Body Fluids and Circulation: questions with answers

Mature mammalian red blood cells generally lack a nucleus and carry haemoglobin for respiratory-gas transport. Platelets are cell fragments derived from megakaryocytes and participate in clotting. White blood cells are nucleated cells; different types have different defence roles. “Formed elements” therefore does not mean that every component is a complete nucleated cell.

Plasma is the fluid matrix of blood and contains clotting factors in an inactive form. Serum is plasma from which clotting factors are absent; it is not unchanged plasma and is not plasma stripped of every protein.

At the first delivery, the source-stated event is: small amounts of Rh-positive foetal blood may expose maternal blood, after which the mother starts preparing anti-Rh antibodies. Rh-negative status does not itself supply anti-Rh antibodies; possible exposure at first delivery can sensitise the mother, creating risk to a subsequent Rh-positive foetus.

Rh-negative status alone does not mean that anti-Rh antibodies are already present. In the chapter’s simplified example, possible exposure to Rh-positive foetal blood at delivery can sensitise an initially unsensitised mother. The chapter describes preventive anti-Rh treatment after delivery to protect later Rh-positive pregnancies. Delivery is only one possible exposure route; real prevention also involves antenatal care and clinical protocols.

The missing position is inactive fibrinogen. Injury-released platelet and tissue factors initiate a linked cascade in which calcium is important; thrombokinase is required for prothrombin to form thrombin, and thrombin converts fibrinogen into fibrin threads.

For the route from blood capillary to tissue spaces, the marked destination contains tissue fluid between cells, depleted of larger proteins and most formed elements compared with blood. Water and many small soluble substances can enter tissue spaces, while lymphatic vessels collect tissue fluid and eventually drain it back to major veins.

Arteries and veins both have tunica intima, tunica media and tunica externa. Their walls are not identical: the smooth-muscle and elastic tunica media is comparatively thinner in veins. Therefore A is the artery and B is the vein in this diagram.

A vessel carrying blood away from the right ventricle to the lungs is the pulmonary artery, even though the blood is deoxygenated. It belongs to the pulmonary circuit, which is distinct from the systemic circuit; together the two circuits make blood pass through the heart twice in a complete circulation.

The corrected account must repair all three independent errors. Chamber number varies among vertebrates, fish have single circulation, and most reptiles have incomplete double circulation with mixing; crocodiles are the four-chambered reptilian exception.

In open circulation, blood passes from large vessels into sinuses or body cavities, as in arthropods and molluscs. In closed circulation, blood remains in vessels, as in annelids and chordates, allowing more precise regulation of flow.

The right atrio-ventricular valve is tricuspid, while the valve at the pulmonary-artery outlet is semilunar. Ventricles are the thick-walled pumping chambers and atria are the relatively thin-walled receiving chambers.

The lower nodal position near the atrio-ventricular septum is the AVN. The SAN initiates the rhythm because it has the greatest intrinsic action-potential frequency, while the AVN relays excitation into the AV bundle and the ventricular conduction network.

The marked component between P and T is the QRS complex. It represents ventricular depolarisation and initiates ventricular contraction; an ECG records electrical activity, not heart sounds or blood pressure.

The strip contains 6 QRS complexes in 5 s. Because QRS represents ventricular depolarisation, the rate is 6 × 60 ÷ 5 = 72 beats min⁻¹; the trace records electrical activity, not sounds or blood pressure.

Heart failure means the heart cannot pump effectively enough to meet body needs; it is not the same as cardiac arrest, when beating stops. A heart attack is sudden damage to heart muscle from inadequate blood supply. Angina is acute chest pain caused when insufficient oxygen reaches heart muscle because blood flow is affected.