8.5 The heart Flashcards
where does oxygenated and deoxygenated blood flow
Deoxygenated blood from the body flows into the right side of the heart, which pumps it to the lungs. Oxygenated blood from the lungs flows into the left side of the heart, which pumps it to the body.
cardiac muscle
The heart is made of cardiac muscle, which contracts and relaxes in a regular rhythm.
It is a myogenic muscle.
It doesn’t get fatigued and doesn’t need to rest like skeletal muscle.
The coronary arteries supply the cardiac muscle with oxygenated blood.
The heart is surrounded by inelastic pericardial membranes, which help prevent the heart over-distending with blood.
Journey in the heart
— Deoxygenated blood enters the right atrium from the upper body in the superior vena cava and the lower body in the inferior vena cava, at relatively low pressure. The atria have thin muscular walls.
— As the blood flows in, slight pressure builds up in until the tricuspid valve opens to let blood pass into the right ventricle.
— When both the atrium and ventricle are filled with blood the atrium contracts, forcing all the blood into the right ventricle.
— As the right ventricle starts to contract, the tricuspid valve closes preventing any backflow of blood to the atrium.
— 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 valve prevents the backflow of blood into the lungs.
— At the same time, oxygenated blood from the lungs enters the left atrium from the pulmonary vein.
— As the pressure in the atrium builds the bicuspid valve opens between the left atrium and left ventricle so the ventricle also fills with oxygenated blood.
— When both the atrium and ventricle are full the atrium contracts, forcing all the oxygenated blood into the left ventricle.
— The left ventricle then contracts and pumps oxygenated blood through the semilunar valves into the aorta and around the body.
— As the ventricle contracts the tricuspid valve closes, preventing any backflow of blood.
why is the left muscular wall of the heart thicker
The muscular wall of the left side of the heart is much thicker than the right side. The right side of the heart has to pump blood a short distance and only has to overcome the resistance of the pulmonary circulation. The left side has to produce sufficient force to overcome the resistance of the aorta and the arterial systems of the whole body and move the blood under pressure to all the extremities of the body.
septum
The septum is the inner dividing wall of the heart which prevents the mixing of deoxygenated and oxygenated blood.
how do the left and right side work together
The right and left side of the heart fill and empty together.
pulmonary vein + artery
Pulmonary vein is the only vein that carries oxygenated blood. Pulmonary artery is the only artery that carries deoxygenated blood.
artery + vein
The arteries carry blood away from the heart.
The veins carry blood towards the heart
what does the cardiac cycle describe
The cardiac cycle describes the events in a single heartbeat, which lasts about 0.8 seconds in a human adult.
the cardiac cycle
In diastole the heart relaxes.
The atria and then the ventricles fill with blood.
The volume and pressure of the blood in the heart build as the heart fills, but the pressure in the arteries is at a minimum.
In systole the atria contract (atria systole), closely followed by the ventricles (ventricular systole).
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 blood in the heart are low at the end of systole, and the pressure in the arteries is at a maximum.
diastole + systole (brief)
Diastole = relaxation
Systole = contraction
aortic pressure
aortic pressure rises when ventricles contract as blood is forced into the aorta. It then gradually falls, but never below 12kPa, because of the elasticity in its wall which creates a recoil action. The recoil produces a temporary rise in pressure at the start of the relaxation phase.
atrial pressure
atrial pressure is always low because the thin walls of the atrium can’t create much force. It is highest when they’re contracting, but drops when the left atrioventricular closes and its walls relax. Then the atria fill with blood gradually increasing pressure until the left atrioventricular valve opens and blood moves into the ventricle.
ventricular pressure
ventricular pressure is low at first but increases as ventricles fill with blood and atria contract. The left atrioventricular valve closes and pressure rises as the thick ventricle walls contract. As pressure rises above that of the aorta, blood is forced into the aorta past the semilunar valves. Pressure falls as the ventricles empty and the walls relax.
ventricular volume
ventricular volume rises as the atria contract and the ventricles fill with blood, and then drops suddenly as blood is forced out into the aorta when the semilunar valve opens. Volume increases again as the ventricles fill with blood.
what is the sound of the heartbeat made from
The sounds of the heartbeat are made by the blood pressure closing the heart valves
where do the sounds of the heartbeat come from
The two sounds of the heartbeat are described as ‘lub-dub’. The first sound comes as the blood is forced against the atrioventricular valves as the ventricles contract, and the second sound comes as a backflow of blood closes the semilunar valves in the aorta and pulmonary artery as the ventricles relax.
benefit of cardiac muscle being myogenic
prevents the body wasting resources maintaining the basic heart rate
how is the basic rhythm of the heart maintained
The basic rhythm of the heart is maintained by a wave of electrical excitation:
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.
The electrical conductivity 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 ventricles.
The bundle of His splits into two branches and conducts the wave of excitation to the apex of the heart.
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 for a more efficient emptying of the ventricles.
what initiates the heartbeat
the Sino-atrial node (SAN)
what does the way in which the wave of excitation spreads through the heart ensure
The way in which the wave of excitation spreads through the heart from the SAN, with AVN delay, makes sure that the atria have stopped contracting before the ventricles start.
what does an electrocardiogram measure
An electrocardiogram (ECG) measures the electrical activity of the heart. An ECG measures the tiny electrical differences in your skin, which result from the electrical activity of the heart.
how is an ECG done
Electrodes are stuck to clean skin to get the good contacts needed for reliable results. The signal from each of the electrodes is fed into the machine, which produces an ECG.
ECG’s are used to diagnose heart problems, for example if someone is having a heart attack recognisable changes take place in the electrical activity of their heart and an ECG can be used to diagnose this
Heart rhythm abnormalities that commonly show up on ECG’s include
tachycardia
bradycardia
ectopic heartbeat
atrial fibrillation
