Early Development: The human embryonic heart
(EHR) begins beating at approximately 21 days after
conception, or five weeks after the last normal menstrual
period (LMP), which is the date normally used to date
pregnancy. The human heart begins beating at a rate near the
mother's, about 75-80 beats per minute (bpm). It then
accelerates linearly for the first month of beating, peaking
at 165-185 bpm during the early 7th week, (early 9th week
after the LMP). This acceleration is approximately 3.3 bpm per
day, or about 10 bpm every three days, an increase of 100 bpm
in the first month.
After peaking at about 9.2 weeks after the
LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15
week after the LMP. After the 15th week the deceleration slows
reaching an average rate of about 145 (+/-25 bpm) bpm at term.
Structure: In the
human body , the heart is normally situated slightly to the
left of the middle of the thorax , underneath the sternum
(breastbone). It is enclosed by a sac known as the pericardium
and is surrounded by the lungs . The apex is the
blunt point at the base of the heart. A stethoscope can be
placed directly over the apex and count the beats. In normal
adults, its mass is 250-350 g , but extremely diseased hearts
can be up to 1000 g in mass. It consists of four chambers, the
two upper atria (singular: atrium ) and the
two lower ventricles .
A septum divides the right atrium and
ventricle from the left atrium and ventricle, preventing blood
from passing between them. Valves between the atria and
ventricles (atrioventricular valves) maintain coordinated
unidirectional flow of blood from the atria to the
ventricles.The ventricular systole consists of the contraction
of the ventricles and flow of blood into the circulatory
system. Again, once all the blood empties from the ventricles,
the pulmonary and aortic semilunar valves close. Finally
complete cardiac diastole involves relaxation of the atria and
ventricles in preparation for refilling with circulating
blood.
Pulse
Rates: A normal pulse rate for a healthy
adult, while resting, can range from 60 to 90 beats
per minute ( BPM ). During sleep,
this can drop to as low as 40 BPM; during strenuous exercise ,
it can rise as high as 200–220 BPM. Generally, pulse rates
are higher in younger persons. A resting heart rate for an
infant is as high as or higher than an adult's pulse rate
during strenuous exercise.