TRANSPORT IN
MAMMALS
Mr. Dejan Suljic
Unit Objectives
• Structure of the mammalian circulatory system
• Structures of arteries, arterioles, veins, venules and capillaries
• The structure and functions of blood
• The formation and functions of tissue fluid
• The structure and function of the heart
Key Objectives
• Study – The Mammalian Circulatory System
• Activity – Student book questions
TRANSPORT SYSTEM IN
ANIMALS
Transport system in animals
A transport system is needed to
distribute oxygen quickly to all the
body cells, and to remove their waste
products. Mammals have greater
requirements for oxygen than most
other animals because they use
respiration to generate heat and
energy inside their bodies.
THE MAMMALIAN
CIRCULATORY SYSTEM
Circulatory System
The main transport system of
mammals is the blood system or
circulatory system. It is made up of a
pump – the heart – and a system of
interconnecting tubes – the blood
vessels.
The blood always remains within these
vessels, and so the system is known as
a closed blood system.
The blood travels twice through the
heart on one complete ‘circuit’. This is
called a double circulation.
Circulatory System
Blood is pumped out of the left
ventricle into the aorta, and travels
from there to all parts of the body
except the lungs. It returns to the right
side of the heart in the vena cava. This
is called the systemic circulation.
The blood is then pumped out of the
right ventricle into the pulmonary
arteries, which carry it to the lungs.
The final part of the journey is along
the pulmonary veins, which return it to
the left side of the heart. This is called
the pulmonary circulation.
BLOOD VESSELS
Blood Vessels
There are three main types of vessel
making up the circulatory system.
Vessels carrying blood away from the
heart are known as arteries, while
those carrying blood towards the heart
are veins.
Small arteries are called arterioles, and
small veins are venules. Linking
arterioles and venules, taking blood
close to almost every cell in the body,
are tiny vessels called capillaries.
ARTERIES AND
ARTERIOLES
Arteries
The function of arteries is to transport blood,
swiftly and at high pressure, to the tissues.
Artery walls are very strong and elastic. Blood
leaving the heart is at a very high pressure.
The thickness and composition of the artery
wall enables it to withstand this pressure.
Both arteries and veins have walls made up of
three layers:
• an inner layer, which is made up of a layer of
endothelium (lining tissue) consisting of a
layer of flat cells (squamous epithelium) fitting
together like jigsaw pieces, plus a layer of
elastic fibers; the endothelium is very smooth,
minimizing friction with the moving blood
• a middle layer containing smooth muscle,
collagen and elastic fibers
• an outer layer containing elastic fibers and
collagen fibers.
Arteries
Elastic arteries, like the aorta, have a
lot of elastic tissue in their middle
layer. This elasticity allows them to
stretch when blood is pumped from
the heart under high pressure,
reducing the risk of rupture.
As blood enters, the artery walls
stretch to accommodate the pressure.
When the pressure drops, the walls
recoil, helping to push the blood
forward and maintain flow.
Arteries
As arteries branch into smaller vessels,
the amount of elastic tissue decreases
and the amount of smooth muscle
increases, transforming them into
muscular arteries.
These arteries transport blood from
elastic arteries and bring it closer to its
final destination. The smooth muscle
in their walls can contract slowly and
steadily, regulating the diameter of the
artery and controlling the blood flow.
Arteries
Muscular arteries further divide into
arterioles, which also contain smooth
muscle. The narrowness of arterioles
helps regulate blood flow to specific
tissues by adjusting their diameter.
The walls of arterioles have a nerve
supply. Nerve impulses from the brain
can cause their smooth muscle to
contract, narrowing the arteriole. This
is called vasoconstriction. When the
muscle relaxes, the diameter of the
arteriole widens. This is called
vasodilation.
CAPILLARIES
Capillaries
The arterioles continue to branch,
eventually forming the tiniest of all blood
vessels, capillaries. The function of
capillaries is to take blood as close as
possible to all cells, allowing rapid
transfer of substances between cells and
blood.
The small size of capillaries is of great
importance in allowing them to bring
blood as close as possible to each group
of cells in the body. A human capillary is
approximately 7 μm in diameter, about
the same size as a red blood cell. The
walls of capillaries are extremely thin
because they are made up of a single
layer of endothelial cells.
VEINS AND VENULES
Veins and Venules
As blood exits capillaries, it gradually
moves into larger vessels called venules,
which then combine to form veins. Vein
function is to return blood to the heart.
By the time blood enters veins, its
pressure is very low. Due to this low
pressure, veins don't require thick walls.
They have three layers like arteries, but
the middle layer is thinner and contains
fewer elastic and muscle fibers.
Whenever muscles contract, they
squeeze inwards on the veins to increase
the pressure. To keep the blood flowing
in the right direction, veins contain half-
moon valves, or semilunar valves, formed
from their endothelium. These valves
allow blood to move towards the heart,
but not away from it.
ACTIVITY
