The structure of the mammalian heart

A few exam questions to start...

Multiple Choice

The following events occur when carbon dioxide enters an erythrocyte in a capillary.

1. 1. Hydrogencarbonate ions diffuse into the plasma from the erythrocyte.

2. 2. Dissociation of carbonic acid.

3. 3. Carbon dioxide reacts with water forming carbonic acid.

4. 4. Chloride ions diffuse into erythrocyte from plasma.
   In which sequence do they occur?

• A

• B

• C

• D

Multiple Choice

Which best describes the chloride shift?

• hydrogen carbonate ions and chloride ions moving into red blood cells

• hydrogen carbonate ions moving out of rbc's and chloride ions moving into rbc's

• hydrogen ions being buffered by chloride ions in red blood cells

• carbonic anhydrase using chloride ions to produce carbonic acid

Multiple Choice

Carbon dioxide release during respiration can affect the % oxygen saturation of haemoglobin.
The tertiary structure of haemoglobin is affected when carbon dioxide reacts with water to form carbonic acid. This reaction releases hydrogen ions.
Which of the statements explains this change?

• The release of hydrogen ions causes the pH to rise, which reduces haemoglobin’s affinity for oxygen.

• The release of hydrogen ions causes the pH to rise, which increases haemoglobin’s affinity for oxygen.

• The release of hydrogen ions causes the pH to fall, which increases haemoglobin’s affinity for oxygen.

• The release of hydrogen ions causes the pH to fall, which reduces haemoglobin’s affinity for oxygen.

Multiple Choice

The table below shows a series of statements about systemic and pulmonary circulation.

Which of the rows, A to D, correctly describes a closed, double circulatory system?

• A

• B

• C

• D

Multiple Choice

The aquatic crustacean Daphnia magna has a heart that pumps a blood-like liquid called haemolymph around the body cavity.
Which of the statements, A to D, describes the circulatory system of Daphnia magna?

• single closed

• single open

• double open

• double closed

The structure of the mammalian heart

• Describe the external and internal structure of the mammalian heart.

• Explain the differences in thickness of the walls of the different chambers of the heart in terms of their functions.

The human heart

The heart is made up of 2 connected pumps.
Deoxygenated blood flows into the right side and is then pumped to the lungs.
Oxygenated blood from the lungs returns to the left side, and is then pumped to the body.
The blood from the two sides of the heart should not mix.

What are the main structures of the heart?

Left atrioventricular
valve

Right atrioventricular
valve

The Heart

Cardiac muscle

The heart mainly consists of cardiac muscle tissue, which contracts involuntarily in a regular rhythm.

The coronary arteries supply the cardiac muscle with oxygenated blood to keep it contracting and relaxing without becoming fatigued.

Cardiac muscle

The heart is surrounded by inelastic pericardial membranes (the pericardium) to stop the heart from over-distending with blood.

The pericardium, which translates as “around the heart,” is a double-layered connective tissue membrane that surrounds the heart like a sac.

Things to remember about the heart

1. Arteries = Away from the heart
2. Veins = back INto the heart
3. Pulmonary = lungs

4. Therefore, the vessel that carries blood AWAY

from the heart to the LUNGS is the …

Pulmonary Artery!

​The pathway of the blood

​LUNGS

​HEART

BODY

​The pathway of the blood

The cardiac cycle and a heartbeat

​The cardiac cycle

​What happens during the cardiac cycle?

Systole

During systole the atria contract, followed by the ventricles. The pressure inside the heart dramatically increases forcing blood out of the heart to the lungs and body.
At the end of systole, the volume and pressure in the heart are low and the pressure in the arteries is at a maximum.

Diastole

During diastole the heart relaxes. The atria and then the ventricles fill with blood. At this stage, the volume and pressure in the heart builds, but the pressure in the arteries is at a minimum.

What creates the lub-dub?

What creates that Lub-dub sound?

•Lub = caused by the closing of the atrio-ventricular valves when ventricles contract

•Dub = caused by the closing of the semi-lunar valves as the ventricles relax

Multiple Choice

Which structure prevents backflow of blood from the left ventricle into the left atrium?

• Bicuspid (mitral) valve

• Tricuspid valve

• Aortic valve

• Pulmonary valve

Multiple Choice

Which stage of the cardiac cycle involves contraction of the atria?

