5. The heart

• • Heart is made of cardiac muscle = which contracts & relaxes in a regular rhythm
  • Coronary arteries supply cardiac muscle with oxygenated blood it needs to keep contracting & relaxing
  • Heart is surrounded by = inelastic pericardial membranes, which help prevent heart from over-distending [swelling due to pressure] with blood 
• FUNCTION OF HEART

  • Deoxygenated blood enters RA through superior & inferior vena cava at a low pressure
  • Atria = have thin muscular walls
  • As blood flows in = slight pressure builds up until tricuspid valve open to let blood into RV
  • When A+V filled with blood = atrium contracts --> forcing all blood into RV + stretching ventricle walls
  • As RV starts to contract = tricuspid valve closes, preventing any backflow of blood into atrium
  • Tendinous cords = make sure valves aren't turned inside out by pressures exerted when ventricle contracts
• FUNCTION OF HEART 2

  • RV contracts fully and pumps blood through semilunar valves into pulmonary artery which transports it to capillary beds of lungs
  • Semilunar valves prevent back flow of blood into heart
  • Oxygenated blood from lungs enters LA from pulmonary vein
  • As pressure builds bicuspid valve open between LA and LV = ventricle fills with oxygenated blood
  • Blood the forced into just LV which then contracts and pumps blood through semilunar valves into aorta and around body
  • As ventricle contracts tricuspid valve closes = preventing backflow
• FUNCTION OF HEART 3

  • Muscular wall on left side thicker as left side has to produce sufficient force to overcome resistance of aorta and arterial systems of whole body
  • Right side has to pump blood a short distance and only has to overcome resistance of pulmonary circulation
  • Septum = inner dividing wall which prevents mixing of deoxygenated and oxygenated blood
• Cardiac cycle + heartbeat

  • Cardiac cycle = events in a single heartbeat which lasts about 0.8 seconds in adult
  • Diastole = heart relaxes

  --> atria + ventricles fill w/blood = volume & pressure of blood in heart build as heart fills, but pressure in arteries is at a min

  • Systole = atria contract [atrial systole] followed by the ventricles [ventricular systole]

  --> pressure inside heart increases a lot + blood is forced out of right side to lungs and from left side to main body circulation

  --> volume + pressure of blood in heart are low at end of systole and blood pressure in arteries is at a max
• Heart sounds

  • Sounds of heartbeat are made by blood pressure closing heart valves
  • 2 sounds of heartbeat

  1. First sound = comes as blood is forced against atrio-ventricular valves as ventricles contract
  2. Second sound = comes as a backflow of blood closes semilunar valves in aorta and pulmonary artery as ventricles relax
• Basic rhythm of heart

  • Cardiac muscle is myogenic = has its own rhythm at around 60 beats per minute [bpm]
  • This prevents body wasting resources maintaining basic heart rate
  • Average resting heart rate of an adult is higher = 70bpm
  • Basic rhythm of heart is maintained by a wave of electrical excitation
• HEART RHYTHM

  1. Wave of electrical excitation begins in pacemaker area called = sino-atrial node [SAN] --> causing atria to contract and so initiating heartbeat

  --> A layer of non-conducting tissue prevents excitation passing directly to ventricles

  2. Electrical activity from SAN is picked up by atrio-ventricular node [AVN] = which imposes a slight delay before stimulating the bundle of His: a bundle of conducting tissue made of fibres [purkyne fibres] = which penetrate through septum between ventricles
• HEART RHYTHM 2

  3. Bundle of His splits into 2 branches + conducts wave of excitation to apex of heart

  4. At apex purkyne fibres spread out through walls of ventricles on both sides

  --> the spread of excitation triggers contraction of ventricles, starting at apex = contraction starting at apex allows efficient emptying of ventricles

  • The way in which the wave of excitation spreads through heart from SAN with AVN delay = makes sure atria have stopped contracting before ventricles start
• Electrocardiograms

  • You can measure spread of electrical excitation through heart as a way of recording what happens as it contracts
  • ECG: recording of the electrical activity of heart

  --> doesn’t directly measure electrical activity of heart

  --> measures tiny electrical differences in skin = which result from electrical activity of heart

  --> to pick up tiny charges = electrodes stuck to skin, signal from electrodes fed into a machine producing ECG
• Tachycardia:

  • When heartbeat very rapid, over 100bpm
  • Often normal when you're exercising, have a fever or scared
  • If its abnormal it may be caused by problems in electrical control of heart + may need to be treated
• Bradycardia:

  • When heart slows down to below 60bpm
  • Many people have this because they're fit = training makes heart more slowly and efficiently
  • Severe bradycardia = can be serious, may need artificial pacemaker to keep heart beating steadily
• Ectopic heartbeat:

  • Extra heartbeats that are out of normal rhythm
  • Most people have at least 1 a day
  • Usually normal but can be linked to serious conditions when they're very frequent
• Atrial fibrillation:

  • An example of arrhythmia = means an abnormal rhythm of heart
  • Rapid electrical impulses generated in atria
  • Contract very fast [fibrillate] up to 400 times a min
  • However, they don’t contract properly + only some impulses are passed onto ventricles = which contract much less often
  • As a result heart doesn’t pump blood effectively