Exchange between Organisms (Mass Transport) - The Heart

Question 1

What is the heart?

Answer 1

The heart is a muscular organ that lies in the thoracic cavity behind the sternum (breastbone). It operates continuously and tirelessly throughout the life of an organism.

Question 2

What is the structure of the human heart?

Answer 2

The human heart is two separate pumps next to each other. The pump on the left deals with oxygenated blood from the lungs, while the one on the right deals with deoxygenated blood from the body.

Question 3

What are the two chambers of each pump of the heart?

Answer 3

• The atrium is thin-walled and elastic and stretches as it collects blood.
• The ventricle has a much thicker muscular wall as it has to contract strongly to pump blood some distance, either to the lungs or to the rest of the body.

Question 4

What are the differences and similarities between the left and right ventricle?

Answer 4

Although the left ventricle has a thicker wall than the right ventricle, their internal volumes are the same. They have to be, otherwise more blood would be pumped out of one side of the heart than the other.

Question 5

Why do we have two separate pumps?

Answer 5

• The blood has to pass through tiny capillaries in the lungs in order to present a large surface area for the exchange of gases.
• In doing do, there is a drop in pressure and so blood flow to the rest of the body would be very slow.
• Mammals therefore have a system in which the blood is returned to the heart to increase its pressure before it is distributed to the rest of the body.
• It is essential to keep the oxygenated blood in the pump on the left side separate from the deoxygenated blood in the pump on the right.

Question 6

What is the function of the right and left ventricle?

Answer 6

• The right ventricle pumps blood only to the lungs, and it has a thinner muscular wall than the left ventricle.
• The left ventricle has a thicker muscular wall, enabling it to contract to create enough pressure to pump blood to the rest of the body.

Question 7

Do the two sides of the heart pump in time with each other?

Answer 7

Although the two sides of the heart are separate pumps and, after birth, there is no mixing of the blood in each of them, they nevertheless pump in time with each other. Both atria contract together and then both ventricles contract together, pumping the same volume of blood.

Question 8

What is between each atrium and ventricle?

Answer 8

Between each atrium and ventricle are valves that prevent the backflow of blood into the atria when the ventricles contract.

Question 9

What are the three valves?

Answer 9

• the left atrioventricular (bicuspid or mitral) valve
• the right atrioventricular (tricuspid) valve
• the semi-lunar valves (half-moon shaped valves in aorta or pulmonary arteries)

they are attached to the cardiac muscle via tendons

Question 10

What are each of the four chambers of the heart connected to?

Answer 10

Each of the four chambers of the heart is connected to large blood vessels that carry blood towards or away from the heart. The ventricles pump blood away from the heart and into the arteries. The atria receive blood from the veins.

Question 11

What are pulmonary vessels?

Answer 11

vessels connecting the heart to the lungs

Question 12

What are the four vessels connected to the four chambers?

Answer 12

• aorta
• vena cava
• pulmonary artery
• pulmonary vein

Question 13

What is the aorta?

Answer 13

The aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.

Question 14

What is the vena cava?

Answer 14

The vena cava is connected to the right atrium and brings deoxygenated blood back from the tissues of the body (except the lungs).

Question 15

What is the pulmonary artery?

Answer 15

The pulmonary artery is connected to the right ventricle and carries deoxygenated blood to the lungs, where its oxygen is replenished and its carbon dioxide is removed. Unusually for an artery, it carries deoxygenated blood.

Question 16

What is the pulmonary vein?

Answer 16

The pulmonary vein is connected to the left atrium and brings oxygenated blood back from the lungs. Unusually for a vein, it carries oxygenated blood.

Question 17

What is the vena cava split into?

Answer 17

• inferior (posterior) vena cava

- superior (anterior) vena cava

Question 18

How is oxygen supplied to the heart muscle?

Answer 18

Although oxygenated blood passes through the left side of the heart, the heart does not use this oxygen to meet its own great respiratory needs. Instead, the heart muscle is supplied by its own blood vessels, called the coronary arteries, which branch off the aorta shortly after it leaves the heart.

Question 19

What happens if the coronary arteries are blocked?

Answer 19

Blockage of these arteries, for example by blood clot, leads to myocardial infarction, or heart attack, because an area of the heart is deprived of blood and therefore oxygen also. The muscle cells in this region are unable to respire aerobically and so die.

Question 20

What are the risk factors associated with cardiovascular disease?

Answer 20

• smoking
• high blood pressure
• blood cholesterol
• diet
• exercise

Question 21

What is the function of the atrioventricular valves and the semi lunar valves?

Answer 21

When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria as there would be a higher ventricular pressure than atrial pressure. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles as there would be a higher atrial pressure than ventricular pressure.

This all maintains the unidirectional flow of blood through the heart. It’s important because blood always flows from a high to a low pressure but in the cardiac cycle, situations do occur when pressure differences would result in blood flowing in the wrong direction.

