Introduction about Heart

INTRODUCTION:
The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest.
STRUCTURE
The heart is situated in the middle mediastinum behind the breastbone in the chest, at the level of thoracic vertebrae T5-T8. The right side of the heart is deflected forwards, and the left deflected to the back.
CARDIAC OUTPUT
The x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.
Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR. The cardiac output is normalized to body size through body surface area and is called the cardiac index.
The average cardiac output, using an average SV of about 70mL, is 5.25 L/min, with a range of 4.0–8.0 L/min.The stroke volume is normally measured using an echocardiogram and can be influenced by the size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload and afterload. Preload refers to the filling pressure of the atria at the end of diastole, when they are at their fullest. A main factor is how long it takes the ventricles to fill—if the ventricles contract faster, then there is less time to fill and the preload will be less. Preload can also be affected by a person’s hydration status. The force of each contraction of the heart muscle is proportional to the preload, described as the Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched. Afterload, or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves (stenosis) or contraction or relaxation of the peripheral blood vessels.