Digestion Flashcards
Describe the function of the digestive system
Is the breakdown of nutrients which can then be absorbed & used by cells to make ATP during cellular r/s & growth & repair
What is the alimentary canal?
A long muscular tube that extend from mouth to anus, also known as GI tract
What are the digestive organs and what are the accessory organs involved?
Digestive: mouth, pharynx, oesophaugus, stomach, small intestine & larger intestine
accessory: tongue, teeth, gallbladder,
Define mechanical & chemical digestion
Mechanical digestion: physical breakdown of food into smaller chunks e.g., teeth, creates larger SA for digestive enzymes, (small intestine segmentation & churning in the stomach)
Chemical digestion: refers to use of enzymes in breakdown of food e.g., lipase, pepsin
Describe the digestion pathway’s 5 stages
- Ingestion: taking in of food via the mouth into GI tract
- Propulsion: movement of food along GI tract, includes swallowing, peristalsis contractions & relaxations of muscle wall, squeezes food along
- Digestion: includes M & C digestion, ezymes secreted to breakdown food further, food is broken to chemical components
- Absorption: occurs in SI, nutrients absorbed via mucosal cells via active/passive transport. LI absorbs water & electrolytes
- Defecation: eliminates waste from the body as faeces via anus
Describe the components of salivia
- 99% water
- lipase enzymes
-defensins
-Amylase
-Antibodies - Lysosomes
-Good bacteria
What are the 4 layers found in the digestive system?
Serosa, muscularis, mucosa & submucosa
Describe the structure & function of the serosa
The outermost layer acts as a barrier, It anchors the GI tract in place.
It is made of squamous epithelium.
Involved in the protection & secretion of serum
Contains adventitia in oesophagus which binds it to surrounding structures
Describe the structure & function of the submucosa
It is external to the mucosa, a loose connective tissue which has lots of BV’s, lymphatic vessels & elastic fibres. EF allows stretch and shape recoil in stomach after digestion of a large meal. It secretes HCO3- to neutralize HCL, it contains the enteric NS
Describe the structure & function of the mucosa
The innermost layer that contains mucus membranes. This layer lines GI from mouth to anus, secreting enzymes, mucus & hormones. It absorbs end products in SI, it contains villi to increase SA for absorption. It contains lymphoid tissue to give protection & immunity
Describe the structure & function of the muscularis
The muscular layer in the GI tract. It is responsible for peristalsis & segmentation. It has an outer longitudinal layer & inner circular layer of smooth muscle cells.
Sphincters form along GI tract to control passage of food.
Extra layer found in stomach lining to aid in churning of food.
Define peristalsis
Peristalsis refers to involuntary wave-like contractions of the muscles in the digestive tract. These contractions help move food and other liquid particles through the digestive system to various processing organs. Peristaltic waves occur in the oesophagus, stomach, and intestines.
Describe the swallowing mechanism
Upper sphincter is contracted, tongue is pressed against the hard palate, forcing the bolus into the oropharynx
Pharyngeal-oesophageal phase begins, the tongue blocks the mouth, soft palate rises closing of nasopharynx, larynx rises so the epiglottis covers the trachea, upper sphincter relaxes, and food enters into the oesophagus
Muscles contract in the pharynx forcing food into the oesophagus, upper sphincter contracts again after food has entered
Peristalsis occurs moving food along to the stomach
GI sphincter opens, after food enters it closes to prevent regurgitation
What is the enteric NS?
GI tracts contains its own nervous system which staffs enteric neurons that communicate with each other about digestion activity.
Consists of nerve plexuses: submucosal & myenteric which run throughout GI tract & provide a major nerve supply & control GI motion
Submucosal found in submucosal layer
Myenteric found between muscular layers
It contains short & long reflex arcs: Short arcs are mediated by the enteric NS plexus in response to stimuli in GI
Long arcs use CNS integration & extrinisic autonomic nerves. Information is sent via the visceral sensory fibres. It recieves sympathetic & parasympathetic fibres from the ANS. These enter intestinal wall & to the plexus. Initiated by stimulation in & outside of the GI tract
Describe the enteric system pathway
Describe the structure & function of the oesophagus
is a muscular tube around 25cm long running through the mediastinum of the thorax, it transports food from the oropharynx to the stomach. Pierces through the diaphragm at the oesophageal hiatus to enter the abdomen. Joins to the stomach at the cardiac orifice.
Oesophagus has mucosa containing non-keratinized stratified squamous epithelial cells which changes to columnar cells in the stomach (needed for secretion). The submucosa contains mucus glands, as the bolus moves it compresses these glands causing secretion of mucus which greases the walls of the oesophagus allowing food bolus smooth passage.
It has a layer of smooth muscle (2/3) & skeletal muscle (1/3), involved in propulsion and peristalsis in the oesophagus. It also has fibrous adventitia (instead of serosa layer) which are made of connective tissue and coated in mucus to prevent friction in GI tract.
