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Circulation and
respiration
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The circulatory
system
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© Zanichelli editore 2016
Functions of the circulatory system
In animals, the circulatory systems has three functions:
• transport of gases, nutrients, metabolic waste, hormones;
• thermoregulation;
• defense (transport of cells and chemicals that help fight
against attacks from pathogens).
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Structure of the circulatory system
The human circulatory system (or cardiovascular system)
consists of the heart, blood vessels and blood.
left
atrium
right
atrium
right
ventricle
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left
ventricle
The heart is a muscle organ, divided
into two atria (right and left) and two
ventricles (right and left).
The blood is pumped from the heart
into arteries and returns to the heart
through veins.
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Pulmonary and systemic circulation
Pulmonary circulation transports
blood from the heart to the lungs,
where it is oxygenated and then
returns to the heart.
Systemic circulation transports
blood from the heart to the rest of
the body.
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Structure of the heart
The walls of the heart are made up of three layers:
• the endocardium is formed by endothelial tissue;
• the myocardium is formed by muscle tissue;
• the epicardium is formed by a protective membrane.
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The cardiac cycle
During a cardiac cycle, two phases alternate:
• a relaxation phase (diastole);
• a contraction phase (systole).
The heartbeat is controlled by electrical activity generated by
the heart itself in the senatorial node (a natural pacemaker).
The senatorial node spontaneously induces the rhythmic and
regular contractions of the heart.
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Blood vessels
Blood vessels include arteries, veins and capillaries – each
have different structures and functions.
Arteries transport blood from the heart to the lungs and the
rest of the body. They have thick and elastic walls.
Veins transport blood back to the heart. They have thinner
and less elastic walls.
Capillaries have very thin walls which allow the exchange of
gases and nutrients. Capillaries control the blood flow to
organs.
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Blood
Blood contains cells (white cells, red cells and platelets), that
are suspended in plasma.
Red cells (erythrocytes) specialize in the transport of
respiratory gases. They also contain hemoglobin.
White cells (leucocytes) are dedicated to the defense of the
organism and are the main component of the immune system.
Platelets are not real cells, but cell fragments. They allow
blood clotting and repair damaged vessels.
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Lymphatic system
The lymphatic system supports the circulatory system:
• it collects fluids that filter from the capillaries;
• it transports white blood cells, defending the body against
pathogens;
• it collects nutrients.
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The respiratory
system
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Functions of the respiratory system
The respiratory system is responsible for pulmonary
ventilation and the exchange of gases with cells. Inside
cells, cellular respiration takes place.
lungs
O2
blood
organ
blood
O2
CO2
CO2
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Anatomy of the respiratory system
The respiratory system includes:
• the upper respiratory tract
(nose, mouth, pharynx, larynx);
• the lower respiratory tract
(trachea, bronchi);
• the lungs, that contain alveoli
where the exchange of gases
occurs.
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Pulmonary ventilation
In humans, pulmonary ventilation (breathing) has two
phases:
• during inspiration (or inhalation), air enters the lungs;
• during expiration (or exhalation), air exits the lungs.
These phases depend on changes of pressure inside the rib
cage.
Control of the rhythm of ventilation is involuntary, but can
also be modified voluntarily.
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Hemoglobin and oxygen
Hemoglobin located in red blood cells takes on oxygen at the
level of the lungs and then transports it through the blood
stream to all the parts of the body.
Hemoglobin releases oxygen once it reaches tissues.
Myoglobin, located in muscle cells, can also carry oxygen.
It is a reserve of oxygen during intense efforts or during apnea.
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