This lesson is only available to users who have subscribed to the Biology course. Please log in, or click here if you would like to gain access to the full course.
Introduction to the lesson
We have now studied how the heart is structured in order to pump blood continually. But how exactly does it do this? In this lesson we will study the cardiac cycle---how the different chambers contract and in what order in order to efficiently pump blood out of the heart and let blood flow into the heart. We will then look at how the cardiac cycle is controlled by the sinoatrial node (SAN), the atrioventricular node (AVN) and the Purkyne tissue. This is a particularly interesting section and will be important in the exams so make sure you learn it well.
The cardiac cycle
Read the section "The cardiac cycle" on p.175-177 of your text book.
Notes on atrial systole
- Study Fig.8.26 (1) on p.175
- Notice that both atria contract at the same time forcing blood from the atria into the ventricles.
- Backflow of blood (into the venae cava or pulmonary vein) is prevented by the semilunar valves.
Notes on ventricular systole
- Study Fig. 8.26 (2) on p.175
- Notice that both ventricles contract at the same time forcing blood from the ventricles into the pulmonary artery and aorta.
- Blood cannot flow back into the atria because the atrioventricular valves are pushed shut by the pressurised blood in the ventricles.
- Notice the direction of both the pulmonary artery and the aorta in relation to the ventricles.
- Notice that the walls of the ventricles are thicker than the atria because the ventricles need to develop much more force when they contract. Their contraction has to push the blood out of the heart and around the body in the case of the left ventricle and to the lungs in the case of the right ventricle. (This explains why the left ventricle has a thicker wall than the right ventricle)
Notes on ventricular diastole
Study Figure 8.26 (3) on page 175
- Notice that in ventricular diastole, the heart muscle is relaxed.
- The relaxation of the muscles causes the pressure in the ventricles to drop.
- During diastole the blood from the veins (venae cava and pulmonary vein) empties into the two atria.
- The values between the atria and ventricle are open so some of the blood will trickle down to the ventricles.
Learning Activity 1
Answer the following questions in your exercise book.
- Refer to Fig. 8.28 on p.176. (cf. Question 8.15 on p.176)
- Which section of the graph represents atrial systole?
- Which section of the graph represents ventricular systole?
- Which section of the graph represents ventricular diastole?
- Explain the action of the semilunar valves and atrioventricular valves in:
- Atrial systole
- Ventricular systole
- Ventricular diastole
- Make summary notes of each of the three stages of the cardiac cycle and illustrate your notes with appropriate diagrams.
- Answer Question 8.16 on p.176.
Control of heart beat
Read the section "Control of heart beat" on p.177-178.
Learning Activity 2
Answer the following questions in your exercise book.
- Using Fig. 8.29 (p.177), explain how the heart contracts.
- Draw an annotated diagram showing how electrical excitation waves move through the heart. Link it to the three stages of the cardiac cycle.
- Write a paragraph describing what happens from the time blood enters the heart until the time it leaves the heart to enter the systemic circuit. Try to integrate everything that you have learned in this passage. Include the following in your description:
- Whether the blood is deoxygenated or oxygenated when it enters and when it leaves (when does it change from being deoxygenated to being oxygenated)
- Which part of the heart blood enters
- All the valves involved at every point
- The sequence of the passage of blood through the atria and ventricles
- The role of the SAN
- The role of the AVN
- The role of the Purkyne tissue
- Atrial systole
- Ventricular systole
- Ventricular diastole
- How each part of the cardiac cycle is represented on an electrocardiogram
- What is fibrillation and how does it happen
Definitions and review
Definitions
- Cardiac cycle
- Atrial systole
- Ventricular systole
- Ventricular diastole
- Myogenic
- Sinoatrial node
- Atrioventricular node
- Purkyne tissue
- Fibrillation
- Electrocardiogram
Mindmap
Add this section to finish your chapter eight mindmap.