Cardiac Output

Slide 1

• How to calculate cardiac output:
• Cardiac Output(CO) = Stroke Volume(SV) x Heart Rate(HR)
• Stroke Volume - Depends on the volume of blood returning to the heart during diastole called the venous return. During exercise, this volume is greater, which stretches the heart muscle more, leading to stronger contractions which expel more blood.
• Heart Rate - Follows a similar trend to stroke volume during & after exercise: rising, staying level, then gradually falling during recovery.
• Athletes who have done endurance training generally have lower heart rates than people who have not, because their hearts are larger and more muscular.
• Cardiac muscle works continuously throughout your life, without fatigue and it is myogenic because it can contract rhythmically without receiving an external impulse from nerve cells.
• Contractions of cardiac muscle is initiated by small changes in the electrical charge of cardiac muscle cells. When these cells have a slight positive charge on the outside, they're said to be polarised. When this charge is reversed, they're depolarised. A charge in polarity spreads like a wave from cell to cell & causes the cells to contract.
• Each heart beat produces a complex signal with several components, each of which is caused by a certain stage in the cycle. Depolarisation of cardiac muscle leads to contractions; when polarised it is relaxed.

Slide 2

• How does the heart beat?
• 1.Depolarisation starts with electrical impulse from the Sinoatrial node(SAN), spreading across the atria walls, causing contraction.
• 2.The impulse also travels to the ventricles via atrioventricular node(AVN) which are specialised cells. From here, the impulse is conducted to the ventricles after a delay of about 0.13 seconds.
• 3.After this delay, impulses pass down to the Purkyne fibres(large specialised fibres) to the heart apex.
• 4.Impulses spread up through the ventricle walls causing contractions from the apex upwards. Blood is squeezed into the arteries.

Slide 3

• 
• P wave - depolarisation of the atria, leading to atrial contraction(atrial systole)
• QRS complex - depolarisation of the ventricles(ventricular systole)
• T wave - Repolarisation of ventricles during the heart's relaxation phase.
• How to measure heart rate:
• small square = 0.04 sec
• 5 small square = 0.2 sec
• 1 large square = 5 small sqre
• 5 large square = 1 sec
• For e.g: 300 divide number of large square = heart rate(BPM)
• An ECG trace can provide information about abnormal heartbeats, areas of damage & inadequate blood flow.

Slide 4

• Nervous control of heart rate
• Heart rate is under the control of the cardiovascular control centre located in the medulla oblongata region of the brain.
• Nerves forming part of the autonomic nervous system lead from the cardiovascular control centre to the heart. 2 nerves are involved:
• 1.Sympathetic nerves - speed up body systems during activity.
• 2.Parasympathetic nerves - slow down body systems during rest.
• Both of the nerves often work against each other to give fine control of involuntary systems. Same case with control of the heart beat. Both nerves connect to the SAN.