The heart (AQA GCSE Biology Combined Science): Revision Notes
The heart
What is the heart?
The heart works as a muscular pump that moves blood throughout your entire body. It's one of the most important organs because it keeps oxygen and nutrients flowing to every part of your body.
The heart beats approximately 100,000 times per day, pumping about 7,000 litres of blood through your circulatory system!
Structure of the heart
The heart has four main chambers that work together to pump blood:
Upper chambers (Atria)
- Right atrium - receives blood coming back from the body
- Left atrium - receives blood coming back from the lungs
Lower chambers (Ventricles)
- Right ventricle - pumps blood to the lungs
- Left ventricle - pumps blood around the body (has thicker muscle walls)
The left ventricle has the thickest walls because it needs to generate enough pressure to pump blood throughout the entire body, while the right ventricle only needs to pump blood the short distance to the lungs.
Important blood vessels
- Aorta - carries oxygen-rich blood from heart to body
- Pulmonary artery - carries blood without oxygen from heart to lungs
- Pulmonary vein - brings oxygen-rich blood from lungs to heart
- Vena cava - brings blood without oxygen from body back to heart
Key features
- Valves - stop blood flowing backwards through the heart
- Coronary arteries - supply the heart muscle with oxygenated blood
The heart's left and right sides are labelled as if you're looking at the person, so left appears on the right side of diagrams.
How the heart pumps blood
Blood flows through the heart in a specific sequence:
- Blood enters the atria
- The atria contract, forcing blood into the ventricles
- The ventricles contract, forcing blood into the arteries
- Blood flows through arteries to organs and returns through veins
Remember the flow direction: Arteries take blood away from the heart, veins bring it back in.
Double circulation system
Humans have a double circulation system with two separate pathways:
Pulmonary circulation (right side)
- Right ventricle pumps blood to the lungs
- Gas exchange happens in the lungs
- Oxygen is added, carbon dioxide removed
Systemic circulation (left side)
- Left ventricle pumps blood around the rest of the body
- Delivers oxygen and nutrients to organs
- Collects waste products
This system is very efficient because blood gets pumped twice on each complete journey around the body, ensuring high pressure delivery to all organs.
Calculating cardiac output
Cardiac output = the volume of blood the heart pumps per minute
Formula:
Where:
- Heart rate = number of heartbeats per minute
- Stroke volume = volume of blood pushed out in one heartbeat
Worked Example: Calculating Cardiac Output
Given data:
- Heart rate = 72 beats/min
- Stroke volume = 0.075 dm³/beat
Step 1: Apply the formula Cardiac output = stroke volume × heart rate
Step 2: Substitute the values Cardiac output = 0.075 × 72
Step 3: Calculate the result Cardiac output = 5.4 dm³/min
The heart rate is naturally controlled by pacemaker cells located in the right atrium.
Key Points to Remember:
- The heart is a muscular pump with four chambers that moves blood around the body
- Arteries carry blood away from the heart, veins bring it back
- The double circulation system pumps blood to lungs AND body separately
- The left ventricle has thicker walls because it pumps blood further
- Cardiac output = stroke volume × heart rate