• Atrial systole

• Ventricular systole

• Diastole

• Atrial diastole

Multiple Choice

Why do ventricles have thicker walls than atria?

• They receive blood at higher pressure

• They contain more valves

• They require less oxygen

• They pump blood over longer distances

Multiple Choice

Which blood vessel carries oxygenated blood away from the heart?

• Pulmonary artery

• Vena cava

• Aorta

• Pulmonary vein

Multiple Choice

Which of the statements about the heart is correct?

• Contraction of the ventricles is known as ventricular systole.

• Deoxygenated blood enters the heart by the pulmonary vein.

• The pressure in the ventricles is always greater than in the atria because of the thicker ventricle wall.

• The wall of the left ventricle is less thick because it only has to pump blood to the lungs

Multiple Choice

The diagram shows the changes in pressure in the left atrium, left ventricle and aorta in a single cardiac cycle. Which letter shows the point at which the semilunar valve opens?

• A

• B

• C

• D

Multiple Choice

The diagram below shows an internal view of the mammalian heart with the atria removed so valves can be seen.
Which of the valves, labelled A to D, is pushed open by oxygenated blood entering a ventricle?

• A

• B

• C

• D

Multiple Choice

When you listen to a human heartbeat through a stethoscope you can hear a two stage ‘lub-dub’ sound.
Which of the following causes the first ‘lub’ component?

• closing of the atrioventricular valves

• sound of blood rushing into the atria

• sound of blood rushing into the ventricles

• closing of semilunar valves

The co-ordination of the cardiac cycle

The co-ordination of the cardiac cycle

•The heart is made of cardiac muscle

•When the cells receive an electrical impulse they contract - causing a heartbeat

•Cardiac muscle is myogenic - it can contract on its own, without needing nerve impulses from elsewhere in the body

•Contraction is triggered by a patch of tissue called the sinoatrial node (SAN aka the pacemaker)

The pathway followed by the wave of excitation

• This SAN is

found on the
upperinside
wall of the
right atrium.

The SAN

•The SA node is known as the pacemaker of the heart and initiates a heartbeat every 0.85 seconds.

•A wave of excitation (an electrical impulse) spreads out from the sinoatrial node across both atria causing them to contract and load the ventricles with blood.

•At the base of the atria is a disc of tissue that cannot conduct the wave of excitation

•Electrical impulse passes to ventricles via the AV node

The pathway followed by the wave of excitation

The AV node is
located on the at
the top of the
interventricular
septum.

The AVN

•Electrical impulse passes to ventricles via the AV node and the Bundle of His (a bundle of heart muscle cells specialised for electrical conduction). They pass the impulse to the base of the ventricles (~ 0.1 s delay).

•Purkyne fibres carry the impulses to the left & right ventricles.

•Ventricles then contract from the bottom upwards.

A Summary

•The mammalian heartbeat is initiated from within the heart muscle itself, which is therefore termed __________. The pacemaker of the heart is the _____________, which lies in the wall of the chamber called the ___________. A wave of excitation causes both _____ to contract. The wave is picked up by another group of specialised cells, called the __________________, which in turn passes it down to the apex of the ventricles via the ______________. Finally the impulse travels through the walls of the ventricles along specialist tissue made up of  ____________.

What are electrocardiograms?

The electrical activity of the heart can be monitored by an electrocardiograph.
Several electrodes are attached to specific places on a person’s chest and limbs. These detect electrical activity generated by the heart.
The leads are connected to a machine that draws an electrocardiogram (ECG).

What are electrocardiograms?

​P =
QRS =
T =

What are electrocardiograms?

​P = Atrial systole
QRS =
T =

What are electrocardiograms?

​P = Atrial systole
QRS = Ventricular systole
T =

What are electrocardiograms?

​P = Atrial systole
QRS = Ventricular systole
T = Diastole

What's wrong with these ECGs?

Multiple Choice

Which of the statements, A to D, explains why diastole follows systole in the mammalian heart?

• Cardiac muscle is myogenic.

• Cardiac muscle takes a short time to repolarise after being stimulated.

• The aorta is capable of maintaining the pressure generated by the left ventricle.

• The SAN receives impulses from the AVN.

Multiple Choice

Which of the options, A to D, on the diagram below shows the time at which the SAN sends out a wave of excitation to initiate a heartbeat?

• A

• B

• C

• D