TOPIC 1. CLASSIFICATION KINGDOM FUNGI
Veins have a larger lumen and less
muscular walls compared to arteries. This is because the blood in the veins
flows at low pressure.
Vein have valves at regular intervals.
The valves prevent the back flow of blood.
The muscles next to the veins
squeeze the veins and help to force blood to flow towards the heart. The
contraction of the ribs during breathing also helps to squeeze some veins and keep
blood flowing.
Capillaries
Capillaries are the smallest blood
vessels. They are narrow and have walls that are one cell thick
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
Red blood cells are formed in the
bone marrow. Their lifespan is about 120 days. The liver and the spleen destroy
old red blood cells and release haemoglobin for the formation of new
cells.
Haemoglobin is the red pigment in
erythrocytes. It has a high affinity for oxygen.
The function of red blood cells is
to transport oxygen and carbon dioxide. The adaptation red blood cells that
make them suited forthis function are the presence of haemoglobin, their large
numbers, biconcave shape and the lack of nucleus which increases the total
surface area of gaseous exchange.
Transport of
oxygen
In the lungs (where there is a high
concentration of oxygen), haemoglobin combines with oxygen to form
oxyhaemoglobin. This is an unstable compound which releases oxygen when it
reaches tissues that have a low concentration of oxygen. The formation of
oxyhaemoglobin and release oxygen and haemoglobin can be shown using the
following equation.
Haemoglobin + oxygen =
Oxyhaemoglobin
Oxygen diffuses out of the red blood
cells, through the capillary walls to the tissues.
Transport of carbon dioxide
In the red blood cells, carbon
dioxide combines with haemoglobin to form carbominohaemoglobin. This compound
is transported to the lungs where carbon dioxide is released and expelled from
body.
White blood cells
Another name
for the white blood cells is
leucocytes. These cells have irregular shapes; milliliter of blood has
approximately 5000 to 10 white blood cells.
