Body Fluids and Circulation

The One Thing

Blood and lymph, together with the heart and blood vessels, form an integrated transport system that maintains homeostasis, supports defence, and distributes oxygen and other materials. The four-chambered heart and double circulation keep oxygenated and deoxygenated blood largely separate, while coordinated electrical activity ensures orderly pumping.

Reactions, Processes and Experiments

Key Terms

• Blood: A fluid connective tissue made of plasma and formed elements that transports materials throughout the body.
• Plasma: The straw-coloured liquid component of blood, forming about 55% of its volume and containing water, proteins, nutrients, hormones, gases, and wastes.
• Formed elements: The cellular components of blood: red blood cells, white blood cells, and platelets.
• Red blood cells: Biconcave, enucleate cells containing haemoglobin that transport oxygen and some carbon dioxide.
• Haemoglobin: An iron-containing respiratory pigment in red blood cells that binds oxygen reversibly.
• White blood cells: Nucleated blood cells that protect the body through immune responses and defence against pathogens.
• Granulocytes: White blood cells containing cytoplasmic granules, including neutrophils, eosinophils, and basophils.
• Agranulocytes: White blood cells without prominent cytoplasmic granules, mainly lymphocytes and monocytes.
• Platelets: Cell fragments formed from megakaryocytes that help stop bleeding by participating in blood clotting.
• Blood coagulation: The process by which a clot forms at an injured site through reactions involving platelets, clotting factors, calcium ions, and thrombin.
• Lymph: A colourless fluid formed from tissue fluid that returns excess fluid and proteins to the blood and supports immunity and fat absorption.
• Heart: A muscular, four-chambered organ that pumps blood through the pulmonary and systemic circulations.
• Atria: The two upper chambers of the heart that receive blood.
• Ventricles: The two lower chambers of the heart that pump blood out of the heart.
• Valves: Flap-like structures that prevent the backward flow of blood.
• Pulmonary circulation: Movement of deoxygenated blood from the right ventricle to the lungs and oxygenated blood back to the left atrium.
• Systemic circulation: Movement of oxygenated blood from the left ventricle to body tissues and deoxygenated blood back to the right atrium.
• Double circulation: A circulatory pattern in which blood passes through the heart twice during one complete circuit.
• Cardiac cycle: One complete sequence of contraction and relaxation of the heart chambers.
• Systole: The contraction phase of a heart chamber.
• Diastole: The relaxation phase of a heart chamber.
• Stroke volume: The volume of blood pumped by one ventricle during one heartbeat.
• Cardiac output: The volume of blood pumped by one ventricle per minute.
• Conducting system: Specialised cardiac tissue that generates and spreads electrical impulses to coordinate heartbeat.
• Sinoatrial node: The natural pacemaker of the heart, located in the right atrium, that initiates the heartbeat.
• Atrioventricular node: A specialised node that receives the impulse from the sinoatrial node and passes it to the ventricles after a short delay.
• Blood vessels: Tubular structures that carry blood; arteries carry blood away from the heart, veins return blood, and capillaries allow exchange with tissues.
• Arteries: Thick-walled, elastic vessels that generally carry blood away from the heart under high pressure.
• Veins: Thin-walled vessels that generally carry blood toward the heart and contain valves to prevent backflow.
• Capillaries: Microscopic, thin-walled vessels where exchange of gases, nutrients, and wastes occurs between blood and tissues.
• Blood groups: Classifications based on specific antigens and antibodies in blood, mainly the ABO and Rh systems.
• ABO system: A blood-group system based on A and B antigens on red blood cells and corresponding antibodies in plasma.
• Rh factor: An antigen, commonly the D antigen, on red blood cells; its presence makes a person Rh-positive and its absence makes the person Rh-negative.
• Electrocardiogram: A graphical recording of the electrical activity of the heart, commonly showing P wave, QRS complex, and T wave.
• Hypertension: Persistently elevated blood pressure, which can increase the risk of heart and blood-vessel disease.

Easily Confused

• Plasma and lymph: Plasma is the liquid component of blood; lymph is colourless fluid formed from tissue fluid that returns excess fluid and proteins to the blood.
• Formed elements and plasma: Formed elements are the cellular components of blood; plasma is its liquid component.
• Granulocytes and agranulocytes: Granulocytes have prominent cytoplasmic granules; agranulocytes do not.
• Atria and ventricles: Atria receive blood; ventricles pump blood out of the heart.
• Pulmonary and systemic circulation: Pulmonary circulation occurs between the heart and lungs; systemic circulation occurs between the heart and body tissues.
• Systole and diastole: Systole is contraction; diastole is relaxation.
• Stroke volume and cardiac output: Stroke volume is blood pumped per heartbeat; cardiac output is blood pumped per minute.
• Arteries and veins: Arteries carry blood away from the heart and veins return blood to it; oxygen content is not an absolute distinction because the pulmonary artery carries deoxygenated blood and pulmonary veins carry oxygenated blood.
• ABO system and Rh factor: ABO classification depends on A and B antigens; Rh classification mainly depends on the presence or absence of the D antigen.
• P wave and QRS complex: The P wave represents atrial depolarisation; the QRS complex represents ventricular depolarisation.
• QRS complex and T wave: The QRS complex represents ventricular depolarisation; the T wave represents ventricular repolarisation.
• Sinoatrial node and atrioventricular node: The sinoatrial node initiates the heartbeat; the atrioventricular node transmits the impulse to the ventricles after a short delay.
• Tricuspid and bicuspid or mitral valves: The tricuspid valve lies between the right atrium and right ventricle; the bicuspid or mitral valve lies between the left atrium and left ventricle.

What Gets Asked

• Composition of blood: Questions may require the proportions of plasma and formed elements, the composition of plasma, or the functions of albumins, globulins, and fibrinogen. A common mark-losing slip is confusing plasma proteins with formed elements.
• Blood-cell functions and counts: Questions may compare red blood cells, white blood cells, and platelets, including red blood cell survival of about 120 days and approximate cell counts. Marks are lost by assigning antibody production to T cells rather than B cells, or by overlooking macrophage formation from monocytes.
• Clotting and lymph: Questions may ask how thrombin converts fibrinogen into fibrin or how lymph returns tissue fluid, transports absorbed fats from intestinal villi, and supports immunity. Do not state that lymph is the same as plasma or omit its role in fluid recovery.
• Heart structure and circulation: Questions may require the route of blood through the chambers, vessels, and valves, or an explanation of double circulation. A frequent error is reversing the pulmonary artery and pulmonary veins or stating that arteries always carry oxygenated blood.
• Cardiac cycle and cardiac output: Questions may require the durations of atrial systole, ventricular systole, and joint diastole, or use Cardiac output = heart rate x stroke volume. Marks are lost by confusing stroke volume with cardiac output or failing to convert 5,250 mL per minute to approximately 5.25 L per minute.
• ECG, blood groups, and disease: Questions may identify the P wave, QRS complex, and T wave; explain ABO or Rh compatibility; or define hypertension. The key slips are assigning the QRS complex to atrial depolarisation, ignoring antigen-antibody reactions in transfusion, and treating 120/80 mm Hg as an unvarying normal value.