Transport in Animals

The One Thing

The circulatory system is a specialised double circulatory system in which the heart, blood and blood vessels work together to transport substances efficiently, exchange materials with cells and protect the body.

Reactions, Processes and Experiments

Key Terms

• Circulatory system: A transport system made of the heart, blood and blood vessels that moves substances around the body.
• Double circulation: A circulation in which blood passes through the heart twice during one complete journey: once between the heart and lungs, and once between the heart and the rest of the body.
• Pulmonary circulation: The pathway from the right ventricle to the lungs and back to the left atrium; it allows blood to gain oxygen and lose carbon dioxide.
• Systemic circulation: The pathway from the left ventricle to the body tissues and back to the right atrium.
• Atrium: An upper chamber of the heart that receives blood.
• Ventricle: A lower chamber of the heart that pumps blood out of the heart.
• Septum: A muscular wall that separates the left and right sides of the heart and prevents mixing of oxygenated and deoxygenated blood.
• Valve: A structure that prevents the backflow of blood and ensures one-way movement.
• Artery: A blood vessel that carries blood away from the heart, usually at high pressure.
• Vein: A blood vessel that carries blood toward the heart, usually at low pressure.
• Capillary: A microscopic vessel connecting arteries and veins where substances are exchanged with cells.
• Plasma: The liquid part of blood that transports cells, dissolved nutrients, hormones, carbon dioxide, urea and heat.
• Red blood cell: A cell specialised to transport oxygen using haemoglobin.
• Haemoglobin: An iron-containing red pigment that combines reversibly with oxygen to form oxyhaemoglobin.
• White blood cell: A cell involved in defending the body against pathogens through processes such as phagocytosis and antibody production.
• Platelet: A small cell fragment that helps blood clot and prevents excessive blood loss.
• Oxygenated blood: Blood containing a relatively high concentration of oxygen and a lower concentration of carbon dioxide.
• Deoxygenated blood: Blood containing a relatively low concentration of oxygen and a higher concentration of carbon dioxide.
• Pulse: The regular stretching of artery walls caused by pressure from heart contractions.
• Cardiac cycle: One complete sequence of heart contraction and relaxation.
• Coronary artery: An artery that supplies the heart muscle with oxygenated blood and nutrients.
• Heart rate: The number of heartbeats per minute.
• Stroke volume: The volume of blood pumped by one ventricle during one heartbeat.
• Cardiac output: The volume of blood pumped by the heart, calculated using cardiac output = heart rate x stroke volume.
• Coronary heart disease: A condition in which fatty deposits narrow coronary arteries, reducing oxygen supply to the heart muscle and possibly causing a heart attack.
• Stent: A device used to widen a narrowed artery.
• Bypass: An alternative route for blood around a blocked coronary artery.

Easily Confused

• Pulmonary circulation and systemic circulation: Pulmonary circulation carries blood between the heart and lungs; systemic circulation carries blood between the heart and body tissues.
• Pulmonary artery and pulmonary vein: The pulmonary artery carries deoxygenated blood, whereas the pulmonary vein carries oxygenated blood.
• Arteries and veins: Arteries carry blood away from the heart at higher pressure; veins carry blood towards the heart at lower pressure.
• Right and left ventricles: The right ventricle pumps blood to the nearby lungs at lower pressure; the left ventricle pumps blood throughout the body at higher pressure.
• Oxygenated and deoxygenated blood: Oxygenated blood has a relatively high oxygen concentration and lower carbon dioxide concentration; deoxygenated blood has the opposite pattern.
• Heart rate and pulse rate: Heart rate is the number of heartbeats per minute; pulse rate measures the pressure waves in arteries and is commonly used to estimate heart rate.
• Stent and bypass: A stent widens a narrowed artery; a bypass creates an alternative route around a blocked coronary artery.
• Plasma and red blood cells: Plasma is the liquid transport medium; red blood cells transport oxygen using haemoglobin.

What Gets Asked

• Trace the route of blood through the heart and circulation. Marks are lost by reversing the order of the chambers, valves or vessels, or by omitting the pulmonary artery, pulmonary vein or aorta.
• Explain why the heart has two sides and a septum. The essential distinction is that the right side pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the body; the septum prevents their mixing.
• Compare arteries, veins and capillaries. Answers must link structure to function: arteries have thick, muscular, elastic walls and a relatively narrow lumen; veins have thinner walls, a wider lumen and valves; capillaries have walls one cell thick and a very narrow lumen.
• Explain adaptations of red blood cells. Relevant features are the absence of a nucleus, haemoglobin, biconcave shape and flexibility; omitting the link to oxygen transport or passage through narrow capillaries loses marks.
• Describe the functions of blood components. Plasma transports dissolved substances, red blood cells transport oxygen, white blood cells defend against pathogens, and platelets enable clotting.
• Calculate or explain cardiac output. The required relationship is cardiac output = heart rate x stroke volume; confusing stroke volume with heart rate gives an incorrect answer.