8.3.28 · Transport in Mammals
The Walls of the Heart — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 18 of 21 1 quiz
Lesson summary
8.3.2 The Walls of the Heart — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Relate the structure of cardiac muscle and the thickness of the chamber walls to their functions.Myogenic — cardiac muscle can start its own contraction without a nerve impulse, so an isola…
- Branched fibres
- Striations
- Intercalated discs
- Continuous activity
- Mitochondria
- Capillary network
Part 1 of 8Cardiac muscle
Myogenic
- Myogenic cardiac muscle can start its own contraction without a nerve impulse, so an isolated heart keeps beating.
- The nervous system alters the rate but does not start each beat.
Branched fibres
- Branched fibres short fibres connect to neighbours, so excitation spreads rapidly through the muscle.
Striations
- Striations they are present, as in skeletal muscle, because of the regular arrangement of actin and myosin.
Nucleus
- Nucleus each cell has a single central nucleus.
Intercalated discs
- Intercalated discs dark transverse lines where adjacent cells join.
- Gap junctions let ions and electrical signals pass directly between cells.
- Desmosomes hold the cells together during forceful contraction.
Part 2 of 8Energy supply
Continuous activity
- Continuous activity cardiac muscle contracts throughout life and must resist fatigue.
Mitochondria
- Mitochondria very large numbers are packed between the myofibrils to supply ATP by aerobic respiration.
Capillary network
- Capillary network a dense network from the coronary arteries supplies oxygen.
Restricted blood supply
- Restricted blood supply it quickly causes damage and pain (angina) because the muscle relies on aerobic respiration.
Long refractory period
- Long refractory period it prevents the muscle being tetanised, so it relaxes and refills between beats.
Part 3 of 8Comparing the three muscle types
| Feature | Cardiac | Skeletal | Smooth |
|---|---|---|---|
| Striations | Yes | Yes | No |
| Nuclei per cell | One, central | Many, peripheral | One |
| Fibre shape | Branched | Long, unbranched | Spindle-shaped |
| Control | Involuntary, myogenic | Voluntary | Involuntary |
| Behaviour | Strong and highly resistant to fatigue | Contracts quickly but tires | Contracts slowly and can stay contracted for long periods |
Part 4 of 8Thickness of the chamber walls
Principle
- Principle wall thickness reflects the pressure the chamber must generate.
Atria
- Atria thin walls, because they only push blood a short distance into ventricles that are already partly filled passively.
Right ventricle
- Right ventricle thicker than the atria, but low pressure protects the delicate alveolar capillaries and allows time for gas exchange.
Left ventricle
- Left ventricle the thickest wall, roughly three times the right, because more muscle gives greater force.
- It drives blood around the whole systemic circuit against considerable resistance.
Pressure and muscle mass
- Pressure and muscle mass pressure is roughly proportional to muscle mass.
- Peak pressure is about 25 mm Hg in the right ventricle.
- Peak pressure is about 120 mm Hg in the left ventricle.
Part 5 of 8Linking structure to function
Linking structure to function
Argument
- Argument link observation, consequence (greater force, higher pressure) and function (blood travels further or against more resistance).
- (thicker wall, more muscle)
Branching
- Branching it lets excitation spread in several directions at once.
Intercalated discs
- Intercalated discs they allow synchronous contraction.
Mitochondria
- Mitochondria they sustain continual aerobic respiration.
Identifying cardiac muscle in a micrograph
- Identifying cardiac muscle in a micrograph look for branching striated fibres;
- the intercalated discs are the most diagnostic feature.
Fibrous tissue
- Fibrous tissue it, not muscle, forms the valves and tendons.
- A non-conducting fibrous band between atria and ventricles blocks electrical impulses.
Part 6 of 8Disease and repair
Heart attack
- Heart attack cardiac muscle cells divide very little, so dead muscle is replaced mostly by non-contractile scar tissue.
- This weakens the wall and reduces stroke volume.
- It can disturb the spread of excitation and cause abnormal rhythms.
High blood pressure
- High blood pressure it forces the left ventricle to work harder, so its wall may thicken abnormally (hypertrophy).
- Efficiency eventually falls and heart failure can follow.
Part 7 of 8More from the specification
1
Cardiac cycle and pressure changes
- During atrial systole, atrial contraction increases pressure to force blood through the atrioventricular valves into the ventricles, followed by ventricular systole where ventricular pressure rises steeply.
2
Muscular thickness of chambers
- The atria have thin muscular walls because they only pump blood a short distance into the ventricles,
- whereas the ventricles possess much thicker muscular walls to generate high pressure.
