Digestion of foods in The Alimentary Canal

Control of Digestive Activity

Mostly controlled by reflexes via the parasympathetic division


Chemical and mechanical receptors are located in organ walls that trigger reflexes

Stimuli include

  • A.Stretch of the organ

  • B.pH of the contents

  • C.Presence of breakdown products

Reflexes include

  • A.Activation or inhibition of glandular secretions

  • B.Smooth muscle activity

Digestive Activities of the Mouth

Mechanical breakdown : Food is physically broken down by chewing (mastication)

Chemical digestion : A.Food is mixed with saliva ; Cooked Starch is broken down into maltose by salivary amylase

Deglutition (Swallowing)

Buccal phase : Voluntary , Occurs in the mouth ; Food is formed into a bolus The bolus is forced into the pharynx by the tongue

Pharyngeal-esophageal phase : Involuntary transport of the bolus ; All passageways except to the stomach are blocked:
  • 1.Tongue blocks off the mouth
  • 2.Soft palate (uvula) blocks the nasopharynx
  • 3.Epiglottis blocks the larynx

Peristalsis moves the bolus toward the stomach ; The cardioesophageal sphincter is opened when food presses against it

Food Breakdown and Absorption in the Stomach

Peristalsis is the major means of moving food

Segmental movements Mix chyme with digestive juices .Aid in propelling food

Gastric juice is regulated by neural and hormonal factors
Presence of food or rising pH causes the release of the hormone gastrin
Gastrin causes stomach glands to produce :
  • Protein-digesting enzymes
  • Mucus
  • Hydrochloric acid

Intestinal glands: Krypts of Lieberkühn and the glands of Brüner.
  • Enzymes are produced by Intestinal cells and Pancreas

  • Pancreatic ducts carry enzymes to the small intestine

  • Bile, formed by the liver, enters via the bile duct

Enzymes from the brush border function to Break disaccharides into monosaccharides

Complete some protein digestion

Pancreatic enzymes play the major digestive function
  • Help complete digestion of starch (pancreatic amylase)

  • Carry out about half of all protein digestion

  • Digest fats using lipases from the pancreas

  • Digest nucleic acids using nucleases

Alkaline content neutralizes acidic chyme

Release of pancreatic juice into the duodenum is stimulated by Vagus nerve and Local hormones : Secretin and Cholecystokinin (CCK)

Hormones travel the blood to stimulate the pancreas to release enzyme- and bicarbonate-rich product

Secretin causes the liver to increase bile output ; CCK causes the gallbladder to release stored bile

Bile is necessary for fat absorption and absorption of fat-soluble vitamins (K, D, A)

Water is absorbed along the length of the small intestine

End products of digestion : Most substances are absorbed by active transport through cell membranes

Lipids are absorbed by diffusion with the aid of bile salts

Substances are transported to the liver by the hepatic portal vein or lymph

Food Breakdown and Absorption in the Large Intestine

No digestive enzymes are produced

Resident bacteria digest remaining nutrients and Produce some vitamin K and B ; Release gases

Water and vitamins K and B are absorbed

Remaining materials are eliminated via feces

Faeces contains : Undigested food residues; Mucus ;Bacteria ; Water

Sluggish peristalsis in colon

Mass movements : Slow, powerful movements occur three to four times per day

Presence of feces in the rectum causes a defecation reflex
  • A.Internal anal sphincter is relaxed

  • B.Defecation occurs with relaxation of the voluntary (external) anal sphincter

Bacterial flora of the colon
Some bacteria remain among the ‘food residues’ as it enters the colon

Many go into action of ‘decomposing’ the content, releasing the gases characteristic of decomposition: CO2 , H2O and methane (CH4)

Depending on the content, other gases such as dimethyl sulphide (protein decomposition -smells like rotten eggs!) N2 , and H2 are also liberated.

Methane bacteria in the colon metabolise the methane released and reduce the gas load, which is an average of 500ml of gas (flatus) per day.

Some resident bacteria, like E.coli produce Vit K .Others synthesise some of the B group Vitamins

Constipation and Bacterial Flora

Some bacteria can digest cellulose over a long period (may take up to 5 days)

If food stays too long in colon and cellulose becomes digested, it loses the ‘grip’ on the water it contained and faeces tend to become harder.

The longer it stays, the more water it loses, the harder it becomes.

The gastrocolic reflex sets off the gastroileac reflex ,both of which are triggered by movement of food in the GIT .I.e. when food enters the stomach, a reflex is sent to the colon to
‘make space’ and mass peristaltic waves propel the food forward to the rectum [GC-reflex].

The GI-reflex propels food from the ileum into the caecum and up the colon...further pushes food to the rectum and anus.


The Fate of the Absorbed Nutrients


Absorbed glucose increases blood sugar levels

Hyperglycemia—excessively high levels of glucose in the blood

Insulin secreted from β cells in pancreas promotes conversion of glucose to glycogen by cells of liver and skeletal muscle

If blood glucose levels are still too high, excesses are converted to fat by liver

Hypoglycemia—low levels of glucose in the blood

Liver breaks down stored glycogen and releases glucose into the blood

Glucagon from α cells in the pancreas converta glycogen to glucose = Glycogenolysis ---- increase glucose levels

Glycogenesis—“glycogen formation” .Glucose molecules are converted to glycogen

Glycogen molecules are stored in the liver and muscle cells

Glycogenolysis—“glycogen splitting” .Glucose is released from the liver after conversion from glycogen

Gluconeogenesis—“formation of new sugar” Glucose is produced from fats and proteins