LEAVING CERT BIOLOGY
Ms. Mc Nelis
The Circulatory System
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The circulatory system in humans is a closed system: blood is contained within a
continuous system of blood vessels
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Materials in the blood can leave the vessels (e.g. food, oxygen) and pass into the cells,
but the blood itself cannot.
Circulatory System consists of:
 Muscular heart
 Blood vessels: arteries, veins, capillaries, venules, arterioles
 Blood
Heart  Arteries  Arterioles  Capillaries  Venules  Veins  Heart
Portal vein: a portal vein is different from other veins because it has capillaries at both ends.
E.g. hepatic portal vein (carries blood from intestine to liver)
Artery
Internal Structure of Blood Vessels
Vein
Capillary
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LEAVING CERT BIOLOGY
Artery
Ms. Mc Nelis
Vein
Lumen
Walls
Valves
Small lumen
Large lumen
Thick muscular walls
Thin muscular walls
Does not contain valves Contains
valves
to
prevent backflow of
blood
Direction of Carries blood away Carries
blood
into
blood flow
from heart
heart
Pressure
High blood pressure
Low blood pressure
Oxygen
Usually carries blood Usually carries blood
rich in oxygen, apart low in oxygen, apart
from pulmonary artery from pulmonary vein
Capillary
Very tiny vessel
Walls only one cell thick
Does not contain valves
Carries
blood
from
arterioles to venules
Medium pressure
Oxygen passes out of
capillary into body cells
Double Circulatory System:

In the pulmonary circuit : blood flows
from the heart to the lungs and back

In the systemic circuit : blood flows
from the heart to the body (all other
organs and cells) and back
Structure of Heart
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LEAVING CERT BIOLOGY
Ms. Mc Nelis
Structure of Human Circulatory System
Heart
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Located in the thoracic (chest) cavity: area inside rib cage
Contains strong double pump
The cardiac muscle of the heart differs from other muscles in the body because it
does not tire (involuntary muscle contraction)
The coronary artery supplies the heart itself with blood – blood comes from aorta and
back through coronary veins
Cardiac Cycle


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The heart is relaxed and blood flows into atria from veins
Bicuspid and tricuspid valves relax (open) and blood flows into ventricles
Atria contract pushing more blood into ventricles
The ventricles contract forcing blood into the arteries and at the same time the
bicuspid and tricuspid valves slam shut (creating ‘lub’ sound)
The ventricles relax and the semilunar valves slam shut to prevent backflow of blood
(creating ‘dub’ sound)
Diastole: the filling phase of the heart with blood. The heart is relaxed.
Systole: the emptying phase when the heart muscles contract.
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LEAVING CERT BIOLOGY
Ms. Mc Nelis
Pacemakers
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Heartbeat can occur independently of the brain.
Heartbeat is controlled by a small bundle of
specialised tissue called a pacemaker (SA and AV
nodes) located in the right atrium.
Sinoatrial (SA) node sends out regular electrical
impulses causing the atria to contract.
It also stimulates the atrioventricular (AV) node.
The AV node then stimulates the ventricles to
contract.
Location:
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•
•
•
•
- SA node is in the wall of right atrium.
- AV node is beside the tricuspid valve
The pacemaker itself controls the heartbeat.
Average rate = 75 beats per minute (bpm)
But nerve signals from the brain and hormones can change the rate at which it
operates.
Factors such as exercise, change in temperature and shock can increase heart rate.
Factors such as relaxation, sleep and alcohol can reduce heart rate.
What causes a pulse?
Pulse: expansion of artery due to pumping of heart
Blood Pressure: the force exerted by blood on the wall of a blood vessel
Effects of External Factors on Heart Rate
Factor
Smoking
Diet
Exercise
Effect
• Nicotine increases heart rate and raises blood pressure
• Carbon monoxide reduces the amount of oxygen carried by the blood
• Chemicals in smoke can cause clots in blood vessels leading to increased
risk of heart attack and stroke
• High salt diet – High blood pressure, greater risk of stroke
• High fat diet – Blocked coronary arteries leading to heart attack
• Heart can become stronger
• Greater oxygen carrying capacity
• Lower resting heart rate
Ways to Maintain a healthy Heart
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Don’t smoke
Eat less fat
Eat less salt
Exercise regularly
Avoid excess stress
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