Page 186 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

7 Heart and Circulation S. Silbernagl

Overview
The left ventricle (LV) of the heart pumps the trol and excretory functions. If there is a risk of
blood through the arterial vessels of the sys- imminent circulatory shock (→ p. 231), renal
temic circulation into the capillaries through- blood supply may be temporarily reduced in
out the body. Blood returns to the heart via favor of the heart and brain. When physical
the veins and is then pumped by the right ven- work is markedly increased, blood flow
tricle (RV) into the pulmonary circulation and through the skeletal muscles is raised to ca. ⁄4
3
thus returns to the left heart (→ A). of the (now greater) CO. During digestion the
The total blood volume is about 4.5–5.5 L gastrointestinal tract receives a relatively large
(ca. 7% of fat-free body mass; → p. 28), of proportion of CO. It is obvious that these two
which about 80% is held in the so-called low groups of organs cannot both have maximal
pressure system, i.e., the veins, right heart, and blood perfusion at the same time. Blood flow
the pulmonary circulation (→ A). Because of its through the skin (ca. 10% of CO at rest) serves,
high compliance and large capacity, the low in the first instance, to remove heat. It is there-
pressure system serves as a blood store. If the fore raised during increased heat production
normal blood volume is increased, e.g., by (physical exercise) and/or at high ambient
blood transfusion, more than 98% of the in- temperature (→ p. 20ff.), but can, on the other
fused volume goes to the low pressure and hand, be reduced in favor of vital organs (pal-
less than 2% to the high pressure system. Con- lor, e.g., in shock; → p. 230 ff).
versely, if the blood volume is decreased, it is The entire CO flows through the pulmonary
almost exclusively the low pressure system circulation, since it is connected in series with
that is reduced. When cardiac and pulmonary the systemic circulation (→ A). Via the pulmo-
function is normal, the central venous pres- nary artery low-oxygen (“venous”) blood
sure (normally 4–12 cm H 2 O) is a good mea- reaches the lungs, where it is enriched with
sure of the blood volume. oxygen (“arterialized”). In addition, a relatively
Cardiac output (CO) is the product of heart small volume of arterialized blood from the
rate and stroke volume and at rest amounts to systemic circulation reaches the lung via the
– 1
ca. 70 [min ] · 0.08 [L], i.e., ca. 5.6 L/min bronchial arteries that supply the lung tissue
(more precisely, a mean of 3.4 L/min per m 2 itself. Both supplies then drain into the left
body surface area, a value called cardiac index atrium (LA) via the pulmonary veins.
(CI). CO can be increased many times over by a Flow resistance in the pulmonary circula-
rise in heart rate and/or stroke volume (SV). tion is only ⁄6 of total peripheral resistance
1
CO is distributed among the organs that are (TPR), so that the mean pressure that has to be
arranged in parallel within the systemic circu- generated by the RV in the pulmonal artery
lation (→ A, Q ˙ values), their share being depen- (ca. 15 mmHg = 2 kPa) is much less than that
dent on how vital they are, on the one hand, which needs to be generated by the LV in the
and on the momentary demands, on the other. aorta (100 mmHg = 13.3 kPa). The main resis-
Maintenance of an adequate blood supply to tance in the systemic circulation is due to the
the brain takes priority (ca. 13% of resting CO), small arteries and arterioles (→ A, upper right),
as this is not only a vital organ, but also be- which for this reason are called resistance ves-
cause it reacts especially sensitively to oxygen sels.
deficiency, and nerve cells, once destroyed,
cannot usually be replaced (→ p. 2f.). Blood
flow through the coronary arteries of the heart
muscle (at rest ca. 4% of CO; → p. 216) must not
fall, because the resulting abnormal pump
function can impair the entire circulation. The
kidneys receive ca. 20–25% of CO. This propor-
176 tion, very high in relation to their weight (only
0.5% of body weight) largely serves their con-
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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