Phys: Cardiac Muscle

Question 1

What is the proportion of deaths in the UK from cardiovascular disease?

Answer 1

27. 4% in Men

25. 2% in Women

Question 2

Why is the heart is dual circuit peristaltic pump?

Answer 2

Blood comes through the heart twice during a complete circulation. Peristaltic becasue is squeezes when contracting

Question 3

How is the contraction viewed?

Answer 3

Short axis - mid ventricular section

Long axis - oblique sagittal plane

Long axis - 4 chamber view

Question 4

Average person cardiac output?

Answer 4

3.5-4 litres in average people during rest

Large rugby payer 6-7 litres

During exercise cardiac output can reach up to 25l.min-1 in average adult and up to 40l.min-1 in cross country skiers

Question 5

What is the average stroke volume for a standard adult?

Answer 5

75ml per contraction

Larger, fitter people will be more in the region of 100ml

During exercise stroke volume can increase to 120ml in average adults

Question 6

What is inotropy?

Answer 6

increased force of contraction (increases stroke volume)

Question 7

What is chronotropy?

Answer 7

Increasing frequency of contractions (heart rate)

Question 8

What are the structural differences between cardiac muscle and skeletal muscle? 4

Answer 8

Fibres are branches by intercalated discs to improve the electricla activity of the heart

Fibres are more vertical on the inside whereas they are more transverse on the outside to create a helical appearance to squeeze in all directions (like twisting a wet flannel)

Contains desmosomes that will attach the fibres together

Low resistance dap junctions are present to increase electrical conductivity

Question 9

How does a contraction start?

Answer 9

Wave of depolarisation occurs from the Sino-Atrial node along the sarcolemmas to the AV Node (holds the impulse for 0.1 seconds to allow ventircles to fill with blood) then down the Bundle of His to the Purkinje Fibres. This then spreads through the ventricular sarcolemma causing a muscular contraction?

Question 10

What are the different aspects of an ECG?

Answer 10

P wave - sum of the electrical signals from both atria (there is a slight difference in time firing but the ECG won’t seperate that

QRS Complex - Contraction of the ventricles in synchronicity

T wave - repolarisation of the myocardium to return membrane to more negative value

Question 11

How does electrical activity form to cause a contraction?

Answer 11

The action potential is propagated into the myocyte by the T-tubules

Sarcoplasmic reticulum also releases Ca2+ ions when activated

Question 12

How does the cardiac and skeletal action potential vary?

Answer 12

Within skeletal muscle the action potential is 3-4milliseconds and the potential is completely gone within 20milliseconds. To make the contraction more powerful, the action potentials summate closer together so the time between firing is is reduced (tetanic contration which is not needed in cardiac muscle)

Within cardiac muscle the depolarisation occurs as normal then pauses as is begins to repolarise. Pauses for 200milliseconds (refractory period to prevent further contractions) then begins to repolarise. Entire potential lasts around 400milliseconds. Prevents tetanus so there are clearly defined and syncronised beats

Question 13

Why does the plateau occur?

Answer 13

a result of an influx of Ca2+ ions from extra-cellular space to the cytosol.

Due to the opening of L-type Ca2+ Channels (L meaning long opening) which open more slowly than Na+ channels to cause a delay in repolarisation of the sarcolemma

Only a small amount of Ca2+ enters the cell (0.02% increase)

Question 14

What is calcium handling?

Answer 14

Calcium induced Calcium release. The opening of L-type Channels causes calcium to enter the cells which releases large quantities of calcium into the cell for myosin-actin interaction for a contraction to occur
75-90% of calcium is released for contraction by CICR and only 10-25% from cell surface so it is important that calcium is tightly regulated

Question 15

How is calcium removed from the muscle to restart another constraction?

Answer 15

ATPase pumps puts calcium back into the sarcoplasmic reticulum (75-90%) and Na+-Ca+ exchange pumps (10-25%) to lower calcium after a successful contraction

Question 16

How is calcium handling differing between cardiac and skeletal muscle cells?

Answer 16

Skeletal - thin t-tubules, extensive SR, Triads(terminal systernae on each side of the t-tubule)

Cardiac - thick t-tubules, limited SR, diads(single t-tubule that is paired with a terminal systernae)

Question 17

How does the cardiac muscle contract?

Answer 17

• Uses ATP to detach and re-energise the myosin head
• A Level muscle contraction (Sliding filament theory)
• At rest only 40% of cross bridges are active
• 6-8kg of ATP used per day is synthesised and utilised by the body

Question 18

Where does the ATP come from?

Answer 18

70% of ATP from oxidation of fats
20% from oxidation of glucose
10% from glycolytic pathway (lactate, ketones and glycolysis)

Question 19

How do the ATP demands affect the structure of cardiac muscle?

Answer 19

Huge amount of mitochondria to use the oxygen to oxidise fat and glucose

Question 20

How do cardiac mitonchondria differ?

Answer 20

They are a continuous reticulum and there are two different types:

• Interfibrillar mitochondira
• Subsarcolemmal mitochondria

Question 21

Why is the left ventricle larger than the right ventricle?

Answer 21

The left ventricle pumps against a higher peripheral resistance as it has to go through the whole body. Right ventricle has a much smaller resistance as it is only pumping through the lungs

Question 22

What occurs when theres an increase in resistance?

Answer 22

Concentric cardiac hypertrophy occurs (gains muscle mass)

Can be caused by hypertension or by long term athletic training

Question 23

What is de-compensation?

Answer 23

Heart failure
-Hypertrophy stops and the heart begins to fall apart with chambers becoming baggy and swollen. This causes ejection fraction to be reduced to around 25% (only a quarter of blood leaves the heart each beat)