cardiac cycle

• cardiac cycle - the sequence of events in one full beat of the heart
• Action of Valves - ensures blood is flowing the right direction; they are opened and closed by changes in the blood pressure in the various chambers of the heart
• Atrio-ventricular valves

  Atrial systole →ventricular systole →diastole

  • the pressure in the ventricles rapidly drops below the;pressure in the atria
  • blood in the atria pushes the atrio-ventricular valves open
  • blood entering the heart flows straight through the atria and into the ventricles
  • the pressure in atria and ventricles rises slowly as they fill with blood
  • the valves remain open while the atria contract, but close when atria begin to relax
• Atrio-ventricular valves 2

  • the closure is caused by a swirling action in the blood around the valves when the ventricle is full
  • as the ventricles begin to contract (systole), the pressure of the blood rises
  • when the pressure rises above the atria , the blood starts to move upwards
  • this movement fills the valve pockets and keeps them closed
  • the tendinous cords attached to the valves prevent them from turning inside out
  • this prevents the blood flowing back into atria
• Semilunar Vavles

  • pre-ventricular contraction, the pressure in the major arteries is higher than the pressure in the ventricles; his means the semilunar valves are closed
  • ventricular systole raises the blood pressure in the ventricles very quickly
  • once the pressure in the ventricles rises above the pressure in the major arteries, the semilunar valves are pushed open; the blood is under a very high pressure, so it is forced out of the ventricles in a powerful spurt
• Semilunar valves 2

  • once ventricle walls have finished contracting, the heart muscle starts to relax (diastole); elastic tissue in the walls of ventricles recoils

  → this stretches the muscle out again and returns the ventricle to its original size, causing pressure in the ventricles to drop quickly

  • as it drops below the pressure in the major arteries, the blood starts to flow back towards the ventricles
  • the semilunar valves are pushed closed by the blood collecting in the pockets of the valves; preventing back flow of blood in ventricles
  • pressure wave created is the 'pulse' we feel
• Pressure changes in the heart chambers

  
• Pressure in the blood vessels

  blood enters the aorta and pulmonary artery but the tissues require blood to be delivered in an even flow

  • the artery walls close to the heart have a lot of elastic tissue (when blood leaves the heart, these walls stretch)
  • as blood moves out of the aorta, the pressure in the aorta starts to drop
• Pressure in the blood vessels 2

  • the elastic recoil of the walls helps to maintain the blood pressure in the aorta; the further the blood flows along the arteries, the more the pressure drops and the fluctuations become less obvious
  • it is important to maintain the pressure gradient between the aorta and the arterioles, as this is what keeps the blood flowing towards the tissues
• Coordination of the cardiac cycle

  the cardiac muscle is described as myogenic as it can initiate its own contraction

  → the muscle will contract + relax even when it’s not connected to the body

  • atria + ventricles also have their own natural frequency of contraction; atrial contracts at a higher frequency

  → this can cause inefficent pumping causing fibrillation ( when contractions aren’t synchronised)

  • At the top right atrium, where the vena cava pumps blood into atrium, where SAN (sino-atrial node) is
• SAN is a small patch of tissue that generates electrical activity. It initiates a wave of excitation at regular intervals; 55-80 times/min for humans

  SAN is also known as the pacemaker
• Contraction of the atria

  • wave of excitation quickly spreads over walls of atria
  • it travels along the membrane of the atria
  • as the wave of excitation passes, it causes the cardiac muscle to contract

  -> this is atrial systole
• Contraction of the atria 2

  the tissue at the base of the atria isn’t able to conduct the wave to ventricles so the AVN (atrio-ventricular node), which is at the top of the intraventricular septum (separating the two ventricles), is the only way to conduct the wave.

  • The wave of excitation is delayed in the AVN node so the atria can finish contraction and allowing the blood to flow down into ventricles before contracting
• Contraction of the ventricles

  after the AVN delay, the blood flows to a specialised conducting tissue called the Purkyne Tissue, runs down the intraventricular septum

  • at the base of the septum, wave of excitation spreads over the ventricles
  • as the excitation spreads upwards from base (apex) of ventricles, the muscles contract
  • this pushes the blood from the apex to the major arteries at the top of heart
• Electrocardiogram (ECG)

  monitors electrical activity

  → this involves attaching a number of sensors on skin

  • some of the electrical activity spreads through the tissues next to the heart + outwards to the skin
  • the sensors of the skin pick up the electrical excitation created by heart + convert it into a trace

  → healthy person trace has a particular shape consisting of P,,Q,R,S,T

  • wave P shows the excitation of atria
  • wave Q,R,S show the excitation of ventricles
  • wave T shows diastole
• Abnormal traces

  sinus rhythm - normal

  bradycardia - slow heart beat

  tachycardia - rapid heart beat

  atrial fibrillation - when no P waves are clear

  ectopic heartbeat - extra or early ventricular beat