BLOOD VESSELS (ARTERIES,
VEINS AND CAPILLARIES)
• The Circulatory System is known as a CLOSED
SYSTEM because the blood is contained within
either the heart or blood vessels at all times.
• The blood vessels that are part of the closed
circulatory system of humans from a vast
network to help keep the Blood flowing in one
direction.
• The Blood Vessels that form this network and are
part of the CIRCULATORY SYSTEM ARE THE
ARTERIES, CAPILLARIES, AND VEINS.
• 5. With the exception of Capillaries and tiny
Veins, Blood Vessels have WALLS made of
THREE LAYERS OF TISSUE, that provides for a
combination of Strength and Elasticity:
• A. THE INNER LAYER IS EPITHELIAL TISSUE.
• B. THE MIDDLE LAYER IS SMOOTH MUSCLE
TISSUE.
• C. THE OUTER LAYER IS CONNECTIVE
TISSUE.
ARTERIES AND ARTERIOLES (SMALL
ARTERIES)
• 1. Arteries carry blood from the HEART TO
CAPILLARIES AND THE REST OF THE BODY.
• 2. The Walls of Arteries are generally THICKER
than those of Veins.
• 3. The Smooth Muscle Cells and Elastic Fibers
that make up the Walls help make Arteries
Tough and Flexible. This enables Arteries to
withstand the high pressure of blood as it is
pumped from the Heart. The force that blood
exerts on the walls of blood vessels is known
as BLOOD PRESSURE.
• 4. EXCEPT FOR THE PULMONARY ARTERIES,
ALL ARTERIES CARRY OXYGEN-RICH BLOOD.
• 5. The Artery that carries Oxygen-Rich Blood
from the LEFT VENTRICLE to all parts of the
body, EXCEPT THE LUNGS, is the AORTA.
• 6. THE AORTA WITH A DIAMETER OF 2.5 cm, IS
THE LARGEST ARTERY IN THE BODY.
• 7. As the Aorta travels away from the Heart, it
branches into smaller Arteries so that all parts
of the body are supplied.
• 8. THE SMALLEST ARTERIES ARE CALLED
ARTERIOLES.
CAPILLARIES
• 1. ARTERIOLES BRANCH INTO NETWORKS OF
VERY SMALL BLOOD VESSELS CALLED
CAPILLARIES.
• 2. IT IS IN THE THIN-WALLED (ONE-CELL IN
THICKNESS) THAT THE REAL WORK OF THE
CIRCULATORY SYSTEM IS DONE.
• 3. The Walls of the Capillaries consist of only
one layer of cells, making it easy for Oxygen
and Nutrients to DIFFUSE FROM THE BLOOD
INTO THE TISSUE.
• 4. Forces of Diffusion drive CO2 and waste
products from the tissue into the Capillaries.
• 5. Capillaries are extremely NARROW; Blood
Cells moving through them must pass in Single
file.
VEINS
• 1. THE FLOW OF BLOOD MOVES FROM
CAPILLARIES INTO THE VEINS. (Figure 47-6)
• 2. Veins form a system that COLLECTS Blood
from every part of the Body and CARRIES it
Back to the HEART.
• 3. The smallest Veins are called VENULES.
• 4. LIKE ARTERIES, VEINS ARE LINED WITH
SMOOTH MUSCLE. Vein walls are thinner and
less elastic than Arteries. Veins though are
more FLEXIBLE and are able to stretch out
readily.
• 5. This flexibility reduces the Resistance the
flow of blood encounters on its way back to
the Heart.
• 6. Large Veins contain Valves that maintain the
one direction flow of Blood. This is important
where Blood must flow against the Force of
Gravity.
• 7. The flow of Blood in Veins is help by
Contractions of Skeleton Muscles, especially
those in the legs and arms. When muscles
contract they squeeze against Veins and help
force Blood Toward the Heart.