Humans have a DOUBLE circulatory system — the blood passes through the heart twice for every complete circuit of the body.
Circuit 1 — PULMONARY CIRCULATION:
Right side of heart → lungs → left side of heart.
Deoxygenated blood is pumped to the lungs to pick up oxygen and lose carbon dioxide.
Circuit 2 — SYSTEMIC CIRCULATION:
Left side of heart → body organs and tissues → right side of heart.
Oxygenated blood is delivered to all body tissues at high pressure.
WHY DOUBLE? Having two separate circuits means oxygenated blood is always kept separate from deoxygenated blood, and the left side can pump oxygenated blood to the body at high pressure — ensuring efficient oxygen delivery to even distant organs.
Structure of the Heart
The heart has FOUR chambers:
Right atrium — receives deoxygenated blood from the body via the VENA CAVA.
Right ventricle — pumps deoxygenated blood to the lungs via the PULMONARY ARTERY.
Left atrium — receives oxygenated blood from the lungs via the PULMONARY VEIN.
Left ventricle — pumps oxygenated blood to the whole body via the AORTA.
The LEFT VENTRICLE has much thicker, more muscular walls than the right ventricle because it must pump blood to the entire body — a much greater distance and requiring much higher pressure than the right ventricle (which only pumps to the nearby lungs).
VALVES prevent blood flowing backwards:
Atrioventricular (AV) valves — between atria and ventricles.
Semilunar valves — in the pulmonary artery and aorta.
Valves open when pressure is higher on one side and snap shut to prevent backflow — this creates the 'lub-dub' heart sound.
The heart is made of CARDIAC MUSCLE — a special type of muscle that contracts and relaxes rhythmically, never getting tired.
The CORONARY ARTERIES supply the heart muscle itself with oxygenated blood. If these are blocked, the heart muscle is deprived of oxygen — causing a heart attack.
Arteries
Arteries carry blood AWAY from the heart.
Key features:
Thick, muscular walls — to withstand the high pressure of blood pumped directly from the heart.
Elastic fibres in the walls — stretch as blood surges through with each heartbeat, then recoil to maintain smooth blood flow.
Narrow lumen (central channel) — helps maintain high pressure.
NO valves — pressure is high enough to keep blood flowing in the correct direction.
Most arteries carry oxygenated blood — EXCEPT the PULMONARY ARTERY, which carries deoxygenated blood from the right ventricle to the lungs.
Memory tip: Arteries = Away from heart.
Veins
Veins carry blood TOWARDS the heart.
Key features:
Thinner walls — blood pressure is much lower in veins (far from the heart).
Wider lumen — accommodates the slower, lower-pressure flow.
VALVES — essential to prevent backflow of blood. Without valves, gravity and low pressure would allow blood to pool in the legs and flow backwards.
Most veins carry deoxygenated blood — EXCEPT the PULMONARY VEIN, which carries oxygenated blood from the lungs to the left atrium.
Memory tip: Veins → towards the heart. Valves in Veins.
Capillaries
Capillaries are the smallest blood vessels — connecting arteries to veins.
Key features:
Walls are only ONE CELL THICK — the shortest possible diffusion distance.
Very narrow — red blood cells must squeeze through in single file.
Form a dense network throughout all body tissues.
Very large total surface area — maximises exchange.
At the capillaries, EXCHANGE of substances takes place:
Oxygen and glucose diffuse OUT of the blood into body cells.
Carbon dioxide and waste products diffuse INTO the blood from cells.
This exchange happens by DIFFUSION down concentration gradients — no energy required.
⚠️ Common Mistake
Arteries do NOT always carry oxygenated blood. The PULMONARY ARTERY carries DEOXYGENATED blood from the heart to the lungs. Veins do NOT always carry deoxygenated blood — the PULMONARY VEIN carries OXYGENATED blood from the lungs to the heart. The rule is: Arteries = Away from heart. Veins = to heart. Not about oxygen content.
📌 Key Note
Left ventricle = thickest wall (pumps to whole body). Right ventricle = pumps to lungs only. Arteries: away, thick walls, high pressure. Veins: to heart, valves, low pressure. Capillaries: one cell thick, exchange site.
🎯 Matching Activity — Match the Heart Structure to its Function
Match each structure to what it does. — drag the symbols on the right to match the component names on the left.
Left ventricle
Drop here
Right ventricle
Drop here
Valves
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Coronary arteries
Drop here
Pulmonary vein
Drop here
Vena cava
Drop here
Pumps deoxygenated blood to the lungs via the pulmonary artery
Carries deoxygenated blood FROM the body TO the right atrium
Supply the heart muscle itself with oxygenated blood
Carries oxygenated blood FROM the lungs TO the left atrium
Pumps oxygenated blood to the whole body via the aorta — thickest walls
Prevent backflow of blood — open and close with pressure changes
⭐ Higher Tier Only
Artificial pacemakers correct irregular heart rhythms. Faulty heart valves can be replaced with biological valves (from pigs or cows) or mechanical valves. Artificial hearts maintain circulation while waiting for a transplant or during recovery. Students should be able to evaluate the advantages and disadvantages of drug treatment vs mechanical devices vs transplant for cardiovascular disease.
🎯 Test Yourself
Question 1 of 4
1. Why does the left ventricle have thicker walls than the right ventricle?
2. The pulmonary artery carries blood from the heart to the lungs. What type of blood does it carry?
3. Why do veins have valves but arteries do not?
4. What makes capillaries efficient at exchanging substances?
⭐ How Well Do You Understand This Topic?
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