Digestion and The Digestive System Notes

The Digestive System

• The digestive system is a long tube running from the mouth to the anus, also known as the alimentary canal.
• Organs associated with the alimentary canal:
  • Pancreas
  • Gall bladder
  • Liver

Processes in Human Nutrition

• Ingestion: Intake of food into the body.
• Digestion: Breakdown of food into soluble form for absorption by the cell.
• Absorption: Diffusion and uptake of soluble substances.
• Assimilation: Incorporation of nutrients into the body cells.
• Egestion: Removal of faeces from the body.

Organs of the Digestive System

• Mouth: Breaks up food particles and assists in producing spoken language.
• Salivary glands: Saliva moistens and lubricates food; amylase digests polysaccharides.
• Pharynx: Swallows.
• Esophagus: Transports food.
• Stomach: Stores and churns food; pepsin digests protein; HCl activates enzymes, breaks up food, kills germs; mucus protects stomach wall; limited absorption.
• Small intestine: Completes digestion; mucus protects gut wall; absorbs nutrients, most water; peptidase digests proteins; sucrases digest sugars; amylase digests polysaccharides.
• Pancreas: Hormones regulate blood glucose levels; bicarbonates neutralize stomach acid; trypsin and chymotrypsin digest proteins; amylase digests polysaccharides; lipase digests lipids.
• Liver: Breaks down and builds up many biological molecules; stores vitamins and iron; destroys old blood cells; destroys poisons; bile aids in digestion.
• Gallbladder: Stores and concentrates bile.
• Large intestine: Reabsorbs some water and ions; forms and stores feces.
• Rectum: Stores and expels feces.
• Anus: Opening for elimination of feces.

Digestion Process

• Digestion begins in the mouth (chewing).
• Digestion ends in the small intestines (absorption).

Two forms of Digestion:

• Mechanical or Physical Digestion (mastication):

  • Chewing in the mouth and churning (backward and forward motion of the stomach walls) to break down large pieces of food into smaller pieces.

• Chemical Digestion:

  • Enzymes are used to breakdown food into smaller molecules that are easily digested.

  • Digestion: The process by which food is broken down from complex substances to simple substances.

  • Ingestion: When food is taken into the mouth.

  • Teeth break down food into smaller pieces by grinding, biting, or slicing.

  • Saliva softens, moistens, and lubricates the food.

The Mouth

• Salivary amylase: The enzyme found in the mouth that breaks down starch into the simple sugar, maltose.
• Amylase is found in the saliva, which is produced in the salivary glands.
• In the mouth, food is partly broken down by the process of chewing (physical) and by the chemical action of salivary enzymes.
• The food is then rolled into a ball, also called a bolus, and swallowed.

The Oesophagus

• The oesophagus is a long tube that runs from the mouth to the stomach.
• After being chewed and swallowed, the bolus enters the oesophagus, where it is pushed along by peristalsis.
• Peristalsis: Rhythmic, wave-like movements caused by the contraction and relaxation of the circular muscles in the walls of the alimentary canal.
• This muscle movement gives us the ability to eat or drink even when we're upside-down.

The Stomach

• The stomach is a large, sack-like organ that churns the food and bathes it in fluid called gastric juices.
• Gastric juice is secreted by gastric glands found in the walls of the stomach.
• Gastric juice contains mucus, hydrochloric acid, and pepsin.
• Pepsin is an enzyme that breaks down proteins to polypeptides.
• Food in the stomach that is partly digested and mixed with stomach acids by the action of the stomach muscles is called chyme.
• At the base of the stomach, there is a ring of muscles called the pyloric muscle that separates the stomach from the small intestine.
• Hydrochloric acid:
  • Provides the acidic medium for maximum enzyme activity.
  • Kills bacteria.
  • Activates pepsin.

The Small Intestine

• After being in the stomach, food enters the duodenum, the first part of the small intestine.

• The duodenum receives juices from two places: the pancreas and the liver.

  1. Pancreas:
     • Pancreatic juice is produced in the pancreas.
     • The juice contains enzymes which breaks down the remaining partially digested food into soluble products. These enzymes include:
       • Lipase: Digests small fat globules/droplets to fatty acids and glycerol (continues to further digest fat after Bile).
       • Trypsin: Continues to digest protein to polypeptides.
       • Amylase: Continues to digest starch to maltose.
       • Pancreatic juice also contains sodium hydrogencarbonate, which is alkaline and so neutralises the acidic chyme from the stomach.
  2. Liver:
     • The liver secretes the digestive juice, bile.
     • Bile is a greenish-yellow alkaline liquid produced in the liver and stored in the gall bladder.
     • It contains bile salts that emulsify fats, that is breaks fats into small globules so they can be digested faster by pancreatic lipase.
     • Bile contains no enzyme.
     • Breaking down of fat into tiny droplets is known as emulsification.
     • Pancreatic juice and bile are alkaline liquids and therefore neutralizes the acid chyme from the stomach and create the optimal pH for these enzymes.

