TOPIC 1.CLASSIFICATION KINGDOM FUNGI
Capillaries are in direct contact
with the tissues of the body. They form a network for the efficient diffusion
of substances. Their thin walls maximize the rate of diffusion.
The thin walls of the capillaries
enable oxygen and nutrients to diffuse from the blood to the cells, carbon
dioxide and other waste products to diffuse from the cells into the blood and
white blood cells to reach sites of infection.
Capillaries join to form venules
(small veins) which join to form veins.
Differences between arteries, veins
and capillaries
Table below gives a summary of the
structural and functional differences between arteries, veins and capillaries.
Differences between arteries, veins
and capillaries
|
Arteries
|
vein
|
Capillaries
|
|
Have narrow smooth
lumens
|
Have wide irregular lumens
|
Have narrow smooth lumens
|
|
Have thick muscular walls
|
Have thin, less muscular walls
|
Have one cell ' thick walls
|
|
Lack valves except where they
are connected to the heart
|
Have valves at regular intervals
|
Lack valves
|
|
Transport blood at high pressure
|
Transport blood at low pressure
|
Transport blood at low pressure
|
|
Transport blood away from the
heart
|
Transport blood towards the heart
|
Transport blood within the tissues
|
|
Transport oxygenated blood, except
the pulmonary artery
|
Transport deoxygenated blood,
except the pulmonary vein
|
Transport either oxygenated or
deoxygenated blood
|
|
Contract and relax to create a
pulse
|
Blood flows smoothly
|
Blood flows smoothly
|
Blood
Blood is a fluid tissue. It consists
of cells (red blood cells and white blood cells) and platelets (fragments of
cells) suspended in a fluid called plasma. An adult human has 4 to 6 liters of
blood. The pH of blood is 7.4.
Plasma
Plasma is a pale-yellow fluid.
Approximately 55% of the blood is plasma. Plasma is mostly made up of water but
it also has dissolved substances such as food nutrients, metabolic wastes,
oxygen, proteins and mineral ions. These solutes make up 8% of the plasma while
water makes up 92%.
The major functions of plasma are
the transportation of:
- nutrients from the digestive system to the whole body
- red blood cells containing oxygen to the tissues
- wastes such as carbon dioxide and urea to the excretory organs
- white blood cells and antibodies to sites of infection
- hormones to the target organs
- mineral ions such as sodium, potassium and chlorides
- Platelets to sites of bleeding.
Plasma is also important for
distributing heat to all parts of the body, regulating the pH of body fluids
and it is where the exchange of nutrients and waste products takes place in the
body.
Red blood cells
Another name for the red blood cells
is erythrocytes. They are red, round biconcave cells with no nucleus.
One milliliter of blood has approximately 5 to 6 million red blood cells
