Circulatory Reading
The circulatory system is centred on the HEART, a muscular organ that
rhythmically pumps BLOOD around a complex network of BLOOD
VESSELS extending to every part of the body. Blood carries the
oxygen and nutrients needed to fuel the activities of the body’s tissues
and organs, and it plays a vital role in removing the body’s waste
products. An average-sized adult carries about 5 litres (9 pints) of
blood.
PARTS OF THE SYSTEM
All the output of blood from the left side of the heart goes into the
aorta, the body’s largest artery. Other arteries branch from the aorta
to supply blood to the head, limbs, and internal organs. The blood is
drained from all these parts by veins into two large vessels, the
inferior and superior venae cava, which deliver the blood back to the
right side of the heart.
HEART
The heart contracts tirelessly – more than 2.5 billion times over an
average lifetime – to pump blood around the body. These contractions
are triggered by electrical impulses that originate in a specialized area
of heart tissue. The signals spread through the muscle in the wall of
the heart via a network of conducting fibres.
INSIDE THE HEART
The heart has two upper chambers, called atria, and two lower
chambers, called ventricles. Blood from the body arrives in the right
atrium. This blood is low in oxygen, and is shown here in blue. The
blood passes to the right ventricle, which pumps it to the lungs to pick
up more oxygen. The left atrium receives oxygen-rich blood (red) back
from the lungs. This passes to the left ventricle, which pumps it by
way of the aorta to the body.
HEART VALVES
At the exit of each heart chamber lies a valve, which ensures the oneway flow of blood through the heart and into the circulation. These
valves are made of flaps that open to allow blood to pass through but
snap tightly shut to prevent backflow. The valves have three flaps,
except for the valve between the left atrium and left ventricle, which
has two.
BLOOD VESSELS
If an adult’s blood vessels were laid end to end, they would stretch out
over 100,000 km (62,500 miles). There are three main types of
vessel. Arteries carry blood from the heart to the body’s tissues, while
veins carry blood back from the tissues to the heart. Small arteries are
called arterioles and small veins are referred to as venules. The third
and smallest type of vessel, capillaries, form a network connecting the
smallest arterioles with the smallest venules.
TYPES OF BLOOD VESSEL
Arteries have relatively thick elastic walls that enable them to
withstand the high pressure of blood pumped from the heart. By the
time the blood has been forced through capillaries and arrived in
veins, its pressure has dropped, so veins have thinner walls.
BLOOD
Blood is composed of a straw-coloured fluid, plasma, and huge
numbers of blood cells that float in the plasma. Of the two main types
of blood cell, red blood cells carry oxygen to the body’s tissues, and
white blood cells help defend the body against infection. Blood also
transports nutrients, proteins needed for blood clotting, and waste
products.
BLOOD CLOTTING
If a blood vessel is damaged, a clot forms to stop blood leaking. First,
platelets stick together to form a plug that stops the leak. At the same
time, a complex sequence of chemical events in the blood leads to the
production of long strands of a protein called fibrin. These bind the
blood cells and debris together to form a gel-like clot that gradually
solidifies. The solid clot remains until the blood vessel has been
repaired.
BLOOD CELLS
A drop of blood contains millions of red cells, and each cell contains
250 million molecules of a substance called haemoglobin. In the lungs,
oxygen binds to haemoglobin, but in the tissues the oxygen is released
again. Several types of white blood cell exist, and all are important to
the body’s immune system. Platelets are tiny cells that are needed for
blood clotting.