FRCS Surgical Physiology — Questions and Answers
Question 1: A patient loses 1500 ml of blood acutely. According to the ATLS classification, this represents which class of haemorrhagic shock?
- Class III (Correct answer)
- Class II
- Class I
- Class IV
Correct answer: Class III
Class III haemorrhage represents 30-40% blood volume loss (approximately 1500-2000 ml in a 70 kg adult). It is characterised by tachycardia, hypotension, tachypnoea, reduced urine output, and altered mental status. Crystalloid and blood transfusion are typically required.
Question 2: In the fasting state, which organ is the primary consumer of ketone bodies for energy?
- Brain (Correct answer)
- Liver
- Skeletal muscle
- Red blood cells
Correct answer: Brain
During prolonged fasting, the brain adapts to utilise ketone bodies (beta-hydroxybutyrate and acetoacetate) for up to 70% of its energy needs. The liver produces ketone bodies but cannot utilise them as it lacks the enzyme succinyl-CoA:3-ketoacid CoA transferase. Red blood cells lack mitochondria and rely solely on glycolysis.
Question 3: A patient with a large bowel obstruction develops metabolic acidosis. Which acid-base disturbance is most likely if the patient also has persistent vomiting?
- Mixed metabolic acidosis and metabolic alkalosis (Correct answer)
- Pure metabolic acidosis
- Respiratory alkalosis
- Pure metabolic alkalosis
Correct answer: Mixed metabolic acidosis and metabolic alkalosis
Large bowel obstruction causes metabolic acidosis from ischaemia, bacterial translocation, and lactate production. Concurrent vomiting causes loss of gastric hydrochloric acid, producing metabolic alkalosis. The combination results in a mixed acid-base disturbance. Arterial blood gas analysis and calculation of the anion gap help differentiate the components.
Question 4: Which of the following is the most important buffer system in the extracellular fluid?
- Bicarbonate-carbonic acid system (Correct answer)
- Phosphate buffer system
- Protein buffer system
- Haemoglobin buffer system
Correct answer: Bicarbonate-carbonic acid system
The bicarbonate-carbonic acid (HCO3-/H2CO3) system is the principal extracellular buffer. It is an open system: CO2 is regulated by the lungs and HCO3- by the kidneys, giving it enormous buffering capacity. Although the phosphate system is important intracellularly and in renal tubular fluid, and haemoglobin is the main intracellular buffer of red blood cells, bicarbonate dominates extracellularly.
Question 5: Following major abdominal surgery, a patient develops oliguria with a urine sodium concentration of 8 mmol/L. What is the most likely cause?
- Appropriate physiological response to surgery (ADH and aldosterone secretion) (Correct answer)
- Acute tubular necrosis
- Post-renal obstruction
- Renal artery thrombosis
Correct answer: Appropriate physiological response to surgery (ADH and aldosterone secretion)
A low urine sodium (<20 mmol/L) indicates avid sodium reabsorption by the kidneys, consistent with a pre-renal cause. Post-surgical stress triggers ADH and aldosterone release, causing sodium and water retention. In acute tubular necrosis, the urine sodium would typically be >40 mmol/L as the damaged tubules lose their ability to reabsorb sodium.
Question 6: During the systemic inflammatory response syndrome (SIRS), which cytokine is primarily responsible for inducing fever?
- Interleukin-1 (IL-1) (Correct answer)
- Interleukin-10 (IL-10)
- Transforming growth factor-beta (TGF-beta)
- Interleukin-4 (IL-4)
Correct answer: Interleukin-1 (IL-1)
IL-1 (along with TNF-alpha and IL-6) is a key pyrogenic cytokine. It acts on the hypothalamic thermoregulatory centre via prostaglandin E2 synthesis, resetting the temperature set-point upwards. IL-10 and TGF-beta are anti-inflammatory, while IL-4 promotes Th2 responses.
FRCS Surgical Physiology
A patient loses 1500 ml of blood acutely.
According to the ATLS classification, this represents which class of haemorrhagic shock?