Digestive System

intracellular digestion

part of metabolism

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involves the oxidation of glucose and fatty acids for energy

extracellular digestion

process by which pure glucose and fatty acids are extracted and obtained from the food we eat

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occurs in the lumen of the alimentary canal

alimentary canal

runs from the mouth to the anus

sphincters

circular smooth muscles around the alimentary canal that can contract to allow compartmentalization of function

digestion

involves the breakdown of food into its constituent organic molecules:

* starches and other carbs → monosaccharides
* lipids (fats) → free fatty acid and glycerol
* proteins → amino acids

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mechanical digestion

physical breakdown of large food particles into smaller particles

chemical digestion

enzymatic cleavage of chemical bonds such as peptide bonds of proteins or the glycosidic bonds of starches

absorption

transport of products of digestion from the digestive tract into the circulatory system for distribution to the body’s tissues and cells

digestive tract pathway

oral cavity (mouth) → pharynx → esophagus → stomach → small intestine → large intestine → rectum

pharynx

shared pathway for both food entering the digestive system and air entering the respiratory system

enteric nervous system

controls the function of the GI system

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regulated by the autonomic nervous system, but can function independently of the brain and spinal cord

peristalsis

rhythmic contractions of the gut tube, in order to move materials through the system

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triggered by neurons in the walls of the digestive tract

parasympathetic division

involved in stimulation of digestive activities, increasing secretions from exocrine glands and promoting peristalsis

hormones involved in feeding behavior and ingestion

ADH and aldosterone: trigger the sensation of thirst

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glucagon and ghrelin: stimulate feelings of hunger

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leptin, and cholecystokinin: stimulate feelings of satiety

where is glucagon and ghrelin secreted

glucagon = pancreas

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ghrelin = pancreas and stomach

oral cavity

involved in chemical and mechanical digestion

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mechanical digestion in mouth: breaking of large food using teeth, tongue, and lips

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chemical digestion: relies on enzymes from saliva produced by the **salivary glands**

mastication

process of chewing

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helps increase surface-to-volume ratio of the food, creating more surface area for chemical digestion as it passes through the gut tub

moderates the size of food particles entering the lumen of the alimentary canal

saliva

produced by 3 salivary glands; innervated by the parasympathetic nervous system

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aids mechanical digestion by lubricating food

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contains salivary amlyase

salivary amylase and lipase

salivary amylase: capable of hydrolyzing starch into simple sugars

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lipase: catalyzes the hydrolysis of lipids

pharynx

cavity that leads from the mouth to the posterior nasal cavity to the esophagus

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3 parts:

* nasopharynx: behind the nasal cavity
* oropharynx: back of the mouth; initiates swallowing
* laryngopharynx: above the vocal cords

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epiglottis

cartilaginous structure that folds down to cover the laryngeal inlet to prevent food from entering the larynx

esophagus

muscular tube that connects the pharynx to the stomach

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top part: skeletal muscle; somatic motor control

middle: skeletal and smooth muscle;

bottom: smooth muscle; autonomic motor control

lower esophageal sphincter (cardiac sphincter)

muscular ring that relaxes and opens to allow the passage of food

stomach

highly muscular organ; uses hydrochloric acid and enzymes to digest food, creating a harsh environment

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anatomy of the stomach

fundus and body: contains most gastric glands

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antrum and pylorus: contain mostly pyloric glands

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gastric glands

respond to signals from the vagus nerve of the PSNS, which is activated by the the brain in response to the sight, taste, and smell of food

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3 types of cells:

* **Mucous cells**: produce bicarbonate-rich mucus that protects the muscular wall from the acidic and proteolytic environment of the stomach
* **chief cells**: secrete pepsinogen, which is the inactive form of pepsin, a proteolytic enzyme
* **parietal cells**: secret H+ as HCL , cleave pepsinogen → pepsin, secrete intrinsic factor, a glycoprotein involved in the absorption of B12

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gastric juice

combination of parietal and chief cells

pepsin

digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments

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activated by acidic environment; most active at low pH

pyloric glands

contain G-cells that secrete gastrin, a peptide hormone that induces the parietal cells to secrete more HCl and signal the stomach to contract, mixing its contents

small intestine

3 segments:

* **Duodenum:** responsible for majority chemical digestion and minor involvement in absorption
* **Jejunum:**
* **Ileum:**

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Duodenum

responsible for majority chemical digestion and minor involvement in absorption

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chyme causes the release of brush-border enzymes

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secretes enteropeptidase, which is involved in the activation of other digestive enzymes from the accessory organs of digestion

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secretes hormones lie secretin and CCK

brush border enzymes

present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers

aminopeptidase

peptidase secreted by glands in the duodenum that removes the -terminal amino acid from a peptide

Dipeptidase

cleave the peptide bonds of dipeptides to release free amino acids

enteropeptidase

enzyme critical for the activation of trypsinogen → trypsin

secretin

peptide hormone that causes pancreatic enzymes to be released into the duodenum

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regulates pH of digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas

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also a enterogastrone, a hormone that slows motility through the digestive tract to increase time for digestive enzymes to act on chyme

cholecystokinin (CCK)

stimulates the release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety

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bile salts

derived from cholesterol

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serve a role in the mechanical digestion of fats and ultimately facilitate the chemical digestion of lipids

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hydrophobic and hydrophilic regions; acts a bridge between lipid and aqueous environments

pancreas

involved in the release of insulin, glucagon, and somatostatin

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bulk of the pancreas is made up of **acinar cells**, exocrine cells that produce pancreatic juices

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**pancreatic amylase** breaks down large polysaccharides into small disaccharides and is responsible for carb digestion

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secretes **pancreatic lipase** which is capable of breaking down fats → free fatty acids and glycerol

liver

receives blood from draining from the abdominal portion of the digestive tract through the hepatic portal vein

* processes the blood before draining into the inferior vena cava on its way to the right side of the heart

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detoxifies endogenous compounds and exogenous compounds

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detoxifies and metabolizes alcohol and mediations

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produces bile

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synthesized certain proteins necessary for proper body function such as **albumin** and **clotting factors**

gallbladder

upon release of CCK, the gallbladder contracts and pushes bile out into the biliary tree

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site of cholesterol or bilirubin stone formation

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jejunum and ileum

involved in the absorption of nutrients

fat soluble vitamins

A,B,E, and K

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dissolve directly into chylomicrons to enter the lymphatic circulation

water soluble vitamins

B complex and C

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taken across the endothelial cells of the small intestine, passing directly into the plasma

large intestine

involved in water absorption

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**cecum**: outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment of the appendix

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**colon**: absorb water and salts from the undigested material left over from the small intestine; forms feces

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**rectum**: storage site for forces