Arteries carry oxygenated blood away from the heart to tissues throughout the body. As arteries branch into smaller vessels, they become capillaries that are only one cell thick, allowing for the exchange of oxygen and waste. Veins then carry deoxygenated blood back to the heart. Arteries have thicker walls than veins to withstand the force of blood being pumped from the heart. Capillaries form a network between arteries and veins and allow for chemical exchange through their thin, single-cell walls.

1. ARTERIES CAPILLARIES VEINS

2. ARTERIES
• Your arteries carry
blood from your heart
out to the rest of your
body.
• They have very thick
walls which allow them
to withstand the
immense pressure
created as your heart
pumps blood forcefully.

3. CAPILLARIES
• As blood travels outward, the
arteries become smaller and
smaller until eventually the
blood enters into what are
known as capillaries.
• Capillaries are so tiny that
blood usually can only get
through one cell at a time. It is
within the capillaries that
oxygen is taken in by the cells,
and waste, such as carbon
dioxide, is released into the
blood.

4. VEINS
• Eventually the
capillaries get bigger
and bigger, and then
the blood enters into
veins. As the blood
travels back to the heart
your veins get larger
and larger

5. TYPES OF BLOOD VESSELS

7. • ARE ARTERIES AND VEINS
DIFFERENT?
• Arteries carry blood from
the heart, while veins bring
it back. Arteries have
thicker walls than veins, to
withstand the force of the
blood pumping directly out
of the heart.

8. • WHAT ARE CAPILLARIES?
• About 98 percent of blood
vessels are microscopically
thin vessels, called
capillaries, that form a
network between arteries
and veins. Their walls are
only one cell thick, so that
chemicals can pass
between the blood and the
body’s tissues.

9. • BLOOD
• A liquid tissue, blood consists
of trillions of cells suspended
in a watery liquid called
plasma. Blood is the body’s
transportation system,
keeping all tissues and organs
supplied with the chemicals
needed for life and removing
waste. Plasma makes up 55
percent of blood, and red
cells around 44 percent.
White cells account for less
than 1 percent of blood.

10. • WHAT DO RED BLOOD CELLS
DO?
• Red blood cells pick up the
life-giving gas oxygen in the
lungs and release it
throughout the body. A single
drop of blood contains about
five million of these tiny cells.
Each cell is packed with the
bright red protein
hemoglobin, which binds
with oxygen and then
releases it where it is
needed.

11. • WHY DO WE HAVE WHITE
BLOOD CELLS?
• White blood cells destroy
germs and damaged tissue. A
drop of blood contains about
7,000 cells, and there are
many different types. Some
patrol the body like soldiers,
swallowing germs. Others
produce chemicals, called
antibodies, that stick to
germs and so make them
easier to kill.