Page 199 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

8 Cardiovascular System

hypoxic damage (! p. 130). Myocardial perfu-
Overview
sion via coronary arteries (approx. 4% of the CO
Blood is pumped from the left ventricle of the at rest) must also be maintained, because any
heart to capillaries in the periphery via the disruption of cardiac pumping function will
arterial vessels of the systemic (or greater) endanger the entire circulation. About 20 to
circulation and returns via the veins to the 25% of the CO is distributed to the kidneys. This
right heart. It is then expelled from the right fraction is very large relative to the kidney
ventricle to the lungs via the pulmonary (or weight (only 0.5% of body mass). Renal blood
lesser) circulation and returns to the left heart flow is primarily used to maintain renal ex-
(! A). cretory and control functions. Thus, renal blood
The total Blood volume is roughly 4–5 L flow may be reduced transiently in favor of
(! 7% of the fat-free body mass; ! table on cardiac and cerebral perfusion, e.g., to ward off
p. 88). Around 80% of the blood circulates impending shock (! p. 218). During strenuous
through the veins, right heart and pulmonary physical exercise, the CO increases and is al-
vessels, which are jointly referred to as the low loted mainly to the skeletal muscle. During
pressure system (! A, left). These highly dis- digestion, the gastrointestinal tract also re-
tensible capacitance vessels function as a blood ceives a relatively high fraction of the CO. Nat-
reservoir in which blood is stored and released urally, both of these organ groups cannot re-
as needed via venous vasoconstriction (! e.g., ceive the maximum blood supply at the same
p. 218). When the blood volume increases— time (! p. 75 A). Blood flow to the skin (ap-
due, for example, to a blood transfusion—over prox. 10% of the resting CO) mainly serves the
99% of the transfused volume remains in the purpose of heat disposal (! p. 222ff.). The cu-
low-pressure system (high capacitance), while taneous blood flow rises in response to in-
only ! 1% circulates in the arterial high-pres- creased heat production (physical work) and/
sure system (low capacitance). Conversely, a or high external temperatures and decreases
decrease will be reflected almost entirely (pallor) in favor of vital organs in certain situa-
by a decrease in the blood stores in the low- tions (e.g., shock; ! p. 218).
pressure system. Central venous pressure The total CO flows through the pulmonary
(measured in or near to the right atrium; nor- circulation as it and the systemic circulation
mally 4–12 cm H 2O) is therefore a good indica- are arranged in series (! A). Oxygen-depleted
tor of blood volume (and ECF volume) in in- (venous) blood is carried via the pulmonary
dividuals with a normally functioning heart arteries to the lungs, where it is oxygenated or
and lungs. “arterialized.” A relatively small quantity of ad-
Cardiac output (CO). The cardiac output is ditional oxygenated blood from the systemic
calculated as heart rate (HR) times stroke circulation reaches the lung tissue via the
volume (SV). Under normal resting conditions, bronchial arteries. All blood in the pulmonary
–1
the CO is approx. 70 [min ] " 0.08 [L] = 5.6 L/ circulation drains via the pulmonary veins.
min or, more precisely, a mean 3.4 L/min per Peripheral resistance. Flow resistance in the
2
m body surface area. An increase in HR (up to pulmonary circulation is only about 10% of the
–1
about 180 min ) and/or SV can increase the CO total peripheral resistance (TPR) in the systemic
to 15–20 L/min. circulation. Consequently, the mean pressure
The distribution of blood to the organs ar- in the right ventricle (approx. 15 mmHg =
ranged in parallel in the systemic circulation 2 kPa) is considerably lower than in the left
.
(! A, Q values) is determined by their ventricle (100 mmHg = 13.3 kPa). Since the re-
functional priority (vital organs) and by the sistance in the lesser arteries and arterioles
current needs of the body (see also p. 213 A). amounts to nearly 50% of TPR (! A, top right),
Maintaining adequate cerebral perfusion (ap- they are called resistance vessels.
prox. 13% of the resting CO) is the top priority,
not only because the brain is a major vital
186
organ, but also because it is very susceptible to
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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