7.5 - The cardiac cycle

Question 1

What are the 2 phases of the beating of the heart

Answer 1

• contraction (systole)
• relaxation (diastole)

Question 2

How many stages is systole described in

Answer 2

2
- because it occurs separately in the ventricles and the atria

Question 3

How many stages is diastole described in

Answer 3

• 1
• because diastole takes place simultaneously in all chambers of the heart and is therefore described as a single phase

Question 4

Describe diastole (relaxation of the heart)

Answer 4

• blood returns to atria of the heart via the pulmonary vein (from the lungs) and the vena cava (from the body)
• atria fills = increased pressure
• when this pressure exceeds that in the ventricles = AV valve opens = allows blood to pass into the ventricles (aided by gravity)
• both the muscular walls of the atria and ventricles = relaxed
• this relaxation of ventricle walls = causes them to recoil = reduces pressure within the ventricle = causes pressure to be lower than that of the aorta + pulmonary artery = so the semi-lunar valves in the aorta and the pulmonary artery close
• this is accompanied by the characteristic ‘dub’ sound of the heart beat

Question 5

What are the 2 stages of systole

Answer 5

• atrial systole
• ventricular systole

Question 6

Describe atrial systole

Answer 6

• the contraction of the atrial walls, along with the recoil of the relaxed ventricle walls = forces remaining blood into the ventricles from the atria
• the semilunar valves are closed
• left and right AV valve are open

Question 7

Describe ventricular systole

Answer 7

• after a short delay (allows ventricles to fill with blood) their walls contract simultaneously
• this increases blood pressure within them, forcing the av valve shut = preventing backflow into the atria
• the with av valves closed, the pressure in the ventricle rises further
• once it exceeds that in the aorta and pulmonary artery, blood is forced from the ventricles into these vessels
• the ventricles have thick muscular walls which mean that they contract forcefully
• this creates the high pressure necessary to pump blood around the body
• the thick wall of the left ventricle has to pump blood to the extremities of the body while the relatively thinner wall of the right ventricle has to pump blood to the lungs

Question 8

How is blood kept flowing in one direction through the heart and around the body

Answer 8

• by the pressure created by the heart muscle
• blood will always move from a region of higher pressure to lower pressure
  —> However there still are some situations in the circulatory system when pressure differences would result in blood flowing in the opposite direction from that which is desirable. This is where valves are used to prevent backflow

Question 9

How do the valves work

Answer 9

• they open whenever the difference in blood pressure either side of them favours the movement of blood in the required direction
• when pressure differences are reversed (when blood would flow in the opposite direction to what is desired) the valves are designed to close

Question 10

What are the 3 examples for valves

Answer 10

• Atrioventricular valves
• semi-lunar valves
• pocket valves

Question 11

Describe the location and job of the atrioventricular valves

Answer 11

• found between both left and right atrium and ventricle
• it prevents backflow of blood when contraction of the ventricles means that ventricular pressure exceeds atrial pressure
• when they close, it ensures that: when the ventricles contract = blood within them moves to the aorta and pulmonary artery rather than back to the atria

Question 12

Describe the location and job of the semi-lunar valves

Answer 12

• found in the aorta and pulmonary arteries
• they prevent backflow of blood into the ventricles when the pressure in these vessels exceeds that in the ventricles
• this arises when the elastic walls of the vessels recoil = increasing the pressure within them and when the ventricle walls relax = reducing the pressure within the ventricles

Question 13

Describe the location and job of the pocket valves

Answer 13

• in veins that occur throughout the venous system
• these ensure that when the veins are squeezed (e.g. when skeletal muscles contract) blood flows back towards the heart, rather than away from it

Question 14

Describe the composition of the valves

Answer 14

• made up of a number of flaps of tough (but flexible) fibrous tissue, which are cusp-shaped
• when pressure is greater on the convex side of these cusps = they move apart to let blood pass between the cusps
• when pressure is greater on the concave side = blood collects within the ‘bowl’ of the cusps = pushing them together to form a tight fit that prevents the passage of blood

Question 15

Describe what having a closed circulatory system means

Answer 15

mammals have one
- the blood is confined to the vessels, allowing the pressure within them to be maintained and regulated

Question 16

What is cardiac output

Answer 16

The volume of blood pumped by one ventricle of the heart in one minute

Question 17

What is cardiac output measured in, what is the equation?

Answer 17

Question 18

Describe the ventricular pressure in this graph

Answer 18

• Ventricular pressure is low at first, but gradually increases as the ventricles fill with blood as the atria contract
• the left av valve closes and pressure rises dramatically as the thick muscular walls of the ventricles contract
• as pressure rises above that of the aorta, blood is forced into the aorta past the semi-lunar valves
• pressure falls as the ventricles empty and then the walls relax

Question 19

Describe the atrial pressure in the graph

Answer 19

• atrial pressure is always relatively low because the thin walls of the atrium can’t create much force
• it’s highest when they’re contracting, but drops when the left av valve closes and its wall relax
• the atria then fill with blood, which leads to a gradual build-up of pressure until a slight drop when the left av valve opens and some blood moves into the ventricle

Question 20

Describe the aortic pressure in this graph

Answer 20

• Aortic pressure rises when ventricles contract as blood is forced into the aorta
• It then gradually falls, but never below around 12kPa, because of the elasticity of its wall, which creates a recoil action
• this is essential if blood is to be constantly delivered to the tissues
• the recoil produces a temporary rise in pressure at the start if the relaxation stage

Question 21

Describe the ventricular volume of the graph

Answer 21

• ventricular volume rises as the atria contract and the ventricles fill with blood, and then drops suddenly as blood is forced out into the aorta when the semi-lunar valve opens
• Volume increases again as the ventricles fill with blood