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Physiology 2130 Lecture Notes - Blood Vessel, Pulmonary Circulation, Theca Interna

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Anita Woods

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Module 9: Blood Vessels
Blood vessels: it is through these that the blood is pumped and distributed to all areas of the
Anatomy General Organization
The circulatory system is essentially a closed system of tubes (blood vessels) filled with fluid
(blood) that is moved around by a central pump (the heart).
The blood vessels consist of :
o Arteries and arterioles that transport the blood away from the heart
o Capillaries where gas exchange takes place
o Venules and veins that return the blood back to the heart.
The large arteries branch into smaller arteries, which eventually turn into smaller arterioles.
These arterioles also branch into smaller vessels that lead to the capillaries.
Capillaries are the smallest of all the blood vessels and are the functional units of the circulatory
system where substances enter and leave.
The capillaries converge into small venules, which get larger and larger to form veins
There are two principal loops that the blood takes through the body.
One loop begins on the right side of the heart and sends blood through arteries to the lungs.
o These blood vessels continually branch into smaller and smaller blood vessels, which
eventually become capillaries.
o Gas exchange takes place in these pulmonary capillaries
o Oxygen diffuses into the blood and carbon dioxide out. The blood then enters venules
and progressively larger veins to eventually return to the left side of the heart.
This loop is called the pulmonary circulation.

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The second loop begins on the left side of the heart.
The freshly oxygenated blood is now pumped to the rest of the body; it travels from the left
ventricle, through the aorta, and into arteries.
o The arteries branch into smaller arterioles that, in turn, branch into capillaries. Again,
the capillaries are the site of gas exchange.
o Oxygen, nutrients, hormones, etc. are delivered to the cells, and carbon dioxide (CO2)
and waste products are picked up. This deoxygenated blood returns to the right side of
the heart through venules and larger veins.
This circulatory loop is called the systemic circulation.
There are two smaller circulatory loops within the larger systemic circulation:
o The hepatic portal loop found in the digestive system
o The hypothalamic-hypophyseal portal system found in the brain
Blood Volume distribution
The total blood volume (TBV) of an average human being is roughly 5 litres (1.3 gallons)
Largest portion (70%) is contained in the veins
o Since they contain the most “capacity,” the veins are referred to as the capacitance
vessels or blood “reservoir”
Heart and lungs contain about 15%
The arteries contain about 10% of TBV
Capillaries (where gas exchange occurs) contain last 5%
Blood velocity and Cross-sectional area of vessels
Just like blood volume, blood pressure, blood velocity, and cross-sectional areas of the blood
vessels also vary throughout the circulation. These characteristics have important functional
significance as you can see below.
The arteries
o Have highest blood pressure and velocity,
o A very low cross-sectional area

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o As a consequence, these vessels rapidly distribute the blood throughout the body.
The arterioles
o Have a lower blood pressure and velocity
o The cross-sectional area is higher
o These vessels are the site of highest resistance in the circulation and help regulate blood
flow to an organ
o The blood velocity is lowest
o Their total cross-sectional area is the highest in the circulation.
o These characteristics help to maximize exchange of substances across these blood
The blood pressure and cross-sectional area decrease while blood velocity increases in the
venules and veins. These vessels return the blood back to the heart while also storing a large
percentage of the total blood volume
Pressure, Flow, and Resistance
You will remember from module 3 that the driving force moving ions during diffusion is a
concentration gradient.
Here, the force that moves the blood through the entire circulatory system is a pressure
The diagram at above shows a large drop in pressure from high (in the aorta) to low (in the
veins). This is the pressure gradient that causes the blood to flow through both the pulmonary
and systemic circulation.
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