Good day everyone, you are welcome to my blog. Today we are discussing fully on the stomach.
Learning outcomes
After studying this section, you should be able to:
- describe the location of the stomach with reference to surrounding structures
- explain the physiological significance of the layers of stomach wall
- discuss the digestive functions of stomach
The stomach is a j-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity.
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1. Organs associated with the stomach
Anteriorly – left lobe liver and anterior abdominal wall
Posteriorly – abdominal aorta,pancreas,spleen,left kidney and adrenal gland
Superiorly – diaphragm,oesophagus and left lobe of liver
Inferiorly – transverse colon and small intestine
To the left – diaphragm and spleen
To the right – liver and duodenum.
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2. Structure of the stomach
The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is
the downward continuation of the oesophagus. Just before the pyloric sphincter it curves upwards to complete the j shape. Where the oesophagus joins the stomach the anterior region angles acutely upwards, curves downwards forming the greater curvature and then slightly upwards towards the pyloric sphincter.
The stomach is divided into three regions: the fundus,the body and the pylorus. At the distal end of the pylorus is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When stomach is inactive the pyloric sphincter is relaxed and open, and when our stomach contains food the sphincter is closed.
Walls of the stomach
The four layers of tissue that comprise the basic structure of the alimentary canal are found in stomach but with some modifications.
3. Muscle layer
This consists of three layers of smooth muscle fibres:
- an outer layer of longitudinal fibres
- a middle layer of circular fibres
- an inner layer of oblique fibres
In this respect, stomach is different from other regions of the alimentary tract as it has three layers of muscle instead of two . This arrangement allows for the churning motion characteristic of gastric activity,as well as peristaltic movement. Circular muscle is strongest between the pylorus and the pyloric sphincter.
mucosa. When the stomach is empty the mucous membrane lining is thrown into longitudinal folds or rugae,and when full the rugae are ‘ironed out’ giving the surface a smooth, velvety appearance. Numerous gastric glands are situated below the surface in the mucous membrane and open on to it . They consist of specialized cells that secrete gastric juice into our stomach.
Blood supply. Arterial supply to our stomach is by the left gastric artery, a branch of the coeliac artery, the right gastric artery and the gastroepiploic arteries. Venous drainage is through veins of corresponding names into the portal vein.
4. Gastric juice
About 2 litres of gastric juice are secreted daily by specialized secretory glands in the mucosa. It consists of :
- water
- mineral salts
- hydrochloric acid
- mucus secreted by mucous neck cells in the glands and surface mucous cells on the stomach surface
- inactive enzymes precursors: Pepsi nogens secreted by chief cells in the glands.
- intrinsic factor
5. Functions of the stomach
These include:
- temporary storage allowing time for the digestive enzymes, pepsins, to act
- chemical digestion- pepsins break proteins into polypeptides
- limited absorption- water,alcohol and some lipid-soluble drugs
- production and secretion of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum
- mechanical breakdown- the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the contents are liquified to chyme. Gastric motility and secretion are increased by parasympathetic nerve stimulation
- preparation of iron for absorption- the acid environment of the stomach solubilises iron salts, essential for iron absorption in the small intestine
- regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pylorus forces small jets of gastric contents through the pyloric sphincter into the duodenum. The sphincter is normally closed, preventing back flow of chyme into our stomach
- secretion of hormones gastrin
- non-specific defence against microbes- provided by hydrochloric acid in gastric juice. Vomiting may occur in response to indigestion of gastric irritants, e.g. microbes or chemicals.
