Explain the physiological factors that prevent gastro-oesophageal reflux
College Answer
For a good answer candidates were expected to give a description of the lower oesophageal sphincter (the intrinsic and extrinsic sphincters and flap-valve), that it maintains a resting pressure of 15-25 mmHg above gastric pressure which prevents gastro-oesophageal reflux, which relaxes after swallowing, that the resting tone is maintained by myogenic and neurogenic mechanisms and the effects of hormones upon the sphincter (ie gastrin, motilin and α adrenergic stimulation increases and secretin, glucagon, VIP and GIP decrease tone).
Recommended sources: Ganong Review of Medical Physiology Chp 26.
Discussion
- Physical factors:
- Upright posture: gravity prevents the upward movement of stomach contents
- Right lateral position decreases reflux, as it puts the pylorus in a dependent position, promoting gastric emptying (Loots et al, 2012)
- Anatomical factors:
- The angle of His is an acute angle between the oesophagus and the gastric fundus, which contributes to the prevention of reflux mainly by creating a mucosal flap valve.
- Mucosal flap valve: this is a "180-degree musculomucosal fold apposite to the lesser curvature of stomach" created by the intraluminal extension of the angle of His. It blocks the oesophageal opening.
- Posterolateral location of the gastric fundus keeps gastric contents away from the oesophageal opening
- Sphincters:
- Lower gastroesophageal sphincter is the smooth muscle that encircles the lower oesophagus.
- Structurally identical to the circular smooth muscle in the rest of the oesophagus
- Seems thicker because it is constantly tonically contracted.
- Diaphragmatic crura (especially the right crus) is a band of skeletal muscle fibres which form the sides of the oesophageal hiatus, the opening through which the oesophagus descends.
- Resting sphincter tone
- Exerts a pressure that is greater than gastric pressure
- Usually 15-25 mmHg in total (but it only needs to be about 2-3 mmHg higher).
- This tone is influenced by neurogenic, myogenic and hormonal factors:
- Neurogenic control is by the vagus nerve:
- Vagally mediated inhibition relaxes the lower oesophageal sphincter by a nitric oxide mediated mechanism
- Myogenic influence on tone is mainly due to functional differences in the smooth muscle cells in the lower oesophagus, as compared to the rest of the oesophagus:
- Lower oesophageal smooth muscle has more α-actin and basic essential light chains
- That smooth muscle is constantly in a state of depolarization because of a higher resting membrane potential, though to be due to its greater chloride conductance
- Hormones that increase lower oesophageal sphincter tone:
- Gastrin
- Motilin
- Catecholamines (α-adrenergic effect)
- Substance P
- Bombesin
- Galanin
- Pancreatic polypeptide
- Hormones that decrease lower oesophageal sphincter tone:
- Secretin
- Glucagon
- VIP and GIP
- Cholecystokinin
- Somatostatin
References
Minami, Howard, and Richard W. Mccallum. "The physiology and pathophysiology of gastric emptying in humans." Gastroenterology 86.6 (1984): 1592-1610.
Hellström, Per M., Per Grybäck, and Hans Jacobsson. "The physiology of gastric emptying." Best Practice & Research Clinical Anaesthesiology 20.3 (2006): 397-407.
Mittal, Ravinder K., and Raj K. Goyal. "Sphincter mechanisms at the lower end of the esophagus." GI Motility online (2006).
Goyal, Raj K., Yanmei Guo, and Hiroshi Mashimo. "Advances in the physiology of gastric emptying." Neurogastroenterology & Motility 31.4 (2019): e13546.