Chapter 15: Body Fluids and Circulation (Human Physiology)

(i) Erythrocytes (RBCs): contain haemoglobin and transport respiratory gases (O₂, and some CO₂). (ii) Leucocytes (WBCs): defence: neutrophils and monocytes are phagocytic; eosinophils resist infections and are linked with allergy; basophils secrete histamine, serotonin and heparin in inflammation; B and T lymphocytes give immune responses. (iii) Platelets (thrombocytes): release factors for coagulation of blood.

Plasma proteins form 6–8% of plasma. Fibrinogen is needed for clotting (converted to fibrin). Globulins are mainly involved in the defence mechanisms of the body (antibodies). Albumins maintain osmotic balance (keep water in the blood). Plasma proteins also help transport substances and keep the viscosity of blood.

(a)–(iii); (b)–(v); (c)–(ii); (d)–(i); (e)–(iv).

A connective tissue has cells in an extracellular matrix, arises from the mesoderm and links or serves other tissues. Blood has cells (RBCs, WBCs, platelets) in a fluid matrix, plasma; it is mesodermal; and it connects all parts of the body, carrying nutrients, gases, hormones and wastes. Hence it is a fluid connective tissue.

Blood is red, contains RBCs, WBCs and platelets and more plasma proteins, flows in blood vessels pumped by the heart and carries O₂, CO₂ and nutrients. Lymph is colourless, has no RBCs or platelets but many lymphocytes, has fewer proteins, flows in lymph vessels and drains into the veins; it returns tissue fluid, absorbs fats from lacteals and helps in immunity.

Blood passes twice through the heart in one complete circuit, along two separate pathways: pulmonary (right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium) and systemic (left ventricle → aorta → body tissues → vena cava → right atrium). Significance: oxygenated and deoxygenated blood never mix, so the tissues get fully oxygenated blood at high pressure; this supports the high energy needs and constant body temperature of birds and mammals.

(a) See Q5. (b) Open (arthropods, molluscs): blood pumped into open sinuses, bathes tissues directly, flow not precisely regulated; closed (annelids, chordates): blood always in vessels, flow precisely regulated. (c) Systole: contraction of a chamber, pushing blood out; diastole: relaxation, when the chamber fills. (d) P-wave: depolarisation of the atria, leading to atrial contraction; T-wave: repolarisation of the ventricles; its end marks the end of systole.

Fishes: 2-chambered heart (one atrium, one ventricle) pumping only deoxygenated blood to the gills: single circulation. Amphibians and reptiles (except crocodiles): 3-chambered (two atria, one ventricle); left atrium gets oxygenated blood, right atrium deoxygenated, but they mix in the single ventricle: incomplete double circulation. Crocodiles, birds and mammals: 4-chambered (two atria, two ventricles) with no mixing: complete double circulation. The trend is toward complete separation of oxygenated and deoxygenated blood.

The heartbeat starts in the heart muscle itself: the nodal tissue (SAN) is autoexcitable and generates action potentials without any nervous stimulus. Nerves and hormones only moderate the rate. Hence the heart is myogenic (not neurogenic).

All nodal tissue is autoexcitable, but the SAN generates the maximum number of action potentials (70–75 per minute). It therefore initiates and maintains the rhythmic contraction of the heart and sets its pace; hence the pacemaker.

The atria and ventricles are separated by a fibrous AV septum that does not conduct impulses. The AVN picks up the impulse from the atria (after atrial systole, with a short delay so the ventricles fill) and the AV bundle (bundle of His) carries it through the septum; its right and left branches and Purkinje fibres spread it through the ventricular muscle, so both ventricles contract together (ventricular systole).

Cardiac cycle: the sequence of events in the heart, cyclically repeated, consisting of systole and diastole of the atria and ventricles; 72 per minute, each about 0.8 s. Cardiac output: the volume of blood pumped out by each ventricle per minute = stroke volume × heart rate = 70 mL × 72 ≈ 5000 mL (5 L).

Two prominent sounds are heard per cardiac cycle with a stethoscope. The first, lub, is due to the closure of the tricuspid and bicuspid valves at the start of ventricular systole. The second, dub, is due to the closure of the semilunar valves at the start of ventricular diastole. They have clinical diagnostic value (e.g. murmurs show faulty valves).

(Draw the ECG of the figure on page 4.) P-wave: depolarisation of the atria, leading to atrial contraction. QRS complex: depolarisation of the ventricles, initiating ventricular contraction, which starts shortly after Q and marks the beginning of systole. T-wave: repolarisation of the ventricles (return to normal); its end marks the end of systole. Counting QRS complexes per minute gives the heart rate; changes in shape indicate heart disease.