Unit 4: Animal
Lesson 9: The Circulatory System
Blood Vessels
If you could join all the blood vessels in your body in a straight line, it would be about 19000km long! Our
blood vessels are not one long tube but a complex network of tubes that branch and rebranch.
Arteries & Arterioles:
 blood vessels that carries blood away from the heart and towards the body tissues
 walls of the arteries have three layers of tissue – connective tissues, smooth muscle and
endothelium
 when the ventricles of the heart contract to pump blood around the body, the arteries
expand slightly to accommodate the increased pressure of the blood within them. (The
outer layer of the arteries include elastin fibres giving the vessel elasticity.)
 when the ventricle relax, the walls of the arteries return to their original size, pushing the
blood farther into the downstream vessels
 when the artery expands from contractions of the heart, it is felt as a pulse
 when arteries branch and rebranch, the smallest artery is called the arterioles
Controlling Blood Flow in Arterioles:
 Because arterioles have smooth muscle, they can be controlled by the nervous system.
 Signals from the nerves can regulate the diameter of the arterioles and control blood flow
to certain parts of the body.
Vasodilation: an increase in the diameter (dilation) of arterioles that increases the blood
flow to tissues. This can be used by the body as a cooling strategy: warm blood close to the
skin loses thermal energy to the surrounding environment.
Vasoconstriction: a decrease in the diameter of arterioles that decreases the blood flow to
tissues. This can be used if the body is cold. Restricting blood flow to the skin prevents
thermal energy loss to the environment. Also, vasoconstriction ensures that the 5L of blood
in your body is distributed where it is needed.
Capillaries:
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arteries branch into arterioles and when those
reach the tissue of the body, they branch further
into smaller blood vessels called capillaries
capillaries form networks of blood vessels that
supply oxygen and nutrients to every cell
throughout the body tissues
capillary walls are only a single cell layer thick
which allows easy diffusion of oxygen and nutrients
into the tissues as well as carbon dioxide and other
waste products to diffuse from the tissue back
into the capillary
There is no smooth muscle in the walls of the capillaries so the diameter of the capillaries
cannot be controlled by the nervous system. Instead, pre-capillary sphincters contract and
relax to regulate blood flow. When the sphincters are relaxed, blood flow to a tissue is
increased.
Veins & Venules:
 capillary networks have an arteriole side and an venule side; (more pressure on the arterial
side)
 venules and veins carry deoxygenated blood containing carbon dioxide and other waste
products; blood is returned to the
heart
 smooth muscles in veins is not as
thick as in arteries and the walls
are not as elastic; this makes the
diameter of veins greater than
arteries which reduces the
pressure

blood pressure in veins is lower than blood pressure in arteries; how does blood return to
the heart? (muscles and the valves)
Blood Pressure:
Blood exerts pressure on the walls of the circulatory system. This is called blood pressure. A
specific volume of blood can be accommodated within the confines of the circulatory system. If
the amount of fluid increases, the pressure on the walls of the blood vessels will increase. Because
of the elasticity of the blood vessels a slight increase in pressure is tolerated, but when blood
vessels are stretched to the limit, the increase in blood pressure poses more serious health risks.
Sphygmomanometers are used to measure blood pressure. The cuff is inflated until blood flow in
the brachial artery is stopped. As the pressure is released from the cuff, the pressure sensors in
the cuff detect the vibrations of the blood flowing through the artery.
Systolic Pressure: this is the first reading which is the pressure of the artery when the heart
contracts. Normal systolic pressure in young adults is 120mmHg. It is the pressure in the arteries
when the heart contracts.
Diastolic Pressure: this is the second reading which is when the heart is relaxed and blood is
flowing through the artery. Normal diastolic pressure is about 80mmHg. It is the blood pressure
in the arteries when the heart relaxes.
The Lymphatic System:
The lymphatic system has two major roles:
1. In the immune system -- filters bacteria and other components of blood.
- Leukocytes gather inside of lymph nodes to destroy disease-causing viruses, bacteria and other
microorganisms. Swollen lymph nodes are signs that the immune system is fighting infections.
2. In the circulatory system -- ensures that blood volume is maintained
-As blood circulates through the capillaries, some proteins leak from the blood into the tissue
fluid. This lowers the pressure of the tissue fluid, and some fluid moves from the blood into the
tissue fluid by osmosis. The excess tissue fluid is collected in lymph vessels and returned to the
blood in the veins. Lymph vessels are distributed throughout the body and carry the fluid called
"lymph". This process maintains a balance of fluids in the circulatory system and ensures that the
blood volume remains relatively stable. Were it not for this system, the body tissue would swell
because of the extra tissue fluid.
Lymph contains fluid, bacteria and other foreign or dead/damaged cells, and fat molecules absorbed
through the lacteals in the villi of the small intestine. These components are either delivered back into
the bloodstream or filtered from the system and removed.
The spleen is the largest organ of the lymphatic system that acts as a filter and a reservoir of
erythrocytes and leucocytes. When you are sick, the body draws on the reservoir to provide additional
disease fighters, or increased levels of oxygen.
Tonsils also serve a similar function as the spleen by filtering out bacteria, viruses and other material.
Infected tonsils become red and
swollen, in order to work better to
trap or stop the diesase-causing
bacteria.
The thymus is a glandular organ of
the lympatic system that secretes
hormones to promote the maturity
lymphocytes. Lymphocytes are a type
leucocyte that recognize and attack
specific foreign invaders and are
essential line of defense against
bacteria, viruses, and other diseasecausing agents.
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The Heart:
The human heart is a remarkable,
muscular organ at the centre of the
circulatory system.
A wall of muscle called the septum
separate the heart into two parallel
pumps, each with an atrium and a
ventricle.
Atria receive blood and pump it into
the ventricles. The ventricles, pump
blood out into two circuits – the
pulmonary circuit (right side of the
heart) and the systemic circuit
(left side of the heart).
Circulation & Heart Sounds:
The familiar “lubb-Dubb” sound is caused by the
closing of heart valves. LUBB sound occurs when the
atrioventricular valves close as the ventricles start to
contract. This is a double sound because the left valve closes
slightly before the right valve.
The second sound, the DUBB part occurs as the
ventricles relax and the semilunar valves snap shut,
preventing blood from flowing back into the ventricles.
QUESTIONS:
1. Use T-chart to identify the three differences between arteries and veins.
2a) What is a pulse?
b) What information can you gather from taking a pulse?
3. How does gravity affect the circulation of blood? What structures in the blood vessels are
designed to counteract the effect of gravity?
4. What type of blood vessel do you think is shown here. Explain.
5. Explain why the rate of blood flow decreases in the capillary networks. Why is this an
advantage?
6. Nicotine is a drug that acts as a vasoconstrictor. Would a smoker likely have high blood
pressure or low blood pressure? Explain.
7. Describe the possible effects of consuming too much sodium.
8. Briefly describe the two primary functions of the lymphatic system.
9. Suggest and explain two possible effects of the removal of the spleen.
10. Explain why there is a difference in the thickness of the walls of the atria and the walls of
the ventricles.
11. Explain the importance of the coronary blood vessels.
12. Why do the atrioventrical valves have chordae tendinaea while the semilunar valves do not?
13. Describe briefly what happens during diastole and systole.
14. The heartbeat is generally described as a lubb-DUBB sound. Explain what is happening in the
heart to produce this sound.
SOLUTIONS:
Questions 1-7:
QUESTIONS 8-9
QUESTIONS 10-14