B4. The Heart

Question 1

The structure of the heart

The right side pumps deoxygenated blood to the lungs and the left side pumps oxygenated blood to the whole body.
Each bit of the heart is adapted to do its job effectively: (5 adaptations)

Answer 1

The left ventricle of the heart has:

• thicker, more muscular walls than the right ventricle this allows it to contract more powerfully and pump blood all the way around the body.
• The right side is less muscular so its contractions are only powerful enough to pump blood to the nearby lungs.

The ventricles have:

• thicker walls than the atria therefore they can push blood out of the heart
• whereas the atria just need to push blood a short distance into the ventricles.

The atrioventricular (AV) valves:

• link the atria to the ventricles
• stop blood flowing back into the atria when the ventricles contract.

The semi-lunar (SL) valves:

• link the ventricles to the pulmonary artery and aorta
• stop blood flowing back into the heart after the ventricles contract.

The cords:

• attach the atrioventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contract.

Question 2

Heart valves
The valves only open ___ ___- whether they’re open or closed depends on the relative __________of the heart chambers. If there’s higher pressure behind a valve, it’s forced ____, but if pressure is higher in front of the valve it’s forced _____. This means that the flow of blood is ________________- it only flows in one direction.

Answer 2

The valves only open one way- whether they’re open or closed depends on the relative pressure of the heart chambers. If there’s higher pressure behind a valve, it’s forced open, but if pressure is higher in front of the valve it’s forced shut. This means that the flow of blood is unidirectional - it only flows in one direction.

Question 3

The cardiac cycle
The cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating round the body. The volume of the atria and ventricles changes as they contract and relax. Pressure changes also occur, due to the changes in chamber volume (e.g. decreasing the volume of a chamber by contraction will increase the pressure of a chamber). The cardiac cycle can be simplified into three stages:… (3 stages, 3, 4, 5 points)

Answer 3

1. Ventricles relax, atria contract:

• The ventricles are relaxed.
• The atria contract, decreasing the volume of the chambers and increasing the pressure inside the chambers.
• This pushes the blood into the ventricles. There’s a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria.

2. Ventricles contract, atria relax:

• The atria relax.
• The ventricles contract (decreasing their volume), increasing their pressure.
• The pressure becomes higher in the ventricles than the atria, which forces the atrioventricular (AV) valves shut to prevent back-flow.
• The pressure in the ventricles is also higher than in the aorta and pulmonary artery, which forces open the semi-lunar (SL) valves and blood is forced out into these arteries.

3. Ventricles relax, atria relax:

• The ventricles and the atria both relax.
• The higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back-flow into the ventricles.
• Blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein.
• In turn this starts to increase the pressure of the atria. As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open.
• This allows blood to flow passively (without being pushed by atrial contraction) into the ventricles from the atria. The atria contract, and the whole process begins again.

Question 4

Interpreting data on the cardiac cycle - Example 1

When does blood start flowing into the aorta?
Why is ventricular volume decreasing at point B?
Are the semi-lunar valves open or closed at point C?

Answer 4

When does blood start flowing into the aorta?
At point A, the ventricles are contracting, which increases the pressure inside them. Once the pressure inside the ventricles is higher than that in the atria, the atrioventricular valves shut, forcing blood into the aorta.

Why is ventricular volume decreasing at point B?
The ventricles are contracting, reducing the volume of the chamber.

Are the semi-lunar valves open or closed at point C? Closed. The ventricles are relaxed and refilling, so the pressure is higher in the pulmonary artery and aorta, forcing the SL valves closed.

Question 5

Interpreting data on the cardiac cycle - Example 2
You may have to describe the changes in pressure and volume shown by a diagram, like the one below.

Answer 5

• In this diagram the AV valves are open.
• So you know that the pressure in the atria is higher than in the ventricles.
• So the atria must be contracting because that’s what causes the increase in pressure

Question 6

Calculating cardiac output
You could be asked to calculate cardiac output (CO). Cardiac output is … It’s calculated using this formula:…

Heart rate -…

Stroke volume -…

Answer 6

Cardiac output is the volume of blood pumped by the heart per minute (measured in cm³ min’)

cardiac output = stroke volume x heart rate

Heart rate - the number of beats per minute (bpm).

Stroke volume - the volume of blood pumped during each heartbeat, measured in (cm3)