8.5 - The heart

Question 1

The human heart

Answer 1

• The heart consists of two pumps, joined and working together
• Deoxygenated blood from the body flows into the right side of the heart, which pumps it to the lungs
• Oxygenated blood from the lungs returns to the left side of the heart, which pumps it to the body.
• The blood from the two sides of the heart does not mix
• The heart is made of cardiac muscle, which contracts and relaxes in a regular rhythm
• IT does not get fatigued and need to rest like skeletal muscle
• The coronary arteries supply the cardiac muscle with the oxygenated blood it needs to keep contracting and relaxing all the time
• The heart is surrounded by inelastic pericardial membranes, which help prevent the heart from over-distending with blood

Question 2

The structure and function of the heart

Answer 2

1) Deoxygenated blood enters the right atrium of the heart from the upper body and head in the superior vena cava, and from the lower body in the inferior vena cava
2) The atria have thin muscular walls. As the blood flows in, slight pressure builds up until the atrio-ventricular valve (the tricuspid valve) opens to let blood pass into the right ventricle
3) When both the atrium and ventricle are filled with blood the atrium contracts, forcing all the blood into the right ventricle and stretching the ventricle walls.
4) As the right ventricle starts to contract, the tricuspid valve closes, preventing any backflow of blood to the atrium
5) The tendinous cords make sure the valves are not turned inside out by the pressures exerted when the ventricle contracts.
6) The right ventricle contracts fully and pumps deoxygenated blood through the semilunar valves into the pulmonary artery, which transports it to the capillary beds of the lungs. The semilunar valves prevent the backflow of blood into the heart.
7) At the same time oxygenated blood from the lungs enters the left atrium from the pulmonary vein. As pressure in the atrium builds the bicuspid valve opens between the left atrium and the left ventricle so the the ventricle also filled with oxygenated blood
8) When both the atrium and ventricle are full the atrium contracts, forcing all the oxygenated blood into the left ventricle
9) The left ventricle then contracts and pumps oxygenated blood through semilunar valves into the aorta and around the body. As the ventricle contracts the tricuspid valve closes, preventing any backflow of blood.The muscular wall of the left side of the heart is much thicker than that of the right.
10) The septum is the inner dividing wall of the heart which prevents the mixing of deoxygenated and oxygenated blood. The right and left side of the heart fill and empty together

Question 3

The cardiac cycle and the heartbeat

Answer 3

• The cardiac cycle describes the events in a single heartbeat, which lasts about 0.8 seconds in a human adult.
• In diastole the heart relaxes. The atria and then the ventricles fill with blood. The volume and pressure in the heart increases but are at a minimum in the arteries.
• In systole the atria and ventricles contract. The pressure inside the heart increases dramatically and blood is forced out of the right side of the heart to the lungs and from the left side to the main body circulation. The volume and pressure of the blood in the heart are low at the end of systole, and the blood pressure in the arteries are at a maximum

Question 4

Heart sounds

Answer 4

• The first sound (‘lub’) comes as the blood is forced against the atrio-ventricular valves as the ventricles contract
• The second sound comes as a backflow of blood closes the semilunar valves in the aorta and pulmonary artery as the ventricles relax

Question 5

The basic rhythm of the heart

Answer 5

• Cardiac muscle is myogenic: it has its own intrinsic rhythm at around 60 beats per minute.
• The basic rhythm of the heart is maintained by a wave of electrical excitation, rather like a nerve impulse
  1) A wave of electrical excitation begins in the pacemaker area called the sino-atrial node (SAN), causing the atria to contract and so initiating the heartbeat. A layer of non-conducting tissue prevents the excitation passing directly to the ventricles
  2) The electrical activity from the SAN is picked up by the atrio-ventricular node (AVN). The AVN imposes a slight delay before stimulating the bundle of His, a bundle of conducting tissue made up of fibres (Purkyne fibres), which penetrate through the septum between the ventricles
  3) The bundle of His splits into two branches and conducts the wave of excitation to the apex (bottom) of the heart
  4) At the apex the Purkyne fibres spread out through the walls of the ventricles on both sides. The spread of excitation triggers the contraction of the ventricles, starting at the apex. Contraction starting at the apex allows more efficient emptying of the ventricles.

Question 6

Electrocardiograms

Answer 6

• You can measure the spread of electrical excitation through the heart as a way of recording what happens as it contracts.
• To pick up changes, electrodes are stuck painlessly to clean skin to get the good contacts needed for reliable results.
• Heart rhythm abnormalities that commonly show up on ECGs include:
  1) Tachycardia - when the heartbeat is very rapid. This is normal when you are frightened, angry, exercising, or have a fever. Can be treated by medication or surgery
  2) Bradycardia - when the heart rate slows below 60 bpm. Many people have it because they’re fit. It can be serious and treated with an artificial pacemaker
  3) Ectopic heartbeat - extra heartbeats that are out of the normal rhythm
  4) Atrial fibrillation - an example of arrhythmia, which means abnormal rhythm of the heart. Rapid electrical impulses are generated in the atria. They contract very fast.