Question

1. Discuss the cardiac conduction system and automaticity of heart
2. Discuss cardiac output and the variables that dictate it.

Answer 1

The hearts pumping action is regulated by an electrical conduction system that cordinates the contraction of the atria and ventricles to work together, alternately contracting and relaxing to pump blood through your heart.The electrical signal is generated by the sinoatrial node (SA node).SA node is special group of cells that fire at regular intervals and cause the heart to beat , these specalised group of cells carry the electrical charge to the rest of the heart, they do not depend on the signal from the brain, the brain regulates the heart rate but does not send signals that cause heart to beat itself, after the atria is activated then the electrical signal travels through the conduction pathways causing the ventricles to contract to pump the blood out, the signals is then sent to the AV node which is a cluster of cells in the center of the heart between the atria and ventricles, which acts like a gate that slows the electrical signal before it enters the ventricles, this delay allows the time for atria to contract before the ventricles do, without this delay the atria and ventricles will contract at same time and the blood wont flow effectively from atria to ventricles. The Bundle of His (Purkinje network) is divided into left and right pathways called the bundle branch to stimulate the right and left ventricles , this forces blood out of the heart to the lungs and to diffrent part of body. The SA node fires another impulse and the cycle begins again.

2.The Cardiac output is the product of heart rate (number of beats per minute) and the stroke volume (which is amount pumped per minute), therfore cardiac output is dependent on two main factors, stroke volume and heart rate.

Stroke volume which is the diffrence between the End diastolic volume (EDV) and End systolic volume (ESV). ESV can be affected by preload, after load, contactility and heart rate. EDV depends on venous return, heart rate and compliance.

Heart rate : Heart rate is initiated by SA node and is modulated by neural and hormonal input, many factors influence a person's heart rate, high temperature and humidity cann increase heart rate, emotion and anxiety, the position of the body during the first 20 seconds after standing up, physical condition like obesity, state of body condition like dehydration etc.