Tag: cardiac cycle

The human circulatory system consists of a muscular four-chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated. Circulation was discovered by William Harvey. He was the first to recognize the full circulation of blood in the human body and to provide experiments and arguments to support this idea.

The human heart is an example of myogenic heart, it possesses cardiac muscle helping the heart to automatically contract spontaneously; which do not need any stimuli as in neurogenic heart. A myogenic heart is capable of generating a cardiac contraction independent of nervous input. In a myogenic heart, contraction is initiated by the cells called myocyte  (Pacemakers)itself instead of an outside occurrence or stimulus such as nerve innervation.

Smaller animals have much higher metabolism; proportionate to body size, smaller animals tend to have a much smaller heart than larger animals. This means that in order to circulate blood at the same rate, the hearts of smaller animals need to pump more times within a specified time because the volume they can pump each time is smaller.

• Heart sounds are noises generated by the closing and opening of the heart valves. In a healthy individual, there are two normal heart sounds called lub and dub. 
  
• Lub is the first heart sound. It is associated with the closure of the tricuspid and bicuspid valves at the beginning of systole. 
• The second heart sound dub is associated with the closure of the semilunar valves at the beginning of diastole.
  

The sinoatrial node (SAN) is the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. 

The rate of heartbeat is inversely proportional to the size of the body. Smaller animals have a greater heart rate due to increased metabolic rate than the large animals. 

A valve is a membranous structure present in a hollow organ. It folds or closes to prevent the return flow of the body fluid passing through it. The valves present in between the auricles and ventricles are meant to prevent the backflow of blood from the ventricles to auricles. Valves are also a part of veins.

Heart rate affects both stroke volume and minute volume by altering the length of diastole and thereby the degree of filling and force of contraction. Minute volume and B.P may rise even if the stroke volume falls. This happens with a moderate rise in the heart rate. But if the heart rate is too high, the stroke volume becomes so low that the minute output falls far below the normal.

Atrial systole occurs at the end of ventricular diastole. After the ventricle has pumped its blood, it begins to relax. There is now negative pressure in the ventricle (since it is now empty), and this pressure helps to draw in the blood from the atrium when it enters into systole. So during atrial systole, the bicuspid and tricuspid valves are open, and the pulmonary/aortic semilunar valves are closed. Then during the diastole, the bicuspid and tricuspid valves are closed.