Transport in humans

Blood vessels : arteries : thick muscular wall : to change the size of the lumen to
regulate blood ow / elastic tissues: allow arteries to distend and recoil to maintain
a continuous ow of blood / thick walls : to withstand high blood pressure from
contraction of cardiac muscles

Veins : valves : prevent back ow of blood to maintain uniform ow / large lumen :

Capillaries: one cell thick endothelium : shortens distance for material exchange

Highly branched capillary network : provide large surface area for rapid material
exchange

Large cross sectional area : reduce speed of blood ow to allow su cient time for
material exchange

Heart

The heart is protected by the pericardial membrane which secretes pericardial uid
that acts as lubricant . The left ventricle is more muscular than the right ventricle as
the left ventricle needs to pump blood for a longer distance to all parts of the body .
The bicuspid valve prevents back ow of blood from the left ventricle to the left
atrium when left ventricle contracts . Tricuspid valve prevent back ow of blood
from right ventricle to right atrium when right ventricle contracts.

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 The blood pressure is high in arteries
due to the contraction of cardiac
muscles . The rate of blood ow is
due to the large pumping force of
cardiac muscles . As the artery
branches o to arterioles , the blood
velocity drops as the total cross
sectional area is larger

The blood pressure in capillaries is lower due to the small lumen of capillaries that
provides large resistance to blood ow . The velocity of blood ow is also the lowest
as the cross sectional area of capillaries is the highest

The blood pressure in veins is nearly zero as the blood has overcome huge
resistance in travelling all over the body . The velocity of blood ow is higher than
that of the capillaries as the contraction of skeletal muscles adjacent to veins
provide pumping force to drive blood ow and the cross sectional area of veins is
much smaller than that of capillaries.

Material exchange : the wall of capillaries is di erentially permeable so nutrients and

Formation of tissue uid : the pressure of blood at the arteriolar end of capillaries is
higher than the hydrostatic pressure of uid surrounding cells so components of
blood eg water, minerals ,hormones ,white blood cells (except for plasma proteins
because they are too large to pass through the capillary walls )are pushed out of the
capillaries to form tissue uid .

As components of plasma like water are pushed out while plasma proteins remain ,
the water potential of blood in capillaries is lower than the tissue uid so water
enters the capillary by osmosis at the venule of the capillary.

Due to hydrostatic pressure of the tissue uid , excess tissue uid is drain into the
lymph vessels ( blunt ended)

The ow of lymph in lymph vessels is maintained by contraction of adjacent

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