• Food then enters the ileum (the last part of the small intestines).

• Ileum: The wall of the small intestines secretes intestinal juice.

• The juice contains enzymes which breaks down the remaining partially digested food into soluble products. These enzymes include:

  • Maltase: Digests maltose to glucose.
  • Sucrase: Digests sucrose to glucose and fructose.
  • Lactase: Digests lactose to glucose and galactose
  • Peptidases: Digests polypeptides to amino acids.

Absorption in the Ileum

• Mostly takes place in the ileum (last part of the small intestines).
• This occurs via the villi (pl; villus, sing.).
• The soluble products of digestion such as glucose, fructose, amino acids, fatty acids and glycerol, are absorbed through the lining of the ileum.

Adaptations for absorption:

1.  The human intestine is about 7 m long, providing enough time for food to be absorbed.
2.  It contains millions of villi, which are tiny, finger-like projections that protrude from the wall of the small intestine, increasing the surface area within the small intestine and speeding up absorption.
3.  The cell membrane or epithelium lining of each villi is covered with very minute projections or folding called microvilli, this further increases the surface area of the cell for absorption.
4.  The epithelium lining is very thin (only one cell think) allowing absorption of nutrients to occur quickly and easily.
5.  Each villus has a dense network of blood and lacteal (lymph vessel) capillaries. The blood vessels absorb and transports sugars, amino acids and water-soluble vitamins, while the lacteal vessel absorbs and transports fat away from the small intestine. Molecules enter by diffusion down a concentration gradient (from high concentration to low concentration) or can also be taken up by active transport.
6.  Needs energy to do so, therefore, many mitochondria are present in the epithelial cells of the small intestines (villi).
7.  Fatty acids and glycerol enter by diffusion into the lacteal and then enter the lymphatic system.

The Large Intestine

• After passing through the small intestine, food passes into the large intestine.
• The large intestine is divided into caecum, colon and rectum.
• The first part of the large intestine is called the cecum (the appendix is connected to the cecum).
• Materials that pass into the large intestine includes: undigested material, mainly cellulose and roughage, water and mucus.
• Some of the water and electrolytes (chemicals like sodium) are removed from the food in the colon, leaving a semi-solid mass which passes into the rectum.
• The main function of the colon is to reabsorb water from the faeces and so food moves slowly along it to ensure maximum water absorption.
• Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process.
• Solid waste is then temporarily stored in the rectum until it is egested via the anus as faeces.
• The exterior through the anus is surrounded by a circle of muscle called the anal sphincter.
• The removal of faeces from the body is called egestion.

Assimilation

• Assimilation is the process of incorporating and making use of the digested food into the body for growth, repair, energy, and development.
• Excess nutrients are stored in the body.
• Sugar – glucose used for respiration, excess converted to glycogen and stored in the liver and muscles, also in the adipose tissue as fats under the skin or around organs. Glycogen can be converted to glucose when the body needs it.
• Amino acids – for growth, repair of damage tissue, used to make enzymes and hormones. Excess amino acid will be toxic if stored, therefore deaminated in the liver. The toxic portion of the amino acid is converted into urea before it is transported to the kidney and excreted in urine, the useful portion of the amino acid is converted to glucose or glycogen in the liver.
• Fatty acids and glycerol recombined to form fat – into the lymphatic system. Used to form new cells, used in respiration if glucose is not available. Excess is Stored in the adipose tissues (under the skin) and around organs.

Egestion vs Excretion

• Egestion is the discharge of undigested food particles or matter from the body of an animal.
• Excretion is the discharge of substances that have gone through one or several metabolic processes inside the body of an animal. Exhaling step of respiration, urination, and sweating are the main excretory processes of an animal.

Constipation

• A diet lacking in fiber can result in a blockage of the alimentary canal.
• Therefore, fiber is necessary in the diet.
• This blockage prevents egestion (constipation).
• Constipation may cause haemorrhoids (swollen or inflamed anal veins) which are the result of force pushing.
• Constipation also increases the chance of colon cancer.

The Teeth

• Food needs to be broken down and chewed before entering the digestive system so that our body can easily absorb nutrients from them.
• Teeth can help us pronounce accurately.
• Teeth can help us look better by giving us a good profile/smile.
• There are four types of teeth:
  • Incisor
  • Canine
  • Premolar
  • Molar
• Each tooth has four types of surfaces:
  • Inner surface.
  • Outer surface.
  • Chewing surface.
  • Adjacent surface.

