Physiological Pharmaceutics

80 Physiological Pharmaceuticsstomach, while the left vagus supplies the anterior part. Sympathetic branches from a nervenetwork called the celiac, or solar plexus accompany the arteries of the stomach into themuscular wall.Gastric secretionThe human stomach secretes between 1.0 and 1.5 litres of gastric juice per day. This juiceis highly acidic because of its hydrochloric acid content, and it is rich in enzymes. Gastricjuice provides a medium for soluble food particles to dissolve and it initiates digestion,particularly for proteins.Acid secretionThe 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. Incomparison, the hydrogen ion concentration in the blood is 40 nmolar. Pure parietal cellsecretion is diluted to between 20 and 60 mmolar by non-parietal cell secretion. Normaladults produce a basal secretion of up to 60 ml per hour containig approximately 4 mmolesof H + . This can rise to more than 200 ml, and between 15 and 50 mmoles per hour, whenmaximally 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. Thebicarbonate produced diffuses back into the bloodstream and after a meal, its concentrationis sufficient to produce a marked alkalinity in the urine called the “alkaline tide”. Thehydrogen 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 mixtureof hydrochloric acid and potassium chloride.Hydrochloric acid is produced by the parietal cells in response to histamine, gastrinor acetylcholine stimulation. Histamine is released from gastric mast cells in response tofood ingestion, and acts on the H 2receptors in the parietal cells, directly causing acidsecretion. Gastrin is released in response to reduced acidity of the gastric contents whenfood enters the stomach, and in particular by the presence of peptides. Acetylcholine isFigure 5.6 Variation in composition of gastric juice with secretion rate