Discover the sequential journey of blood through the systemic and pulmonary circulations, understanding how the heart pumps blood to every corner of the body. Explore the distribution of cardiac output, from the brain to the muscles, and learn how different organs regulate blood flow based on their...
Circulatory System
[Warning: some pics have the color code of oxygenation, red = oxygenated blood, blue = non
oxygenated, instead some others that have red for arteries and blue for veins, regardless of the
blood oxygenation state.]
The circulatory system behaves as a system of pipes with very heterogeneous elastic, contractile and
permeability properties.
There are two circulations, and they are sequential: the path starts from systemic circulation, enters
the right atrium and ventricle and then gets to the pulmonary circulation. Everything that passes and is
pumped through one part of the heart then must be pumped in the other part too (an exception is the
small amount of blood that goes through the coronary circulation to perfuse the heart tissue).
When we consider the whole amount of blood, there are
some numbers that we need to keep in mind: about 5/6 L
of blood (in total) and 5 L/min of Cardiac Output
(taken as a reference an adult male of 70 Kg, the “standard”
individual). If we have about 70 bpm, the stroke volume
should be around 80 mL.
The most part (84%) of the blood is found in systemic
circulation, mostly in the veins (64%), only 13 % in arteries,
7% in arterioles and capillaries (microcirculation), 7% in
the heart and 9% in pulmonary circulation.
Normally, blood would go from the left ventricle into the
aorta, then through arteries and arterioles reach a capillary
bed and then get back to the right part of the heart. The
path is not always so simple: there are some specific
exceptions.
In the kidneys we have an arterial capillary bed which is
the glomerulus: “arterial” means that it goes from
arteries to capillaries and the blood is oxygenated, but it
also means that the pressure is higher (the whole capillary
bed is in arterial range pressure, between 70 and 120
mmHg [compared to the venous one: between 8 and 12
mmHg]). This element in the kidney is used to produce
filtration in the glomerulus: the content of the blood
is arterial, and the pressure is maintained high (no
exchange of metabolites yet).
The hepatic circulation is more complex as there are at least three entrances:
- The hepatic artery brings the arterial blood directly to the liver.
- The portal vein also brings blood to the liver and the blood that it brings comes from the capillary
beds generated by the splenic and mesenteric arteries (from the abdomen). The portal vein is
a physiologically completely different vein: it does not only carry the blood that has equilibrated with
the tissue to release glucose, oxygen and take away CO2 and metabolites but (in this case, this vein)
also brings to the liver whatever we have absorbed by the intestine. The composition of the blood in
the portal vein is completely different from other veins. (Except if we have been fasting for a while).
The portal blood and the hepatic artery mix in the hepatic lobule and then gets out of the liver through
the hepatic vein.
Autonomous control
The renal organization and the cerebral organization of vessels have a specificity: it consists in
the fact that the regulation of the flow in the renal arteries (in particular in the afferent arteries that bring
blood into the glomerulus) and the regulation of the cerebral arteries are mostly dependent on
autonomous mechanisms (not autonomic): all the resistances are under control of the autonomic
nervous system, but we have mechanisms of autoregulation for kidney and cerebral circulation. This
is to say that whenever we change contraction of the arterial system and displace the blood (e.g.) from
skin to muscle or from muscle to gut etc. the circulation through the brain and the kidneys needs to be maintained
constant.
79 Body At Work II
, Enrico Tiepolo
Distribution of the cardiac output
Blood flow to the various systems is differently organized in terms of
percentage but also of volume. Some are largely perfused, and some
others are very little perfused.
E.g., the brain receives 13% of blood flow → in a person that weighs
70 kg it means that the brain receives as its weight were 10 kg which
means that the brain is perfused 7 times more than the
abdomen (as we do not have 10 kg of brain).
The skin is the largest organ in the body regarding volume: the skin
changes massively the amount of blood that circulates there based
on the temperature of the environment and of the body.
It is important to remember that the kidney receives about ⅕ (20%)
of the entire circulation. The kidney filters about ⅕ of the latter
amount of blood.
Of course, different organs receive different amount of blood depending on the functional state they are:
• The liver and the gastrointestinal tract are going to receive blood based on whether we are
starving or digesting.
• The heart receives a significant amount of flow, 4% → 2,8 Kg and the heart weights around
200 g.
Arteries
The arteries are high/quick-flow, elastic, low-compliance vessels. They can be classified as
“resistance vessels” because their tone contributes to sustaining blood pressure. They have strong
walls (especially in systemic circulation) to face high blood pressure.
Their structure may change with time: hypertrophy may be produced by high pressure, catecholamine
stimulus, vasopressin and aldosterone activity, and such hypertrophy mostly hardens the walls increasing
the fibrous rather than the muscular component.
The epithelium of the arteries is subject to shear forces that may damage it, especially in the long run, if
the wall loses elasticity (due to persistently high pressure: this is the reason why diastolic, rather than
systolic, hypertension is so important) this may lead to deposition of cholesterol and the formation of
atheroma; platelets may adhere to the lesions, generating thrombi, which are yellowish rather than red
because the flux is rapid, and erythrocytes tend to flow in the middle of the vessel rather than sticking to
the walls. Venous thrombi instead, have been proposed as mostly being enriched in fibrin and
erythrocytes, therefore having a reddish appearance.
Resistance of a vessel
The resistance of a vessel is strictly related to its radius as we
need to consider the friction that the walls of the “pipe” exert
on the flow of the fluid.
The portions of the fluid that will be next to the walls will
flow with a smaller velocity because they are affected by the
friction that the walls exert.
The most lateral lamina of fluid flows against the wall, then
the inner one is quicker and, as you go towards the inside,
you increase the velocity. This brings about the fact that the
larger the vessel, the more freely the central
portion of the fluid flows.
If the vessel is sufficiently big, then the erythrocytes will be in the center with a big flow and they will
produce no hindrance, no shear. Instead, if the vessel is small, the erythrocytes are packed on each other,
they will hit the walls, and everything will be slowed down.
Essentially, the flow is proportional to the fourth power of the radius (it means that the resistance
decreases with the fourth power of the radius). This suggests that, as you go from artery to arteriole to
capillary, the resistance of each single vessel will increase enormously. However, while they become
80 Body At Work II
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