Passage of food occurs via flattening of mucosa folds, mucous glands & loose & elastic submucosa
Describe the structure & function of the stomach
stores food and breaks it down mechanically & chemically, it can hold around 4L of food & water & extend to the pelvis. When stomach collapses mucosa & submucosa fold into rugae. The fundus is the dome-shaped top of the stomach covered by the diaphragm. The mid-portion of the stomach is referred to as the body, which continues down the pyloric part made of the pyloric canal & pyloric antrum.
Pyloric antrum narrows to form pyloric canal leading down to the pyloric sphincter which controls emptying of the stomach.
Stomach contains greater & lesser curvature which omentum extends from. Lesser omentum runs with continuous peritoneum which covers the stomach. The greater omentum covers the small intestine, has fat deposits & lymph nodes.
It has an extra layer of the muscularis which extends from the oblique muscle
Stomach secretes HCL, pepsin & mucus
Describe the structure & function of the 4 cells types involved in the stomach lining
Mucus neck cells- produce a thin acidic layer of mucus different from other mucus cells
Parietal cells- found amongst chief cells, secrete HCL, have microvilli to provide a large SA for HCL secretion into stomach lumen. HCL provides acidic conditions needed for pepsin enzyme to digest proteins. Harsh enough to denature cell walls in plants & proteins as well as bacteria. HCL also controls harmful bacteria
Chief cells- found in basal regions of gastric gland, they produce pepsinogen which is the inactive form of pepsin. When stimulated pepsinogen is released & activated by HCL. Once pepsin is present it catalyses conversion of pepsinogen to pepsin, a positive feedback mechanism facilitated by levels of pepsinogen. It also secretes lipases which breakdown lipids.
G-cells (enteroendocrine cells)- release hormones into interstitial fluid e.g., histamine, which act as paracrine (activates different nearby cells), Gastrin hormone is secreted which regulates stomach secretion & motility
Describe the steps involved in digestion of food in the stomach
- Propulsion: peristaltic waves move from the fundus to the pylorus (connects to duodenum)
- Grinding: most vigorous peristalsis waves towards pylorus. Pyloric end acts as a pump to deliver small amounts of chyme to duodenum
- Retropulsion: Peristaltic wave closes pyloric valve, forcing contents of pylorus back into the stomach
Describe the structure & function of the small intestine
Has the 4 layers
Small intestine has circular folds which are deep permanent folds in the mucosa & submucosa which force chyme to spiral through to the lumen, slowing its movement giving more time for absorption.
Small intestine contains villi which are finger-like projections on the mucosa, they are large in the duodenum and gradually shorten & narrow throughout the small intestine. Each villus has a capillary bed & a lacteal (type of lymphatic vessel). Nutrients are absorbed into the capillary blood & the lacteal
Small intestine has microvilli which are long densely packed extensions on villi, they give mucosa a fuzzy appearance, they contain brush border enzymes on the border which complete carbohydrate and protein digestion
Describe the structure & function of the duodenum
Duodenum contains the most features including duodenal papilla (where bile duct & pancreatic duct merge & empty their contents into the duodenum. It is a hollow c-shaped structure, 10 inches in length.
It contains the Brunner’s glands (duodenal glands) are tubuloalveolar, branched glands that open into the base of the crypts of Lieberkühn. Brunner’s glands manufacture a seromucous fluid rich in bicarbonates that acts to buffer the acidic chyme delivered from the stomach.
The duodenum allows the absorption of mineral & nutrients. Pyloric sphincter found between the duodenum and the stomach to control the passage of food. between the duodenum and the jejunum is the duodenojejunal flexure, which does not close and allows food to pass unobstructed.
Duodenum function: Cholecystokinin (CCK) is a hormone produced by I-cells in the lining of the duodenum.
It is released in response to the presence of proteins and fats to digest. CCK causes the increased production of hepatic bile, and stimulates the contraction of the gall bladder and the relaxation of the sphincter of Oddi, resulting in the delivery of bile into the duodenal part of the small intestine.
Describe the structure & function of the jejenum
Jejunum extends from duodenum to the ileum, it is 2.5 metres long
Jejenum function:
Main Absorption of important nutrients such as sugars, fatty acids, and amino acids.
Vigorous and quick peristalsis to move nutrients through the digestive system.
Lined with longest villi to facilitate nutrient absorption.
Contains mucosal folds (rugue) to increase SA for absorption
Cleavage of nutrients by enzymes.
Absorption of lipophilic nutrients (proteins, fats, cholesterol, and fat-soluble vitamins).
Contains few brunners glands. PH ranges between neutral to alkaline
Describe the structure & function of the ileum
The ileum is twisted & joins the large intestine at the ileocecal valve, it is 3.6m long
Ileum function:
Absorption of vitamin B 12 and conjugated bile salts.
Reabsorption of whatever products of digestion were not absorbed by the jejunum.
Secretion of protease and carbohydrate enzymes which help break down carbohydrates into simple sugars, and lipids into glycerol and fatty acids so they can be more easily absorbed by the body.
Release of certain digestive hormones, including cholecystokinin, gastrin, and secretin
Absorption of water and electrolytes.
Has lots of lymphatic tissue
How does the small intestine receive blood?
The superior mesenteric artery supplies the small intestine with oxygenated blood, deoxygenated blood collected by the mesenteric veins drain back to the hepatic portal vein in the liver.
