitamins, enter the lacteals

• the substances that are absorbed into the blood capillaries are carried by the hepatic portal vein to the liver, where they may be removed for further processing, or may remain in the blood to be carried to other body cells

• substances that are absorbed into the lacteals are transported in the lymph system, which eventually empties into the blood through veins in the upper part of the chest

Describe the large intestine and what is it made up of?

• about 1.5m long and larger in diameter than the small intestine

• made up of the:

  • caecum - a pouch, about 6cm long, where the small intestine joins the large intestine

  • colon - longest part of the large intestine shaped like an inverted U

  • rectum - last part of the large intestine, where semi-solid material left in the colon after water absorption is pushed here by peristalsis

  • anus - external opening at the end of the rectum

  • appendix - small tube attached to the caecum

What is the function of the large intestine?

• no villi in the large intestine and no digestive enzymes are secreted, even though the lining does secret a large amount of mucus

• movement of material through the large intestine is fairly slow, taking 18-24 hours for material to pass through, allowing most of the remaining water to be absorbed so the contents become more solid

• bacteria in the large intestine break down much of the remaining organic compounds

  • some bacteria produce vitamins, which are then absorbed, along with mineral nutrients, through the walls into the blood

• the semi-solid material left after water absorption and bacterial action makes up the faeces

What does faeces contain?

• water

• undigested food (cellulose)

• bacteria

• bile pigments (which gives the faeces its colours)

• remains of cells that have broken away from the internal lining of the alimentary canal

What is excretion?

• excretion - the removal of metabolic waster (waste that has been produced by chemical activity of the body cells)

  • the contents of faeces are not metabolic waste except for bile pigments

  • ∴ defecation is referred to as elimination

What is constipation?

• reduced movement in the large intestine, causing more water to be absorbed, making harder faeces and more painful/difficult defecation

• causes:

  • lack of roughage (cellulose/insoluble fibre from plants) in diet

  • lack of exercise

  • emotional problems

• ways to reduce likelihood:

  • increase amount of fibre in diet

  • increase sufficient exercise

What is diarrhoea?

• increased peristalsis in the large intestine, causing less water to be absorbed, making faeces more watery

• if it is severe, it may lead to dehydration through loss of water from the intestines and may even cause death

• cause:

  • irritation by viruses/bacteria (e.g. salmonella, e.coli)

• ways to reduce:

  • safe/hygienic food practices to reduce contamination by viruses and bacteria

What is bowel/colorectal cancer?

• uncontrolled growth of cells in the wall of the large intestine

• research suggests that bowel cancer may be linked to diet, high alcohol consumption and smoking

• causes:

  • a diet in high red and processed meat and low in fibre (fruit/vegetables)

  • being overweight/obese

  • physical inactivity

  • alcohol consumption

  • smoking

• ways to reduce:

  • high fibre diet

  • no smoking/alcohol

  • healthy BMI

What is coeliac disease?

• inherited autoimmune condition, causing inability to tolerate gluten and the immune system to destroy villi in response

• causes:

  • genetic condition

  • symptoms result as nutrients cannot be efficiently absorbed

• ways to reduce:

  • gluten free diet