Реферат: Heart Essay Research Paper HEARTThe human heart

resultant heart rate. In adults at rest this is between 60 and 74 beats a minute. In infants and young

children it may be between 100 and 120 beats a minute. Tension, exertion, or fever may cause the rate of

a healthy heart to vary between 55 and 200 beats a minu!

te.

The output of the heart is expressed as the amount of blood pumped out of the heart each minute: the

heart minute-volume (HMV). This is the product of the heart rate and the stroke volume (SV), the amount

of blood pumped out of the heart at each contraction.

EVOLUTION OF THE HEART

The hearts of primitive vertebrates apparently had only one atrium and one ventricle. Since their body

temperature and metabolic rate fluctuated with the environmental temperature, they did not need as

efficient a circulatory system as mammals and birds. The two-chamber heart is retained by modern fish,

but oxygen-rich blood does not mix with oxygen-poor blood, because the blood is aerated at the gills and

goes directly into systemic circulation, not to the heart. As the primitive lung evolved in amphibians,

two circulatory systems arose. The problem of mixing oxygenated and deoxygenated blood was resolved in a

number of amphibians such as the FROG, in which the single atrium is divided into two separate chambers.

Thus there is only a slight mixing of the bloods in these three-chambered hearts. This adaptation appears

to help the frog when it is under water, since the skin provides oxygen when the lungs cannot be used. In

SIRENS a partial division takes place in the ventricle !

as well.

As animals became larger and more active on land, they needed more pressure to provide faster flow. The

sides of the heart were separated when a septum formed to divide the ventricle into two chambers. Birds

and mammals have completely separate chambers and have more blood per tissue weight and more pressure,

because the tissues of birds and mammals (warm-blooded vertebrates) require a constant perfusion of

oxygen-rich blood in order to maintain their high metabolic rates and constant body temperature.

HEART EXAMINATION

The closure of the heart valves and the contraction of the heart muscle produce sounds that can be heard

through the thoracic wall by the unaided ear, although they can be amplified by means of a STETHOSCOPE.

The sounds of the heart may be represented as lubb-dupp-pause-lubb-dupp-pause. The lubb sound indicates

the closing of the valves between the atria and ventricles and the contracting ventricles; the dupp sound