Question 22

What is the function of the heart tendons?

Answer 22

The heart tendons stabilise valves, helping to maintain the unidirectional flow of blood through the heart. The tendons and papillary muscles tether the atrioventricular valves to the ventricular walls. This allows the valves to close properly and not bulge into the atria under high pressure.

Question 23

Explain how you think the atrioventricular valves work.

Answer 23

A heart valve allows blood flow in only one direction through the heart, and the combination of the atrioventricular and semi-lunar heart valves determines the pathway of blood flow. Valves open or close based on pressure differences across the valve. The atrioventricular valves separate the atria from the ventricles on each side of the heart and prevent backflow of blood from the ventricles into the atria. When blood fills the atria, there is a high pressure on the valves, forcing them open and allowing the blood into the ventricle. The ventricle then contracts, forcing blood against the valves, closing them.

Question 24

Explain the significance in the difference of thickness between the atria, the left and the right ventricle.

Answer 24

The atrium is thin-walled so that it is elastic and can stretch as it collects blood via the vena cava.

The ventricle has a much thicker muscular wall as it has to contract strongly to pump blood some distance, either to the lungs or to the rest of the body. The right ventricle pumps blood only to the lungs, and it has a thinner muscular wall than the left ventricle. The left ventricle has a thicker muscular wall, enabling it to contract to create enough pressure to pump blood to the rest of the body. Although the left ventricle has a thicker wall than the right ventricle, their internal volumes are the same. They have to be, otherwise more blood would be pumped out of one side of the heart than the other.

Question 25

What is the apex?

Answer 25

bottom of the heart

Question 26

What is the atrioventricular node?

Answer 26

a part of the electrical conduction system of the heart that electrically connects the upper and lower parts of the heart

Question 27

What is the bundle of His?

Answer 27

transmits impulses from the atrioventricular node down the septum to the purkinje fibres at the base of the ventricles

Question 28

What are tendinous chords?

Answer 28

cord-like tendons that connect the interior structure of the heart muscle to the atrioventricular valves, thus preventing the valves from inverting

Question 29

What is myogenic?

Answer 29

electrical activity originating in the heart tissue resulting in contraction of the heart muscle without (necessarily) needing input from the nervous system

Question 30

What is the sinoatrial node?

Answer 30

an area of the heart muscle in the right atrium that controls and coordinates the contraction of the heart, also known as the pacemaker

Question 31

What is wave of depolarisation?

Answer 31

electrical impulses

Question 32

What is a mammal’s heart made of?

Answer 32

Almost entirely of muscle. It’s called cardiac muscle.

Question 33

What happens if you have a hole in your heart?

Answer 33

• congenital (at birth) between atria or ventricles
• allows mixing of oxygenated and deoxygenated blood
• blood passes from RA to LA
• some blood does not enter ventricles and pulmonary artery
• reduces systolic pressure

Question 34

What happens if a hole in the heart is left untreated?

Answer 34

• dangerous
• pressure reduced
• some blood bypasses lungs
• less oxygen carried
• less respiration
• decreased oxygen supply to cells
• less energy
• weakness/anaemia

Question 35

What is the cardiac cycle?

Answer 35

The heart undergoes a sequence of events that is repeated in humans around 70 times each minute when at rest. This is known as the cardiac cycle.

Question 36

What are the two phases to the beating of the heart?

Answer 36

• contraction (systole)

- relaxation (diastole)

Question 37

Where does contraction occur?

Answer 37

Contraction occurs separately in the ventricles and the atria and is therefore described in two stages.

Question 38

Where does relaxation occur?

Answer 38

For some of the time, relaxation takes place simultaneously in all chambers of the heart and is therefore treated as a single phase.

Question 39

What is diastole?

Answer 39

Relaxation of the heart.

• Blood returns to the atria of the heart through the pulmonary vein and the vena cava.
• As the atria fill, the pressure in them rises. It is originally low because of the elastic recoil of the heart, and the fact that blood has just left through the aorta or pulmonary artery.
• When the pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles.
• The passage of blood is aided by gravity.
• The muscular walls of both the atria and ventricles are relaxed at this stage.
• The relaxation of the ventricle walls causes them to recoil and reduces the pressure within the ventricle.
• This causes the pressure to be lower than that in the aorta and the pulmonary artery, and so the semi-lunar valves in the aorta and the pulmonary artery close.
• This is accompanies by the characteristic ‘dub’ sound of the heart beat.

Question 40

What is atrial systole?

Answer 40

Contraction of the atria.

The contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria. The muscle of the ventricle walls remains relaxed, and the semi-lunar valves stay closed.

Question 41

What is ventricular systole?

Answer 41

Contraction of the ventricles.

