Case 02: Health and Illness Behaviour

This case examines the haematopoietic system and the pharmacology of antiemetic agents through the lens of a patient recovering from trauma and surgery. Understanding blood cell production, the mechanisms of drug-induced nausea and vomiting, and the pharmacology of agents used to control these symptoms is central to safe perioperative management.

Haematopoiesis — the production of all blood cells — occurs in the red bone marrow of the axial skeleton and proximal limb bones in adults. All blood cells derive from pluripotent haematopoietic stem cells, which commit irreversibly to either the myeloid or lymphoid lineage. The myeloid lineage produces erythrocytes, platelets (via megakaryocytes), granulocytes (neutrophils, eosinophils, basophils), and monocytes. The lymphoid lineage produces T lymphocytes, B lymphocytes, and natural killer (NK) cells.

Nausea and vomiting following opioid administration, surgery, or illness are mediated through the chemoreceptor trigger zone (CTZ), located in the area postrema of the medulla oblongata. This region is highly permeable to the blood–brain barrier, allowing circulating emetic substances (including opioid metabolites such as morphine from codeine metabolism) to stimulate dopamine D2 and serotonin 5-HT3 receptors, triggering the vomiting centre. Additional pathways include afferent vagal signals from gastrointestinal mechanoreceptors and chemoreceptors stimulated by gut distension.

Three commonly used antiemetics work through distinct mechanisms. Metoclopramide is primarily a dopamine D2 receptor antagonist (with lesser 5-HT3 and muscarinic antagonist activity). It suppresses the CTZ directly and also promotes gastric motility by blocking the D2-mediated inhibition of acetylcholine release in the gut, increasing peristalsis and gastric emptying. Because it crosses the blood–brain barrier, it can cause adverse neurological effects (extrapyramidal symptoms). Domperidone shares metoclopramide's D2 antagonism mechanism but does not penetrate the BBB, avoiding central neurological side effects. Ondansetron is a selective 5-HT3 receptor antagonist, blocking receptors both centrally in the CTZ and peripherally in the gut wall and vagal nerve terminals. It is particularly effective for chemotherapy-induced and postoperative nausea.

Morphine (derived from codeine via CYP2D6 metabolism) induces nausea via several mechanisms: direct stimulation of CTZ mu-receptors triggering dopamine and serotonin release; inhibition of gut motility (decreased peristalsis, increased pyloric muscle tone, delayed gastric emptying) leading to distension that stimulates visceral mechanoreceptors; and stimulation of the vestibular apparatus. In the context of haemorrhagic shock, morphine also releases histamine, causing vasodilation and further drops in MABP.

Host factors significantly influence wound healing and repair. Nutritional status is critical: vitamin C is required for collagen synthesis (required for both skin and bone healing), vitamin D for bone mineralisation, and vitamin B12 and folate together for nucleic acid synthesis and cellular proliferation (particularly erythropoiesis). Metabolic conditions such as diabetes mellitus and renal failure impair healing. Steroid use reduces collagen synthesis and suppresses the inflammatory phase of healing. Infection — including that caused by smoking-related immunosuppression — is the most common cause of delayed healing. Excessive mechanical movement at the fracture site disrupts callus formation.

The broader illness behaviour of the patient — how they interpret symptoms, seek care, comply with treatment, and adapt to illness — is shaped by psychological, social, and cultural factors. Health behaviour change models such as the Stages of Change (Prochaska and DiClemente) and motivational interviewing provide frameworks for supporting patients through recovery and encouraging adherence to treatment regimens.