By Nerea Campillo
Physiology of Emergency Medicine
Medical Physiology: Chapters 14, 20, 21, 32 (p. 169-178, 245-269, 429-432)
Including a shortened summary of Pocket Companion to Medical Physiology:
Chapters 9, 10, 17, 20, 21, 24
Pressure, flow and resistance
Medical Physiology: Chapter 14 (p. 169-178)
The circulation
There is a systemic circulation and a pulmonary circulation. In both, the arteries
transport blood under high pressure to the tissues. Arteries have strong walls
and the blood flows at high speed. In the pulmonary arteries and the aorta, the
pressure is pulsatile, in all other vessels it isn’t. After the arteries come the
arterioles, with strong muscular walls that can close completely. Then the blood
reaches the capillaries where fluid, nutrients, electrolytes and hormones are
exchanged. The blood volume is low there and it also flows slowly due to their
large cross-sectional area (blood flow → 1/cross-sectional area). The venules and
then the veins transport the blood back to the heart. The venous walls are thin
but their lumen can increase greatly. Their overall cross-sectional area is much
larger than that of the arteries. By the time the blood reaches the right atrium
again, the pressure is down to 0 mmHg.
Basic principles of the circulation
The blood flow to the tissues is regulated based on the nutrient demand of the tissue. This is
done per tissue with the help of the constriction of microvessels, so the blood flow in the
whole body doesn’t need to increase just because your little toe needs more oxygen. The
cardiac output is the sum of all the local tissue flows, about 5000 ml/min ( = 5 L/min).
The arterial pressure is regulated separately from the cardiac output and the local blood flow
control. Nervous signals increase the contractility of the heart, decrease the compliance of
the veins (blood is squeezed out of the veins to increase the venous return) and all arterioles
are slightly contracted so more blood is present in the large arteries. The kidneys help with
the long term control of arterial pressure.
Blood flow
Blood flow is determined by the pressure difference between the two ends of the vessel and
by the vascular resistance. Resistance occurs as a result of friction with the vessel wall.
Ohm’s law:
blood flow = pressure difference / resistance
Laminar and turbulent flow
When blood flows steadily through a long, smooth blood vessel it shows laminar flow. This
means that each ‘layer’ of blood remains at the same distance from the vessel wall (it flows
straight). The layers that are the closest to the vessel wall, flow the slowest because they
adhere most to the vessel wall (= the parabolic profile for velocity of blood flow).
Different things can disrupt the laminar flow of blood; blood that flows too fast, obstruction in
vessel, sharp turns, rough vessel surface. All this causes a turbulent flow.The blood forms
whorls, called eddy currents (NL: draaikolken). The eddy currents cause the blood to flow
,By Nerea Campillo
with more resistance. The tendency for turbulence to occur (Reynolds’ number) is calculated
in the following way:
Reynolds’ number = (velocity of blood flow x diameter blood vessel x density of the blood) /
viscosity of the blood
When Reynolds’ number is higher than 200-400, there is turbulence in some of the branches
of the vessel. This happens normally in large arteries (so there is always some turbulence),
mainly in the proximal aorta and the pulmonary arteries.
The viscosity of the blood is mainly regulated by the amount of red blood cells. The
hematocrit is the proportion of blood that is red blood cells.
Resistance
1 peripheral resistance unit (PRU) means that the pressure difference between two points is
1 mmHg and the flow is 1 ml/sec.
Conductance is the measure of the blood flow through a vessel for a given pressure
difference. This depends on the resistance: the higher the resistance, the lower the
conductance. Small changes in the diameter of the vessel change a lot in the conductance.
Because in small vessels all the blood is near the vessel wall, so all the blood flows slow.
Blood vessels are parallel to each other, which means that the total resistance is way less
than the resistance of a single blood vessel. When one of those parallel ways is removed
(for example when a limb is amputated or a kidney is removed), the total resistance
increases a lot and the total blood flow and vascular conductance are reduced.
Pressure
When the arterial pressure is increased, the blood is pushed through the vessels with more
force and the vascular resistance increases as a compensatory measure. Blood flow
autoregulation is the ability of each tissue to adjust its vascular resistance and maintain
normal blood flow although the arterial pressure changes. This autoregulation eventually
overrides the effects of vasoconstrictors so the blood flow matches the requirements of the
tissue.
When the arterial pressure falls below the critical closing pressure, blood flow ceases as the
blood vessels are completely collapsed.
, By Nerea Campillo
Cardiac output and venous return
Medical Physiology: Chapter 20 (p. 245-258)
Cardiac output: the amount of blood that flows into the aorta each minute. Stroke volume x
heart rate = cardiac output.
Venous return: the amount of blood that flows into the right atrium each minute.
Average cardiac output
Factors that influence the average cardiac output for an individual:
● Basic level of body metabolism
● Exercise vs. rest
● Age
● Size of body
Cardiac output at rest is 5,6 L/min in men and 4,9 L/min in women. Max. cardiac output is
2,5x times the normal venous return.
Cardiac index is the cardiac output per square meter of body surface. Average is 3
L/min/m2.
Peripheral influences on cardiac output
In everyday life, cardiac output is mainly regulated by peripheral factors that determine the
venous return.
Venous return
Venous return influences the cardiac output because of the Frank-Starling mechanism. This
states that increased filling of the heart stretches the myocytes and causes them to contract
with more force.
Venous return also influences the stretch of the sinus node, which causes the Bainbridge
reflex that increases the heart rate. This influences cardiac output too.
Local blood flow
The regulation of cardiac output is the sum of the regulation of all local blood flows through
the different organs. When the peripheral resistance increases, the cardiac output falls (as
long as arterial pressure stays the same). The arterial pressure is maintained the same due
to nervous stimulation.
Changes in max. cardiac output
Hypereffective heart
A hypereffective heart can be caused by:
Nervous stimulation: sympathetic stimulation + parasympathetic inhibition = increased heart
rate, increased strength of contraction.
Hypertrophy: Can be caused by long-term increased workload (exercising).
Hypoeffective heart
A hypoeffective heart can be caused by:
● Increased arterial pressure
● Inhibition of nervous excitation
● Abnormal heart rhythm or rate
● Coronary artery blockage
● Valvular heart disease
● Congenital heart disease
