4.3 - The Heart Flashcards
What is the structure of the heart?
- is a muscular pump
- the right-hand side of the heart pumps deoxygenated blood to the lungs
- the left-hand side of the heart pumps oxygenated blood to the rest of the body
CHAMBERS:
2 atria (collect blood from the body and lungs)
2 ventricles (pump blood to body and lungs)
VALVES:
atrioventricular valves (between atria and ventricles + bicuspid valve on left and tricuspid on right)
semi-lunar valves (between ventricles and arteries + aortic valve on the left and pulmonary valve on right)
BLOOD VESSELS:
vena cava (inferior and superior) feeds into the right atrium and returns deoxygenated blood from the body
pulmonary artery connects to the right ventricle and sends deoxygenated to the lungs
pulmonary vein feeds into the left atrium and returns oxygenated blood from the lungs
aorta extends from the left ventricle and sends oxygenated blood around the body.
- located in the chest cavity
- protected in the chest cavity by the pericardium (tough and fibrous sac)
What are the roles of the valves in the heart and how do they work?
Roles:
- to prevent backflow of the blood by opening and closing due to pressure differences
How do they work?
- they are forced open when the pressure of blood behind them is greater than the pressure in front of them
they are forced close when the pressure of blood in front of them is greater than the pressure behind them
What is the structure of arteries?
- Arteries carry oxygenated blood away from the heart. This filters into arterioles, which lead to the rest of the body.
- Artery walls have thick muscular layers. Thick, muscular walls are needed to withstand the high pressure that arteries are put under
- Thick elastic layers too. Elastic tissue allows the walls to stretch and recoil, to keep in line with the pulsating flow through which the blood travels through.
- Smooth muscle helps blood flow. Smooth muscle lined with smooth endothelium reduces friction and creates less restriction for the blood to move through.
- There are no valves in arteries. This is because the blood is always under high pressure, so it doesn’t flow backwards. Except in the case of the arteries leaving the heart, which have valves.
What is the structure of veins?
- Veins transport deoxygenated blood from the body back to the heart.
- The muscle and elastic layers are relatively thin. The muscle layer is thin because constriction isn’t needed to control the flow of blood to the tissues as veins take blood back to the heart. The elastic layer is thinner because the blood is transported slowly and under low pressure, so the veins won’t burst.
- Veins have valves. As blood pressure in the veins is so low, the valves ensure that blood doesn’t flow backwards.
- They have a wide lumen. This maximises the volume of blood that is carried to the heart.
What is the structure of capillaries?
- Capillaries exchange substances between the blood and body tissues. They are the smallest of the blood vessels.
- Capillary walls are just one cell thick. They are made up of a single layer of endothelial cells which allows for rapid diffusion of substances.
- There are many capillaries throughout the body and they are highly branched. This means that there is a large surface area for the exchange of substances and that all cells are very close to a capillary.
- Capillaries are extremely narrow. Red blood cells are flattened against the side of the capillary because they are so narrow. This decreases the diffusion distance between the red blood cells and the cells that need oxygen, increasing the rate of diffusion.
- Small spaces are left between the endothelial cells that make up the capillary wall. These spaces allow white blood cells to leave the capillaries and destroy infections in tissues.
What are the advantages of a double circulatory system in mammals?
- Blood can be pumped faster around the body – this allows more oxygen to be delivered to tissues for respiration and maintaining their body temperatures and metabolic requirements
- Blood can be pumped around the body at a higher pressure – the blood returning to the heart has already been loaded with oxygen so the heart can pump the blood around the body immediately.
What is the cardiac cycle?
- is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating through the body
- control of the basic heartbeat is therefore myogenic as the heart can generate its beat without outside stimulation
Explaining what the cardiac cycle is?
it is a sequence of events that make up a single heartbeat
the cardiac cycle is repeated about 72 times per minute
includes periods of heart muscle contraction and relaxation
another follows one cardiac cycle in a continuous process (no gaps between cycles where blood stops flowing)
the contraction of the muscles in the wall of the heart reduces the volume of the heart chambers and increases the pressure of the blood within that chamber
when the pressure within a chamber exceeds that in the next chamber, the valves are forced open and blood moved through
when the muscles in the wall of the heart relax they recoil which increases the volume of the chamber and decreases the pressure so that the valves close
What are the three stages of the cardiac cycle?
1) Atrial systole
2) Ventricular systole
3) Cardiac diastole
Explain atrial systole
ventricles are relaxed
blood under low pressure flows into the left and right atria from the pulmonary veins and vena cava
this increases the pressure of the blood against the atrioventricular valves (bicuspid and tricuspid)
as a result atrioventricular valves open
semilunar valves are closed (so no blood can be pumped to the lungs or body at this stage)
blood begins to leak into the ventricles
atria then contracts which forces more blood into the the ventricle
(atrial systole = atria contracts)
Explain ventricular systole
atria relax
ventricles contract from the base of the heart upwards
this increases the pressure in the ventricles (pressure becomes higher than the pressure in the atria)
this forced the atrioventricular valves to close (preventing backflow to the atria) - producing the first heart sound ‘lub’
contraction of the papillary muscles pulls heart tendons preventing the valve from being inverted
this high pressure in the ventricles forces open the semi-lunar valves
blood can now flow under high pressure into the pulmonary artery (to the lungs) and the aorta (to the body)
Explain Cardiac Diastole?
Walls of atria and ventricles both relax (elastic recoil)
higher pressure in the pulmonary artery and aorta which causes semi-lunar valves to close (preventing backflow)
Atria fills with blood increasing pressure due to the higher pressure in the vena cava and pulmonary vein
ventricles continue to relax, and their pressure falls below the pressure in the atria
atrioventricular valves open
blood flows passively without being pushed by atrial contraction into the ventricles from the atria
the atria contract and the phases begin again
Explain myogenic stimulation of the heart
it means that the heart can contract without any input from the nervous system, and the signal for cardiac compression is raised within the heart tissue itself
How heart action is initiated and coordinated?
the heartbeat is initiated by a group of specialised muscle cells in the right atrium = ‘sinoatrial node’
sinoatrial node acts as a pacemaker
SAN is a group of cardiac muscle cells connected to nerve endings which form part of the involuntary nervous system, these nerves can alter the basic rhythm of the heart in line with the body’s requirements.
SA node triggers roughly 60-100 cardiac contractions per minute (normal sinus rhythm)
if the SA node fails, a secondary pacemaker (AV node) may maintain cardiac contractions at roughly 40-60 bpm
If both fail, a final tertiary pacemaker (Bundle of His) may coordinate contractions at a constant rate of roughly 30-40 bpm
Electrical conduction of a heartbeat
SA node sends a wave of electrical activity to the atrial walls (depolarisation)
this impulse directly caused the left and right atria to contract which initiates the heartbeat
A band of non-conducting collagen tissue (atrioventricular septum) prevents the waves of electrical activity from being passed from the atria to the muscles
The only conducting route for the impulse to the ventricles is via the atrioventricular node
there is a delay of approx 0.15s in conduction from SAN to AVN, meaning the atrial systole is completed before ventricular systole begins
AV node is connected to a strand of modified cardiac fibres called the bundle of His
The bundle of His splits into two branches which carry the impulse on into finer branches of cardiac fibres called the purkyne tissue
the finer fibres of purkyne tissue carry the electrical impulse down the septum and through the ventricles
this impulse causes both the ventricles to contract from the base upwards (ventricular systole)
ventricles contract 0.20s after the atria contract giving sufficient time for blood to be fully squeezed into ventricles from the atria before they start to contract
ECG
PRESSURE GRAPHS
