Acid Base and GIT: part 1

GIT and Acid base homeostasis

Acid- base changes in the GIT are actually in sync and infact only to bring about efficient absorption of the dietary component and hence do not play a role in acid-base homeostasis (unlike kidney). The small amount of alkali lost as a by-product of acid-base transport events in the guts is easily regenerated by renal net acid excretion. Abnormal gut function brings about a upheaval in maintenance of acid-base homeostasis.

Normal Physiology of Gut Fluid and Electrolyte Transport

During the course of each day, secretion as well as absorption of fluid and electrolytes occurs along the gastrointestinal tract.

Normally 7 to 8 L of fluid is secreted each day, far exceeding dietary consumption, and almost all of these secretions, as well as any ingested fluid, are absorbed by the end of the colon.

Saliva

Via parotid, submandibular, and sublingual glands; in addition, there are many very small buccal glands. The parotid glands secrete almost entirely the serous type of secretion, while the submandibular and sublingual glands secrete both serous secretion and mucus. The buccal glands secrete only mucus. The submandibular glands contribute about two thirds of resting salivary secretion, the parotid glands about one fourth, and the sublingual glands the remainder. Saliva has a pH between 6.0 and 7.0, a favorable range for the digestive action of ptyalin.

At rest salivary secretion is low, amounting to about 30 mL/hr. Stimulation increases the rate of salivary secretion, most notably in the parotid glands, up to 400 mL/hr. The most potent stimuli for salivary secretion are acidic-tasting substances ( citric acid),  smell of food and chewing. Anxiety, fear, dehydration, and medications (e.g., antihistamines) inhibit secretion.

Primary secretion (from the acini)  is isotonic as plasma as if formed as a result of ultrafiltration. But actually as the final secretion (ductular) occurs, Cl^- and Na⁺  are reabsorbed and and K⁺ and HCO₃^-  are secreted (via CFTR channel??). The changes in the ductular cells result in a hypotonic secretion. When salivary flow is rapid, there is less time for ionic composition to change in the ducts.  Aldosterone increases the K⁺ concentration and reduces the Na⁺ concentration of saliva in an action analogous to its action on the kidneys.

The hypotonicity facilitates taste sensitivity and results in various organic compounds form a protective coating on the oral mucosa. Salivary bicarbonate , calcium and phosphate act as buffer and  neutralize acids that would otherwise destroy tooth minerals 

For details readers are advised to read an excellent paper by Melvin JE (1999)

Gastric secretion of acid

Acid is secreted by parietal cells. Various transport proteins are present in the parietal cells.

1.       H⁺/ K⁺ ATPase

2.       K⁺ selective channel

3.       Cl^- channel

4.       Cl^-/ HCO₃^- exchanger  :   At the serosal surface

5.       Na⁺/ K⁺ ATPase   :     At the baso- lateral surface  

The pH and volume of the gastric secretion is regulated primarily by gastrin so maximum acid secretion occurs only after a meal [ pH= 1, volume= 7 ml/ min].

A fall in pH also increases the amount of HCO₃ absorbed [ alkaline tide]. Eventually HCO₃^- secreted back into GI tract by pancreas.

It is believed that K⁺ is merely recycled at the canalicular membrane on the luminal side otherwise intracellular K⁺ levels may rise to dangerous levels.

         

Rest of  the details  will be discussed in the next post.

Poorvi