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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
The Blood Vessels
The circulatory system has three types of blood vessels.
• Arteries: carry blood away from the heart to the capillaries
• Capillaries: permit exchange of material with the tissues
• Veins: return blood from the capillaries to the heart
All three blood vessel types have an inner endothelium, a
simple squamous epithelium attached to a connective tissue
basement membrane that has elastic fibres.
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
The Arteries
The largest artery in the body is the
aorta, which carries O2-rich blood
from the heart to other parts of the
body.
The arterial wall has three layers.
• Inner layer: endothelium
• Middle layer: smooth muscle
that contracts and relaxes to
regulate blood flow and pressure
• Outer layer: fibrous connective
tissue
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Figure 10.1 Blood vessels. The walls of arteries
and veins have three layers. The inner layer is
composed largely of endothelium, with a basement
membrane that has elastic fibres; the middle layer is
smooth muscle tissue; the outer layer is connective
tissue (largely collagen fibres).
a. Arteries have a thicker wall than veins because
they have a larger middle layer than veins.
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Chapter 10: Circulatory System and Lymphatic System
Arterioles are small arteries that branch off
from an artery.
Arterioles have three layers.
• Inner layer: endothelium
• Middle layer: some elastic tissue but mostly
smooth muscle
o Smooth muscle contracts: blood vessel
constricts, resulting in higher blood
pressure
o Smooth muscle relaxes: blood vessel
relaxes, resulting in lower blood pressure
• Outer layer: fibrous connective tissue
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Section 10.1
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Chapter 10: Circulatory System and Lymphatic System
The Veins
Veins and venules (small veins) take
blood from the capillary beds to the
heart.
• Veins and venules have the same
three layers as arteries, but there
is less smooth muscle and
connective tissue
• Veins have valves that prevent
backflow
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Section 10.1
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
• Veins act as a blood reservoir
o Since their walls are thinner, they can expand to a
greater extent
o About 70% of blood is in the veins
• The largest veins in the body are the venae cavae (superior
vena cava, inferior vena cava), which deliver O2-poor blood
to the heart
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Chapter 10: Circulatory System and Lymphatic System
The Capillaries
Capillaries are narrow blood
vessels that join arterioles to
venules.
• Composed of a single layer
of epithelium with a
basement membrane
• Form vast networks
(capillary beds) throughout
the body
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Section 10.1
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
• Only certain capillary beds are open at any given time.
o After eating, capillary beds that serve the digestive system are
open, and those that serve the muscles are mostly closed
o Sphincter muscles relax to open the bed and allow blood flow
o Sphincter muscles contract to close the bed and prevent blood
flow
o When the bed is closed, blood flows through anastomoses
(arteriovenous shunts) directly from arterioles to venules,
bypassing the bed
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
Figure 10.2 Anatomy of a capillary bed. A capillary bed forms a maze of capillary
vessels that lies between an arteriole and a venule. When precapillary sphincter
muscles are relaxed, the capillary bed is open, and blood flows through the capillaries.
When sphincter muscles are contracted, blood flows through a shunt (anastomosis)
that carries blood directly from an arteriole to a venule. As blood passes through a
capillary in the tissues, it gives up its oxygen. Therefore, blood goes from being O2-rich
in the arteriole (red colour) to being O2-poor in the vein (blue colour).
CAPILLARY FLOW
Two forces control movement of fluid through capillary walls:
Osmotic pressure: draw of fluid to the dissolved solutes
that can’t cross semipermeable membrane
Tends to cause water to move from tissue fluid to blood
Created by salts and plasma proteinds
Hydrostatic Pressure (Blood Pressure): tends to cause
water to move in the opposite direction.
Capillary Exchange
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Chapter 10: Circulatory System and Lymphatic System
Section 10.2
Lymphatic Capillaries
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1
• Exchange of substances takes place across the thin walls of
the capillaries.
o Oxygen and nutrients diffuse out of the capillary and into
the tissue fluid that surrounds cells
o Wastes (carbon dioxide) diffuse into the capillary
o Some water leaves the capillaries, and excess is picked
up by lymphatic vessels
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Chapter 10: Circulatory System and Lymphatic System
Section 10.2
Capillary Exchange
Fluid in the blood is called plasma. When blood reaches a
capillary, the movement of fluid in the blood through the
capillary wall is controlled by:
• Osmotic pressure (causes water to move from the
tissue fluid to the blood)
• Blood pressure (causes water to move from blood to
tissue fluid)
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Chapter 10: Circulatory System and Lymphatic System
Section 10.2
Arterial End of Capillary
• Blood pressure (hydrostatic pressure) is higher than osmotic
pressure of blood
• Water exits capillary
Midway Along the Capillary
• Blood pressure and osmotic pressure cancel each other out
• No net movement of water
• Solutes diffuse according to concentration gradient
o Nutrients and oxygen diffuse out of the capillary; wastes
and carbon dioxide diffuse into the capillaries
• Small substances leaving capillaries contribute to tissue fluid
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Chapter 10: Circulatory System and Lymphatic System
Section 10.2
Venous End of Capillary
• Osmotic pressure is greater than blood pressure
• Water moves into capillary
• Excess tissue fluid is collected by lymphatic capillaries, where
it becomes lymph
Figure 10.10 Lymphatic
capillaries. A lymphatic
capillary bed (shown here
in green) lies near a blood
capillary bed. When
lymphatic capillaries take
up excess tissue fluid, it
becomes lymph.
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Chapter 10: Circulatory System and Lymphatic System
Check Your Progress
1. Describe how blood flow is controlled in each of the
three major types of blood vessels.
2. List several specific substances that diffuse across
capillary walls.
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Section 10.1
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Chapter 10: Circulatory System and Lymphatic System
Section 10.1