HEART AND BLOOD VESSEL DISORDERS - Medicine for Mountaineering and Other Wilderness Activities - page 215

CHAPTER 17
HEART AND BLOOD VESSEL DISORDERS
Blair D. Erb, Sr., M.D.
Blair D. Erb, Jr., M.D.
Principal Contributor s
The heart and blood vessels circulate blood, which transports oxygen and nutrients to the
body tissues and carries away carbon dioxide and waste. The heart consists of a four-
chambered pump: right atrium, right ventricle, left atrium, and left ventricle. Blood travels
fromtheleftventricle throughtheaortaandthroughrepeatedly branchingsmaller andsmal-
ler arteries to capillaries barely large enough to permit passage of single red blood cells. In
the capillaries, oxygen, carbon dioxide, and other substances are exchanged between blood
and tissue. From the capillaries, blood continues its journey as it flows through larger and
larger veins to return through the right atrium into the right ventricle. Blood is then pumped
from the right ventricle through the lungs, where it releases its acquired carbon dioxide and
is replenished with oxygen. It returns through the pulmonary veins and the left atrium to the
left ventricle. The result is a figure-eight circuit: one loop carrying blood through the lungs
where it is capillaries venous blood right atrium right ventricle arterial blood left ventricle
left atrium oxygenated, the other carrying blood to the rest of the body ( Fig. 17-1 ).
Four valves permit the heart to function as an efficient pump. The tricuspid and mitral
valves in the right and left ventricles close during cardiac contraction (systole), preventing
reflux of blood into the atria. Simultaneously the pulmonic and aortic valves open, permit-
ting blood to be pumped into the pulmonary artery and aorta. After systole has been com-
pletedandtheheartmusclerelaxes(diastole),thepulmonicandaorticvalvesclose,prevent-
ingrefluxofbloodintotheventricles, andthetricuspid andmitral valvesopen,allowing the
ventricles to fill.
Peripheral veins in the legs are compressed by the contracting muscles during exercise
such as walking, squeezing blood toward the chest. Low pressure in the chest produced by
the inspiratory phase of respiration results in a pressure gradient that moves blood from the
extremities toward the heart. Consequently, an individual can be said to have “two hearts”:
one in the chest that pumps blood through the arteries to the extremities, the second the
musclesinthelegsthatpumpbloodthroughtheveinsbacktothechest.Delicate valvesloc-
ated in the veins prevent backflow during this process.