The heart (AQA GCSE Biology): Revision Notes
The heart
What is the heart?
The heart is a muscular organ that acts like a pump. Its main job is to push blood around your body so that all your cells get the oxygen and nutrients they need to stay alive.
The heart is roughly the size of your fist and beats approximately 100,000 times per day, pumping about 7,500 litres of blood through your body!
Structure of the heart
The heart has four main chambers that work together to pump blood effectively.
The four chambers
Upper chambers (atria)
- Right atrium - receives blood that has little oxygen (deoxygenated blood) from the body
- Left atrium - receives blood that is rich in oxygen (oxygenated blood) from the lungs
Lower chambers (ventricles)
- Right ventricle - pumps deoxygenated blood to the lungs
- Left ventricle - pumps oxygenated blood around the body (this chamber has thicker muscle walls because it needs to pump blood further)
Important blood vessels
The heart connects to several major blood vessels that carry blood to and from different parts of the body:
- Vena cava - brings deoxygenated blood from the body back to the right atrium
- Pulmonary artery - carries deoxygenated blood from the right ventricle to the lungs
- Pulmonary vein - brings oxygenated blood from the lungs to the left atrium
- Aorta - carries oxygenated blood from the left ventricle to the body
- Coronary arteries - supply the heart muscle itself with oxygenated blood
Remember: arteries carry blood away from the heart, while veins bring blood back to the heart. The pulmonary artery is unusual because it carries deoxygenated blood, while the pulmonary vein carries oxygenated blood.
Valves
Valves are like one-way doors in the heart. They stop blood from flowing backwards and make sure it only moves in the right direction through the heart chambers.
Without valves, blood would flow backwards when the heart relaxes, making the pumping action much less efficient and potentially dangerous.
How the heart works
The heart works by squeezing and relaxing in a regular pattern. This creates the pumping action that moves blood around your body.
The pumping sequence
- Blood enters the atria - both atria fill up with blood at the same time
- Atria contract - this squeezes blood down into the ventricles below
- Ventricles contract - this powerful squeeze pushes blood out into the arteries
- Blood flows to organs - arteries carry blood to where it's needed, then veins bring it back to the heart
This sequence repeats about 70 times per minute when you're resting.
The heart's rhythm is controlled by a natural pacemaker called the sinoatrial (SA) node, which sends electrical signals to coordinate the contractions.
Double circulation
Your body has a special circulation system called double circulation. This means blood goes through the heart twice during each complete journey around your body.
Two separate circuits
Right side of heart (to lungs)
- The right ventricle pumps blood to your lungs
- In the lungs, blood picks up oxygen and gets rid of carbon dioxide
- This oxygenated blood returns to the left side of the heart
Left side of heart (to body)
- The left ventricle pumps oxygenated blood around the rest of your body
- Body organs use up the oxygen and produce carbon dioxide as waste
- This deoxygenated blood returns to the right side of the heart
This double system is very efficient because it keeps oxygenated and deoxygenated blood completely separate.
Double circulation allows for higher blood pressure in the systemic circuit (to the body) while maintaining lower pressure in the pulmonary circuit (to the lungs), protecting the delicate lung tissues.
Calculating cardiac output
Cardiac output tells us how much blood the heart pumps in one minute. This is important for understanding how well the heart is working.
The formula
Cardiac output = stroke volume × heart rate
- Heart rate - the number of heartbeats per minute
- Stroke volume - the amount of blood pushed out of the heart in one beat
Worked Example: Calculating Cardiac Output
If someone's heart rate is 72 beats per minute and their stroke volume is 0.075 dm³:
Step 1: Identify the values
- Heart rate = 72 beats/min
- Stroke volume = 0.075 dm³
Step 2: Apply the formula Cardiac output = stroke volume × heart rate Cardiac output = 0.075 × 72 = 5.4 dm³/min
Answer: This means their heart pumps 5.4 litres of blood every minute.
Key Points to Remember:
- The heart has four chambers: two atria (top) and two ventricles (bottom)
- The left ventricle has thicker walls because it pumps blood around the whole body
- Double circulation means blood passes through the heart twice - once to the lungs and once to the body
- Arteries carry blood away from the heart, veins bring blood back to the heart
- Cardiac output = stroke volume × heart rate