The stomach can also be divided into three anatomical regions (Figure 5.1). The uppermost part is the fundus, which after a meal is often seen to contain gas. It also produces slow sustained contractions which exert a steady pressure on the gastric contents gradually pressing them in an aboral direction. The largest part of the stomach is the body which acts as a reservoir for ingested food and liquids. The antrum is the lowest part of the stomach. It is almost funnel-shaped, with its wide end joining the lower part of the body of the stomach and its narrow end connecting with the pyloric canal. The pyloric portion (the antrum plus the pyloric canal) of the stomach tends to curve to the right and slightly upward and backward and thus gives the stomach its J-shaped appearance.
Lower oesophageal sphincter
Lower oesophageal sphincter
Figure 5.1 Structure of the stomach
Figure 5.1 Structure of the stomach
The stomach adapts to increasing volumes of food by receptive relaxation, which allows the stomach to expand with little variation in intragastric pressure. The distal or antral portion of the stomach has a wall composed of thicker muscle and is concerned with regulation of emptying of solids by contraction, and by acting as a gastric homogenizer and grinder. It is co-ordinated with the body in the propulsion of gastric contents towards the pylorus. The pyloric sphincter has two functions. It sieves the chyme and prevents large particles of food from being emptied from the stomach before solid masses have been sufficiently reduced in size. It also prevents reflux of duodenal material containing bile and pancreatic enzymes which may damage the gastric mucosa.
The mucosa of the stomach (Figure 5.2) is thick, vascular and glandular and is thrown into numerous folds or rugae, which for the most part run in the longitudinal direction, and flatten out when distended. The mucosal surface of the stomach is lined by a single layer of simple columnar epithelium, 20 to 40 pm in height. Approximately 3.5 million gastric pits (foveolae) puncture the lining, each of which serves approximately 4 gastric glands (Figure 5.3). The distribution of gastric glands varies throughout the stomach. The first region, 1.5 to 3 cm in length, around the gastric cardia or gastro-oesophageal junction, contains the cardiac glands. The second region, the fundus and body, contains the acid-secreting glands. The third region, which contains the pyloric or antral glands, includes the pylorus and extends past the antrum to the lesser and greater curvatures.
The surface of the mucosa is always covered by a layer of thick tenacious mucus that is secreted by the columnar cells of the epithelium. Gastric mucus is a glycoprotein which lubricates food masses, facilitating movement within the stomach, and forms a protective
layer over the lining epithelium of the stomach cavity. This protective layer is approximately 140 pm thick in humans and is a defence mechanism to prevent the stomach from being digested by its own proteolytic enzymes (Figure 5.4). The barrier is enhanced by the secretion of bicarbonate into the surface layer from the underlying mucosa. As the hydrogen ions diffuse across the mucus layer from the lumen, they meet bicarbonate secreted from the underlying mucosa, thus setting up a pH gradient. The mucus is continually digested from the luminal surface and is continually being replaced from beneath. It has been estimated that the turnover time of the mucus layer, i.e. from production to digestion, is in the order of 4 to 5 hours. However, it may be slower since any interaction with the mucosa causes it to secrete copious amounts of mucus, making accurate measurements problematic.
The gastric glands are located beneath the surface epithelium and open into small pits or foveolae gastricae. There are approximately 100 gastric pits per square millimetre of surface epithelium. Between three and seven glands open into each pit.
The gastric mucosa contains five different types of cells. In addition to the tall columnar surface epithelial cells mentioned above, the other common cell types found in the various gastric glands are:
(1) Mucoid cells which secrete mucus and are found in all the gastric glands. They predominate in the gastric glands in the cardiac and pyloric areas of the stomach. The necks of the glands in the body and fundus are also lined with mucoid cells.
(2) The chief or zymogen cells are located predominantly in the gastric glands in the body and fundus. These cells secrete pepsinogen, the precursor for the enzyme pepsin.
(3) Hydrochloric acid is secreted by the parietal or oxyntic cells which are mainly located in the body and fundus of the stomach (Figure 5.5). They are also responsible for secreting most of the water which is found in the gastric juice and a protein called intrinsic factor. Parietal cells are almost completely absent from the antrum.
(4) Endocrine cells or endocrine-like cells. The endocrine cells throughout the antrum secrete the acid-stimulating hormone gastrin. The endocrine cells are scattered, usually singly, between the parietal and chief cells.
Arterial blood is brought to the stomach via many branches of the celiac trunk. The celiac trunk is a short, wide artery that branches from the abdominal portion of the aorta, the main vessel conveying arterial blood from the heart to the systemic circulation. Blood from the stomach is returned to the venous system via the portal vein, which carries the blood to the liver.
Both parasympathetic and sympathetic divisions of the autonomic nervous system supply the stomach. The parasympathetic nerve fibres are carried in the vagus (10th cranial) nerves. As the vagus nerves pass through the opening in the diaphragm together with the oesophagus, branches of the right vagus nerve spread over the posterior part of the
stomach, while the left vagus supplies the anterior part. Sympathetic branches from a nerve network called the celiac, or solar plexus accompany the arteries of the stomach into the muscular wall.
The human stomach secretes between 1.0 and 1.5 litres of gastric juice per day. This juice is highly acidic because of its hydrochloric acid content, and it is rich in enzymes. Gastric juice provides a medium for soluble food particles to dissolve and it initiates digestion, particularly for proteins.
The composition of the gastric juice varies according to the stimulus and the secretion rate (Figure 5.6). It is a mixture of water, hydrochloric acid, electrolytes (sodium, potassium, calcium, phosphate, sulphate, and bicarbonate) and organic substances (mucus, pepsins, and protein). Parietal cells can secrete hydrogen ions at a concentration of 150 mmolar. In comparison, the hydrogen ion concentration in the blood is 40 nmolar. Pure parietal cell secretion is diluted to between 20 and 60 mmolar by non-parietal cell secretion. Normal adults produce a basal secretion of up to 60 ml per hour containig approximately 4 mmoles of H+. This can rise to more than 200 ml, and between 15 and 50 mmoles per hour, when maximally stimulated.
Hydrogen ions are produced by metabolic activity in the parietal cells (Figure 5.7). The key reaction between water and carbon dioxide is catalysed by carbonic anhydrase. The bicarbonate produced diffuses back into the bloodstream and after a meal, its concentration is sufficient to produce a marked alkalinity in the urine called the "alkaline tide". The hydrogen ions are actively pumped into the stomach lumen in exchange for potassium ions. Potassium also diffuses passively out of the cell, hence pure parietal secretion is a mixture of hydrochloric acid and potassium chloride.
Hydrochloric acid is produced by the parietal cells in response to histamine, gastrin or acetylcholine stimulation. Histamine is released from gastric mast cells in response to food ingestion, and acts on the H2 receptors in the parietal cells, directly causing acid secretion. Gastrin is released in response to reduced acidity of the gastric contents when food enters the stomach, and in particular by the presence of peptides. Acetylcholine is
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