MMED1005 Lecture Notes - Lecture 25: Bilirubin, Lecithin, Carboxypeptidase
7 Jun 2018
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The processes as food moves through the GI tract
**dot orry aout salia as it otais oly ery fe ezyes. The oesophagus is there to
get food into the stomach to be safely handled**
Gastric (stomach) function:
- Motility:
o Upper stomach (fundus and corpus) act as a reservoir
▪ Food gets broken down into a thin paste with large surface area
o Lower stomach acts as a pump and grinding mechanism
- Secretion – exocrine (i.e.: released out of the body)
o Acid gastri aid ad itrisi fator sereted fro parietal cells (or
oxyntic ells i the upper stomach
o Pepsinogen from Chief cells, largely in upper stomach
o Msucous from goblet cells throughout gut, acts to lubricate. Does not stop
gastric acid from getting onto the epithelial layer, just slows the rate.
o Bicarbonate-rich solution from all enterocytes (epithelial cells). This solution
neutralises the gastric acid. surface of the epithelium is almost neutral pH
- Secretion – endocrine (hormones) and paracrine (local mediators) and neurocrine
(from enteric nerve cells)
o At as three loks hih ust all e opeed efore the parietal ells i the
upper stomach release gastric acid
▪ Gastrin from G cells (in the mucosa of the distal lower stomach.
Remember the gastric acid is secreted from upper stomach)
▪ Histamine from ECL cells (enterochromaffin-like-cells) paracrine
▪ Acetylcholine from enteric nerve cells – come from enteric neurones
The stomach:
- The lower stomach acts as a pump in terms of motility, with waves of contraction
that squeeze contents into the duodenum
- The upper stomach acts like a reservoir, holding contents in low pressure
Gastric acid: HCl
- pH in the full stomach can lie between pH 2-4
- acid has several functions:
o kills bacteria/fungi
o activates pepsinogen to form pepsin
o helps break up food particles, together with grinding movements of motility
- acid production is tightly controlled – activated by ACh, histamine, and gastrin.
Inhibited by somatostatin
- acid does not usually damage the mucosa because of the layer of mucous and
underlying HCO3--rich solution
find more resources at oneclass.com
find more resources at oneclass.com
Gastric mucosa:
- in the upper stomach, the oxyntic (acid-producing) mucosa has deep pits in the
mucosa contain the cells (parietal cells or oxyntic cells) which secrete the acid
- surrounded by ECL cells
- gastrin (a hormone) is produced by G cells in similar pits, but these pits do not
contain parietal cells
- *there are also somatostatin-releasing D cells near the parietal cells
Control of gastric acid:
- acid release is tightly controlled – it is energy-expensive and risky
- histamine from ECL cells excites parietal (oxyntic) cells
- ACh is released from enteric neurons onto parietal cells: the enteric neurons are
mostly activated by parasympathetic neurons from the brain via the vagus nerve
- Gastrin is released from G cells in the mucosa of the distal stomach in the presence
of peptides from food. It is a hormone, switching on parietal cells
- Somatostatin is released from D cells when the gastric contents become too acidic; it
directly inhibits parietal cells, and inhibits gastrin and histamine release from G cells
and ECL cells
o Negative feedback
- Release happens in 3 phases:
o The cephalic phase (sight and smell of food)
o Gastric phase – food in the stomach
o Intestinal phase – food in the duodenum initially stimulates acid secretion,
then inhibits it as digestion proceeds
What gastric acid does:
- Take crudely chopped food that has been swallowed, acidifies it and kills off bugs,
and starts the breakdown of proteins. Together with the churning movements of the
lower stomach, the lumpy paste becomes a smoother paste with smaller particles
- Stomach squeezes dollops of food into the duodenum at an appropriate rate for the
amount of nutrients contained
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
The processes as food moves through the gi tract. **do(cid:374)(cid:859)t (cid:449)orry a(cid:271)out sali(cid:448)a as it (cid:272)o(cid:374)tai(cid:374)s o(cid:374)ly (cid:448)ery fe(cid:449) e(cid:374)zy(cid:373)es. The oesophagus is there to get food into the stomach to be safely handled** Motility: upper stomach (fundus and corpus) act as a reservoir, food gets broken down into a thin paste with large surface area, lower stomach acts as a pump and grinding mechanism. Does not stop gastric acid from getting onto the epithelial layer, just slows the rate: bicarbonate-rich solution from all enterocytes (epithelial cells). Surface of the epithelium is almost neutral ph. Remember the gastric acid is secreted from upper stomach: histamine from ecl cells (enterochromaffin-like-cells) paracrine, acetylcholine from enteric nerve cells come from enteric neurones. The lower stomach acts as a pump in terms of motility, with waves of contraction that squeeze contents into the duodenum. The upper stomach acts like a reservoir, holding contents in low pressure.