• After a short delay to allow the ventricles to fill with blood, their walls contract simultaneously.
• This increases the blood pressure within them, forcing shut the atrioventricular valves and preventing backflow of blood into the atria.
• The ‘lub’ sound of these valves closing is a characteristic of the heart beat.
• With the atrioventricular valves closed, the pressure in the ventricles rises further.
• Once it exceeds that in the aorta and pulmonary artery, blood is forced from the ventricles into these vessels, as the semi-lunar valves are forced open.
• The ventricles have thick muscular walls which mean 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.
• Pressure falls in the atria and ventricles, and the cycle starts again.

Question 42

Why do we have valves?

Answer 42

Blood is kept flowing one direction through the heart and around the body by the pressure created by the heart muscle. Blood will always move from a region of higher pressure to one of lower pressure.

However, there are situations within the circulatory system when pressure differences would result in blood flowing in the opposite direction from that which is desirable. In these circumstances, valves are used to prevent any unwanted backflow of blood.

Question 43

How do the valves open and close?

Answer 43

Valves in the cardiovascular system are designed so that 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, that is, when blood would tend to flow in the opposite direction to that which is desirable, the valves are designed to close.

Question 44

What are atrioventricular valves?

Answer 44

• Between the left atrium and ventricle, and the right atrium and ventricle.
• These prevent backflow of blood when contraction of the ventricles means that ventricular pressure exceeds atrial pressure.
• Closure of these valves ensures that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than back to the atria.

Question 45

What are semi-lunar valves?

Answer 45

• In the aorta and pulmonary artery.
• These 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 46

What are pocket valves?

Answer 46

• 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 47

What is the structure of the valves?

Answer 47

• They are 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 rather than on the concave side, they move apart to let blood pass between the cusps.
• This opens the valves.
• When pressure is greater on the concave side, blood collects within the ‘bowl’ of the cusps.
• This pushes them together to form a tight fit that prevents the passage of blood.

Question 48

What does having a closed circulatory system mean?

Answer 48

Mammals have a closed circulatory system. This means the blood is confined to vessels, and this allows the pressure within them to be maintained and regulated.

Question 49

In the left side of the heart during the cardiac cycle, how does ventricular pressure change?

Answer 49

• Ventricular pressure is low at first, but gradually increases as the ventricles fill with blood as the atria contract.
• The left atrioventricular valves close and pressure rises dramatically as the thick muscular walls of the ventricle contract.
• As pressure rises above that of the aorta, blood is forced into the aorta past the semilunar valves.
• Pressure falls as the ventricles empty and the walls relax.

Question 50

In the left side of the heart during the cardiac cycle, how does atrial pressure change?

Answer 50

• Atrial pressure is always relatively low because the thin walls of the atrium cannot create much force.
• It is highest when they are contracting, but drops when the left atrioventricular valve closes and its walls relax.
• The atria then fill with blood, which leads to a gradual build-up of pressure until a slight drop when the left atrioventricular valve opens and some blood moves into the ventricle.

Question 51

In the left side of the heart during the cardiac cycle, how does aortic pressure change?

Answer 51

• 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 (essential if blood is to be constantly delivered to the tissues).
• The recoil produces a temporary rise in pressure at the start of the relaxation phase.

Question 52

In the left side of the heart during the cardiac cycle, how does ventricular volume change?

Answer 52

• 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 semilunar valve opens.
• Volume increases again as the ventricles fill with blood.

Question 53

What is cardiac output?

Answer 53

Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute. It is usually measured in dm3min-1 and depends upon two factors.

Question 54

What 2 factors does cardiac output depend on?

Answer 54

• the heart rate (the rate at which the heart beats, per minute)
• the stroke volume (the volume of blood pumped out at each beat)

cardiac output = heart rate x stroke volume

Question 55

Why do we need contractions and relaxations?

Answer 55

Because fluids flow from a region of higher pressure into a region of lower pressure.

Question 56

What does myogenic mean?

Answer 56

The contraction of the heart is myogenic, meaning that the signal for cardiac compression arises within the heart tissue itself. In other words, the signal for a heart beat is initiated by the heart muscle cells (cardiomyocytes) rather than from brain signals.

Question 57

What is the sinoatrial node?

Answer 57

In the wall of the right atrium, there is a region of specialised fibres called the sinoatrial node which is the pacemaker of the heart. This initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time.

Question 58

Why do the ventricles not start contracting until the atria have finished?

Answer 58

Due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation (septum).

Question 59

Where does the electrical wave go after passing through the atria?

Answer 59

The electrical wave eventually reaches the atrioventricular node located between the two atria which passes on the excitation to ventricles, down the bundle of His to the apex of the heart.

This causes the ventricles to contract, thus emptying them. The ventricles contract at the apex in order to force the most blood possible upwards out of the aorta and pulmonary artery.