MRCS Part B Revision · Applied surgical science and critical care
Gastric Outflow — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Gastric Outflow is an applied surgical science and critical care station. Three of the seventeen examined stations in the MRCS Part B OSCE fall in this area. These stations ask you to interpret data and manage a sick surgical patient: a chart, a blood gas, an imaging study or a deteriorating patient on the ward, and the physiology underneath the decision.
What you need to know for the Gastric Outflow station
A man with a peptic ulcer history and twelve hours of vomiting, whose gas shows a hypochloraemic, hypokalaemic metabolic alkalosis. The station moves from interpreting that to the physiology of gastric acid.
The blood results
A metabolic alkalosis (pH 7.62, bicarbonate 56) with respiratory compensation (carbon dioxide 54), hypochloraemia (72) and hypokalaemia (3.0). The picture is gastric outlet obstruction from a scarred pylorus: the vomit is pure gastric juice, so hydrogen and chloride are lost without the duodenal bicarbonate that would normally balance them.
Why the alkalosis, and why paradoxical aciduria
Losing hydrochloric acid leaves bicarbonate behind in the blood. Volume depletion then drives the kidney to keep sodium at any cost: aldosterone exchanges sodium for potassium and hydrogen in the distal tubule, and with little chloride available the proximal tubule reabsorbs sodium with bicarbonate instead. The kidney therefore excretes hydrogen ions into an already alkaline blood, so the urine is acid while the plasma is alkaline. The potassium falls from loss in vomit, from the aldosterone-driven exchange, and from the shift of potassium into cells as hydrogen moves out to buffer the alkalosis. The treatment is saline with potassium; the chloride is what corrects the alkalosis.
The cells of the stomach
Parietal (oxyntic) cells in the body and fundus secrete hydrochloric acid and intrinsic factor. Chief cells secrete pepsinogen. Mucous neck cells and surface cells secrete mucus and bicarbonate. G cells in the antrum secrete gastrin; D cells secrete somatostatin; enterochromaffin-like cells secrete histamine.
What acid does
Activates pepsinogen to pepsin and gives it the pH it works at, denatures protein, kills swallowed organisms, releases iron and B12 from food, and triggers secretin release in the duodenum.
How it is made
Carbonic anhydrase in the parietal cell makes carbonic acid from carbon dioxide and water. The hydrogen ion is pumped into the canaliculus by the H+/K+ ATPase (the proton pump) in exchange for potassium; chloride follows through its own channel. The bicarbonate leaves the basolateral side in exchange for chloride, which is the “alkaline tide” after a meal. The pump is stimulated by acetylcholine (vagus), gastrin and histamine, and inhibited by somatostatin and prostaglandins; proton pump inhibitors block it directly.
Phases of secretion
Cephalic (about 30%): the sight, smell and taste of food act through the vagus. Gastric (about 60%): distension and peptides in the stomach release gastrin and acetylcholine. Intestinal (about 10%): a small stimulation early, then inhibition by secretin, cholecystokinin and gastric inhibitory peptide once acid and fat reach the duodenum. Basal secretion is greatest in the evening.
Why the stomach does not digest itself
A layer of mucus holding bicarbonate at the surface, tight junctions between the cells, rapid turnover of the epithelium every few days, prostaglandin-maintained blood flow that carries acid away, and pepsinogen kept inactive until it reaches the lumen. Aspirin, NSAIDs and Helicobacter break this down.
Volumes
About 2 litres of gastric juice a day, part of roughly 7–8 litres of secretions entering the gut, nearly all reabsorbed.
What are you asked at the Gastric Outflow station?
The station runs to 22 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- Blood Gas PaO2 75 mmHg (>75 mmHg) on air pH 7.618 (7.35 – 7.45) PaCO2 54.4 mmHg (35 – 45 mmHg) HCO3- 56.2 mmol l-1 (22 – 26 mmol l-1) BE - 30.1 mmol l-1 (+/- 2 mmol l-1) Electrolytes Na+ 135 mmol l-1 (134-146 mmol l-1) K+ 3.0 mmol l-1 (3.4-5.0 mmol l-1) Cl- 72 mmol l-1 (98-108 mmol l-1) Applied Sciences: Gastric Outflow
- What do the patient’s blood results demonstrate?
- Can you explain what might be causing the above results and how?
- The patient is also noted to have an aciduria. Why is this?
- Why is the patient's potassium low?
- Describe the cells of the stomach and what they secrete
- What roles does HCL perform when secreted?
- Describe how HCL is secreted by parietal cells
- How is HCL secretion regulated?
- Describe the process and phases of gastric acid secretion
- Why does HCL secreted not damage the stomach?
- What is the volume of gastric secretion daily?
And 10 more at this station.
How is the applied surgical science and critical care station marked in MRCS Part B?
Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
FAQ
What does the Gastric Outflow station ask?
It opens with "Blood Gas PaO2 75 mmHg (>75 mmHg) on air pH 7.618 (7.35 – 7.45) PaCO2 54.4 mmHg (35 – 45 mmHg) HCO3- 56.2 mmol l-1 (22 – 26 mmol l-1) BE - 30.1 mmol l-1 (+/- 2 mmol l-1) Electrolytes Na+ 135 mmol l-1 (134-146 mmol l-1) K+ 3.0 mmol l-1 (3.4-5.0 mmol l-1) Cl- 72 mmol l-1 (98-108 mmol l-1) Applied Sciences: Gastric Outflow" and runs to 22 questions over nine minutes. Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
What counts as applied surgical science in MRCS Part B?
Three of the seventeen examined stations: critical care management, interpretation of clinical data such as blood results and charts, and interpretation of visual information such as imaging and traces.
How is an applied science station marked?
Out of 20, with 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. Unlike the communication stations, none of the marks are for how you say it.
How much physiology do I need?
Enough to explain the decision you are making. The station rewards applying physiology to the patient in front of you rather than reciting it, so practise talking through why a number changes your management.
How many stations are in the MRCS Part B OSCE?
Seventeen examined stations of nine minutes each, with a minute to read the task before each one. Two preparation stations and at least one rest station bring the circuit to about twenty, and the exam takes about three and a half hours.
What is the pass mark for MRCS Part B?
There is no published pass mark. The cut score is set separately for Applied Knowledge and Applied Skills, for each circuit, by borderline regression. Published pass rates across the 2024/25 diets ranged from 51% to 66%.
Can I fail a station and still pass?
Yes. There is no rule about how many stations you may fail: the cut score applies to your total mark in each component, so a weak station costs the marks you lost on it and strong stations elsewhere can make them back. Applied Knowledge and Applied Skills are passed separately and must both be passed at the same sitting, so a strong anatomy performance cannot rescue a weak communication one.
Dr Richard Miller, MBChB FRCS
Station summaries are reviewed against the current intercollegiate MRCS syllabus and the published marking blueprint. Guidance changes between diets: check the royal colleges' own pages before relying on a date, a fee or a threshold.
Practise this station
The question bank carries the model answer to every question above, with the rest of the applied surgical science and critical care stations.
More applied surgical science and critical care stations