Gastrointestinal Physiology
Don't panic, we're not writing you a textbook on GI physiology in a single post.
This is a rapid revision post of the key facts on the alimentary canal that get examined in both the Primary and Final FRCA exams.
The liver and spleen are covered in separate posts (linked below).
What is saliva?
- A watery secretion from salivary glands that lubricates the oral cavity, initiates digestion, protects the teeth and mucosa and facilitates taste and speech
- 99% water
- Electrolytes (Na, K, Cl, HCO3)
- pH is 8
- Mucins (lubrication)
- α-amylase (carbohydrate digestion)
- Lingual lipase (fat digestion)
- Haptocorrin (binds and protects vitamin B12 from stomach acid)
- Lactoferrin
- Lysozyme
- Immunoglobulin A
It is produced by three paired major salivary glands:
- Parotid - enzyme-rich serous fluid
- Submandibular - serous and mucous
- Sublingual - mainly mucous
Parasympathetic stimulation makes higher volume, watery saliva, while sympathetic stimulation makes drier, thicker saliva, hence glycopyrrolate causes a dry mouth.
What is swallowing?
- Swallowing is a coordinated neuromuscular process, partly voluntary and partly reflexive, that transfers a bolus from the mouth to the stomach while protecting the airway
- It involves cranial nerves V, VII, IX, X, XII
- It has three phases - oral, pharyngeal and oesophageal
The oral phase from mouth to pharynx is voluntary
- Food is mixed with saliva
- It is then pushed posteriorly by the tongue to the oropharynx
The pharyngeal phase from pharynx to oesophagus is involuntary
- Triggered when food reaches the oropharynx (CN 9)
- Soft palate lifts to seal the nasopharynx
- Larynx elevates, glottis closes and epiglottis folds down (airway protection)
- Cricopharyngeus relaxes (upper oesophageal sphincter)
- Superior and middle pharyngeal constrictors contract
- Brief apnoea of one or two seconds
The oesophageal phase from cricopharyngeus to stomach is also involuntary
- Peristalsis pushes bolus towards the stomach
- Cricopharyngeus closes
- Lower oesophageal sphincter relaxes
What type of muscle is found in the oesophagus?
- Upper 1/3 - striated muscle
- Lower 2/3 - smooth muscle
The upper oesophageal sphincter has a barrier pressure of 30-60 cmH2O, so vigorous facemask ventilation can easily insufflate the stomach.
The lower sphincter has a lower pressure of 20-40 cmH2O.
What factors influence lower oesophageal sphincter tone?
- The lower oesophageal sphincter is a functional sphincter
- This means it is a physiological high-pressure zone that controls flow between two regions without a distinct anatomical ring of muscle
Factors that increase LOS tone:
- Acetylcholine (parasympathetic)
- Histamine
- Gastrin
- Motilin
Factors that decrease LOS tone:
- Dopamine
- Oestrogen
- Cholecystokinin
- Secretin
What is nausea?
- An uncomfortable sensation in the stomach that may accompany an urge to vomit