Four different types of teeth:

• Incisors:
  • Shape: Chisel shaped
  • Root: 1 root
  • Function: cut & bite bits of food
  • Location: front of the mouth
• Canine:
  • Shape: pointed or daggered shaped
  • Root: 1 root
  • Function: tear foods
  • Location: behind the incisors
• Premolar:
  • Shape: broad bumpy surfaced
  • Root: 2 root and 2 pointed cusps.
  • Function: crush and grind food
  • Location: just behind the canine teeth
• Molars:
  • Shape: similar to premolars but are larger and wider at the surface
  • Root: 2 or 3 root and 4-5 cusps.
  • Function: crush & grind foods
  • Location: behind the premolars

Permanent Teeth

• The adult human mouth contains 32 permanent teeth (16 per jaw).
• Teeth types:
  • 4 third molars (also called wisdom teeth)
  • 8 molars
  • 8 premolars
  • 4 canine
  • 8 incisors
• In each jaw teeth are arranged symmetrically & identically.
• Most people have a complete set of adult teeth by the time they reach their teenage years.
• It is common for adults to have their wisdom teeth removed because there is not always enough room for them to grow in comfortably or without causing misalignment of other teeth.

Deciduous Teeth

• A child's mouth contains 20 temporary teeth called primary teeth, baby teeth, milk teeth or deciduous teeth.
• Teeth types:
  • 8 premolars
  • 4 canine
  • 8 incisors
• Deciduous teeth can reserve spaces for permanent teeth.
• They start to fall out when a child is about 5-7 years old.
• Once the permanent teeth start to erupt, the deciduous teeth will fall out and give room for permanent teeth.

Structure of the Teeth

• The teeth is made up of 2 main parts:

  • The Crown:
    • Found above the gum & is covered in an enamel (hard white coat).
    • The outer coating of the enamel is the hardest substance in our bodies!! ( much harder than bones)
  • The Root:
    • Its securely fixed in the socket of the jaw bone by dental cement.
    • The smaller teeth (canine & incisors) have a single root & the larger teeth (premolars & molars) have 2 or 3 roots.

• The cross section of a tooth consists of 3 main layers:

  • Enamel: The hardest, white outer part of the tooth. Enamel is mostly made of calcium phosphate, a rock-hard mineral.
  • Dentine (its hard & bone-like little harder than bone) : A layer underlying the enamel. It is a hard tissue that contains microscopic tubes. When the enamel is damaged, heat or cold can enter the tooth through these paths and cause sensitivity or pain.
  • Pulp cavity: The softer, living inner structure of teeth. Blood vessels and nerves run through the pulp of the teeth.

• Other layer:

  • Cementum: A layer of connective tissue that binds the roots of the teeth firmly to the gums and jawbone.

How to care for the Teeth

Keep your teeth healthy by:

• Eating less sweets & sweet foods.
• Chewing sugar free gums between meals (this produce saliva which neutralizes any acids).
• Brush at least twice a day with fluoride toothpaste for at least two minutes, especially first thing in the morning and before bedtime
• Floss every day – usually at bedtime
• Visit your dentist every six months for an oral exam and professional cleaning.

Why Clean our Teeth

• Why clean teeth? To prevent decay!!
• Plaque: A sticky, colorless film made of bacteria and the substances they secrete. It begins to form a coat over the teeth after a meal. Plaque develops quickly on teeth after eating sugary food but can be easily brushed off.
• Bacteria in the mouth feed on the sugar and starches in the food, this produces a lot of acid waste. The acid dissolves the enamel coat of the teeth.
• The bacteria themselves do not cause tooth decay.
• Over time this acid will eat away the enamel eventually creating holes or cavity in the dentine of the tooth. The bacteria can then enter the tooth, reproduce and feed on the nerves, thus causing a tooth ache.
• Once, this doesn’t happen too quickly the tooth can repair the enamel.
• The build up of plaque between teeth & gum causes gum disease, as plaque rots the gum.
• It takes 24hrs for bacteria in the plaque to build up to a destructive level.
• Therefore, brushing the teeth once, a day prevents tooth decay!!!

Other teeth Conditions

• Cavities (caries): Bacteria evade removal by brushing and saliva and damage the enamel and deeper structures of teeth. Most cavities occur on molars and premolars.
• Tooth decay: A general name for disease of the teeth, including cavities.
• Periodontitis: Inflammation of the deeper structures of the teeth (periodontal ligament, jawbone, and cementum). Poor oral hygiene is usually to blame.
• Gingivitis: Inflammation of the surface portion of the gums, around and between the crowns of the teeth. Plaque and tartar buildup can lead to gingivitis.
• Tartar: If plaque is not removed, it mixes with minerals to become tartar, a harder substance. Tartar requires professional cleaning for removal.

Digestive enzymes

pH of the alimentary canal

• Mouth: Slightly alkaline to neutral
• Stomach: Very acidic; pH 2 (dilute hydrochloric acid)
• Duodenum: Very alkaline; (bile)
• Illeum: alkaline

Digestion and Absorption

Carbohydrate

• Digestion begins in the mouth by salivary amylase and completed in the small intestine by pancreatic amylase.

Protein

• Proteins are broken down to peptides by pepsin in the stomach
• Trypsin in the small intestines – peptides to amino acids

Fat

• Bile breaks down large lipid droplets into smaller droplets, by a process called emulsification.
• Fat digestion occurs by pancreatic lipase in small intestine.

Vitamins

• Fat-soluble vitamins are absorbed and stored along with fats.
• Most water-soluble vitamins are absorbed by diffusion or mediated transport.