3
Ventricular differences in circulation
- The left ventricle has thicker muscular walls than the right ventricle because it must pump blood into the high-resistance systemic circulation via the aorta,
- while the right ventricle pumps into the lower-resistance pulmonary circulation.
4
Diastole and chamber filling
- During cardiac diastole, both atrial and ventricular muscle relaxes,
- causing ventricular pressure to drop below atrial pressure so blood refills the chambers from the veins.
5
Cardiac cycle and pressure changes
- During atrial systole, atrial contraction increases pressure to force blood through the atrioventricular valves into the ventricles, followed by ventricular systole where ventricular pressure rises steeply.
6
Muscular thickness of chambers
- The atria have thin muscular walls because they only pump blood a short distance into the ventricles,
- whereas the ventricles possess much thicker muscular walls to generate high pressure.
7
Ventricular differences in circulation
- The left ventricle has thicker muscular walls than the right ventricle because it must pump blood into the high-resistance systemic circulation via the aorta,
- while the right ventricle pumps into the lower-resistance pulmonary circulation.
8
Diastole and chamber filling
- During cardiac diastole, both atrial and ventricular muscle relaxes,
- causing ventricular pressure to drop below atrial pressure so blood refills the chambers from the veins.
9
Cardiac cycle and pressure changes
- During atrial systole, atrial contraction increases pressure to force blood through the atrioventricular valves into the ventricles, followed by ventricular systole where ventricular pressure rises steeply.
10
Muscular thickness of chambers
- The atria have thin muscular walls because they only pump blood a short distance into the ventricles,
- whereas the ventricles possess much thicker muscular walls to generate high pressure.
11
Ventricular differences in circulation
- The left ventricle has thicker muscular walls than the right ventricle because it must pump blood into the high-resistance systemic circulation via the aorta,
- while the right ventricle pumps into the lower-resistance pulmonary circulation.
12
Diastole and chamber filling
- During cardiac diastole, both atrial and ventricular muscle relaxes,
- causing ventricular pressure to drop below atrial pressure so blood refills the chambers from the veins.
13
Cardiac cycle and pressure changes
- During atrial systole, atrial contraction increases pressure to force blood through the atrioventricular valves into the ventricles, followed by ventricular systole where ventricular pressure rises steeply.
14
Muscular thickness of chambers
- The atria have thin muscular walls because they only pump blood a short distance into the ventricles,
- whereas the ventricles possess much thicker muscular walls to generate high pressure.
15
Ventricular differences in circulation
- The left ventricle has thicker muscular walls than the right ventricle because it must pump blood into the high-resistance systemic circulation via the aorta,
- while the right ventricle pumps into the lower-resistance pulmonary circulation.
16
Diastole and chamber filling
- During cardiac diastole, both atrial and ventricular muscle relaxes,
- causing ventricular pressure to drop below atrial pressure so blood refills the chambers from the veins.
17
Atrium
- Upper chamber of the heart that receives blood.
18
Systole
- Contraction of the heart, and diastole = its relaxation.
19
Diastole
- Relaxation phase of the cardiac cycle.
20
Artery
- Vessel carrying blood away from the heart at high pressure.
21
Cardiac cycle
- The sequence of events in one complete heartbeat.
22
Atrial systole
- Contraction of the atria
23
Pulmonary artery
- Vessel carrying deoxygenated blood from the right ventricle to the lungs.
24
Atrioventricular valve
- Tricuspid or bicuspid valve between an atrium and ventricle.
Part 8 of 8More on this topic
- Check
- one cycle of 0.7 s gives a rate of about 86 beats per minute.
Key terms
| Term | Meaning |
|---|---|
| Cardiac muscle | Striated, branched, involuntary muscle forming the heart wall. |
| Myogenic | Able to contract without a nerve impulse. |
| Intercalated disc | Junction between cardiac cells containing gap junctions and desmosomes. |
| Hypertrophy | Abnormal thickening of a muscle wall. |
| Stroke volume | Volume of blood pumped by a ventricle in one beat. |
| Angina | Chest pain caused by too little oxygen reaching the heart muscle. |
Avoid these mistakes
1
Saying the nervous system starts every heartbeat — the heart is myogenic and nerves only change the rate.
2
Saying cardiac muscle has no striations — it is striated like skeletal muscle.
3
Saying the right ventricle is thinner because it holds less blood — it is thinner because it pumps only to the lungs at lower pressure.
4
Saying cardiac muscle fibres are multinucleate — each has one central nucleus.
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