Physiological Processes - Digestive System

6 Processes in Digestive System

1. Ingestion

2. Propulsion

3. Mechanical digestion

4. Chemical digestion

5. Absorption

6. Defecation

Process 1:

Ingestion (oral cavity)

Just entry of food into the mouth.

Process 2:

Propulsion (Entire GI track)

Voluntary propulsion of food (swallowing/deglutition) and involuntary peristalsis.

• Peristalsis occurs through the entire GI track past the pharynx

• Sequential contraction and relaxation of smooth muscles lining the GI track to propel food/bolus/chyme/stool

Process 3:

Mechanical Digestion (Teeth, stomach, intestines)

• Chewing

• Churning of stomach

• segmentation of small intestine

  • Localised contraction of the circular folding in the small intestine

  • Stops propelling chyme through the small intestine, rather they squash and break up the chyme, slowing it down to increase the time allotted to digestion via enzymes, increasing absorption.

All processes increases access of enzymes to extract nutrients from food/bolus/chyme/stool

Process 4:

Chemical Digestion (Salivary glands, stomach, liver/gall bladder, pancreas, small intestine)

• Saliva - Salivary amylase

• Stomach acid - Hydrocholric acid

• Liver/gallbladder - Bile

• Pancereas - Pancreatic juice

All chemicals contribute to the water present in the digestive system and breakdown of food/bolus/chyme

Process 5:

Absorption (Primarily Small intestine)

Movement of nutrients from chyme present in lumen, into the bloodstream.

Small intestine:

• Mostly water absorbed

• Mostly nutrients absorbed

Large intestine:

• Mostly water absorbed

• Causes stool to become firm

Process 6:

Defecation (Anus)

Undigested material (fecal matter/stool) excreted from the body.

2 Forms of Digestion regulation

1. Neural regulation

2. Hormonal regulation

Digestion Regulation:

Neural Regulation

The entire GI track has nerves lining the walls. These nerves interact with the CNS.

• Stimulation of mechanoreceptors, chemoreceptors, osmoreceptors (sense concentration changes), presence of food may change pH

1. Long reflexes

Central (central nervous system) and autonomic (peripheral nervous system), respond to extrinsic stimuli.

• e.g walking past a bread shop, smelling tasty bread triggers you to salivate, stomach to release gastric acids & grumble.

1. Short reflexes

   Central (central nervous system) and autonomic (peripheral nervous system), respond to intrinsic stimuli.

   Activation of the parasympathetic nervous system (rest & digest response).

   • Increased secretion of digestive enzymes

   • Increases stomach motility (spontaneous movement)

     • Mechanoreceptors trigger stomach distension (stretch)

   • Decreased movement of the small intestine

   Activating the sympathetic nervous system (fight or flight response)

   • inhibits GI track activity.

Digestion Regulation:

Hormonal Regulation

GI hormones (gastrin & CCK) secreted by:

• stomach wall cells

• enterocytes in small intestine (wall cells)

Gastrin:

• Gastrin is secreted in response to the presence of food

• Gastrin stimulates secretion of gastric acid by parietal cells in stomach lining.

CCK (Cholecystokinin):

• Secreted by enterocytes in duodenum (small intestine).

• Stimulates release of bile from the gallbladder which breaks down fat & protein

  • Greek: “Chole” - gall bladder

  • Greek: “cysto” - sac

  • Greek: “kinin” - move

  • Greek: “Cholecystokinin” - movement of gall bladder sac

Secretin:

• Secreted by enterocytes in duodenum (small intestine).

• Maintains pH of the duodenum, neutralising acidic chyme

3 Digestion Phases

1. Cephalic phase

2. Gastric phase

3. Intestinal phase

Digestion Phase 1:

Cephalic Phase

Sight & Smell:

Starts before food enters the mouth and involves long reflex control of digestion

• Long reflexes of the nervous system can trigger the cephalic phase, activating parasympathetic nervous system.

  • Gastrin, in turn gastric acid is released from stomach lining to prepare the stomach for food.

Digestion Phase 2:

Gastric Phase

Distension, pH & hormones:

The entry of bolus into the stomach and involves short reflex control of digestion

• Short reflexes & hormones

• Stomach responds in the presence of proteins

Digestion Phase 3:

Intestinal Phase

Distension, pH & hormones:

Entrance of chyme into the intestines and involves inhibitory signals to the stomach and excitatory signals to the intestines

• Short reflexes

• Pyloric sphincter contracts to stop excess chyme from entering the small intestine

• Small intestine responds in the presence of fatty acids

  • pancreatic enzymes & bile released