C2 - ECG, cardiac cycle control, oxygen dissociation curve

sino-atrial node (3)

an area of the heart muscle in the right atrium, that initiates a wave of electrical excitation across the atria, to generate contraction of the heart muscle

atrio-ventricular node (3)

the only conducting area of tissue in the wall of the heart, between the atria and ventricles, through which electrical excitation passes from the atria to conducting tissue in the walls of the ventricles

Explain stage 1 of the electrical excitation process (3)

a wave of electrical stimulation arises at the SAN, this electrical signal spreads over both atria, causing them to contract together

How are ventricles electrically insulated from the atria?

a thin layer of connective tissue

Explain stage 2 of the electrical excitation process (3)

due to the AVN electrical stimulation only spreads to the ventricles from this point, AVN introduces a delay in the transmission of the electrical impulse, the muscles of the ventricles do not start to contract until the muscles of the atria have finished contracting

Why is it important that the AVN introduces a delay in the transmission of the electrical impulse?

it ensures the atria are emptied before the ventricles contract

Explain stage 3 of the electrical excitation process (2)

AVN passes the electrical excitation down the nerves of the bundle of His, the left and right bundle branches and to the apex of the heart

Explain stage 4 of the electrical excitation process (2)

the excitation is transmitted to the purkinje fibres in the ventricle walls, which carry it upwards through the muscles of the ventricle walls

Explain stage 5 of the electrical excitation process (2)

the impulse causes the cardiac muscle in each ventricle to contract simultaneously, from the apex upwards

Why do the ventricles contract from the apex upwards? (2)

it pushes the blood up to the aorta and pulmonary artery, emptying the ventricles completely

bundle of His (2)

non-conductive tissue in the septum of the heart, where the electrical impulse passes straight down to the bottom of the ventricles

What occurs during atrial systole? (2)

atria contract, ventricle relax

What occurs to the valves between atrial systole and ventricular systole? (2)

bicuspid/tricuspid valves close, semi-lunar valves open

What occurs during ventricular systole? (2)

ventricles contract, atria relax, aorta and pulmonary artery are filling with blood

What is occurring to the valves between ventricular systole and diastole? (2)

semi-lunar valves close, bicuspid/tricuspid valves open

What occurs during diastole?

atria relax, ventricles relax

What does the ‘lub’ sound represent?

bicuspid/tricuspid valves closing

What does the ‘dub’ sound represent?

semi-lunar valves closing

What is an electrocardiogram and how does it work? (2)

a trace of the voltage changes produced by the heart, detected by electrodes on the skin

What is a P wave on an ECG?

atrial depolarisation

Explain what a P wave is as shown on an ECG (3)

first part of the trace, showing the voltage change generated by the SAN, causing the atria to contract

Why are P waves small?

atria have less muscle than ventricles

Explain what a PR interval is as shown on an ECG (3)

the time between the start of the P wave and the start of the QRS complex, the time taken for the excitation to spread from the atria to the ventricles, through the AVN

What is a QRS complex on an ECG? (2)

ventricular depolarisation, atrio repolarisation hidden on ECG by QRS complex

Explain what a QRS complex is as shown on an ECG

the depolarisation and contraction of the ventricles

Why does the QRS complex have a bigger amplitude than the P wave?

ventricles have more muscle than the atria

Explain what an ST segment is as shown on an ECG

lasts from the end of the S wave to the start of the T wave

What is the T wave on an ECG?

ventricular repolarisation

Explain what the T wave is as shown on an ECG

the repolarisation of the ventricle muscles

isoelectric line (2)

the line between the T wave and P wave of the next cycle, the baseline of the trace

How do you calculate heart rate in bpm?

60 / length of cardiac cycle (s)

rhythm

the regularity of the pattern of the trace

Explain what an ECG would look like for an individual with atrial fibrillation and why (2)

they have a rapid heart rate, so they may lack a P wave

Explain what an ECG would look like for an individual with enlarged ventricle walls

they may have a QRS complex showing greater voltage change

Explain what an ECG would look like for an individual with blocked coronary arteries or atherosclerosis and why (2)

changes in the height of the ST segment and T wave, as they may have insufficient blood being delivered to the heart muscle

Explain what an ECG would look like for an individual who has had a heart attack

they may have a wide QRS complex

How do you work out the length of a cycle on an ECG?

time between equivalent points on the trace

Order the blood vessels from highest blood pressure to lowest blood pressure (5)

aorta and large arteries, arterioles, extensive capillary beds, veins

Explain high blood pressure in aorta and large arteries (2)

where blood pressure is highest, rises and falls rhythmically, with ventricular contraction

Explain the progressive drop in pressure from aorta and large arteries to arterioles (3)

progressive drop in pressure in arterioles due to friction between the blood and vessel walls, and the large total surface area, despite their narrow lumen

What else does blood pressure in arterioles depend on?

if they are dilated or constricted

Explain the further reduction in blood pressure from arterioles to extensive capillary beds

fluid leaks from the capillaries to the tissues

Finish the sentence: In arteries and capillaries, the higher the blood pressure, the faster the blood flows …

so both pressure and speed fall as the distance from the heart increases