As main agents important in the circulatory system are:

As the surface area per volume ratio of an
organism increases, so would it’s need for oxygen and other components, and it
would be harder to effectively distribute these components. Which is why multi-cellular
organisms have to adapt and develop a specialized exchange surface, one of
these being a good blood supply which is part of the circulatory system I will
be talking about below.

The circulatory system is a closed system
which passes through the heart twice during a complete circuit. It has 2
circuits which are: Pulmonary (pumps blood to lungs for oxygenation) and
Systemic (pumps oxygenated blood to head and body.  Three main agents important in the
circulatory system are: the arteries( they have thick, muscular and elastic
walls to whist and the pressure and to exert force which is what gives you your
pulse, and they serve to take blood away from the heart at high pressure); the
capillaries( which allow exchange of materials between the tissues and the
blood with they permeable walls) and veins (they have a large diameter to offer
least flow resistance and valves to prevent back flow, they  returns low pressure oxygenated blood to
heart). The main component of the circulatory system is “blood”, it
is a connective tissue which plays three main important functions which are:
Transportation, regulation and protection.

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Transportation: The human body
requires: oxygen and carbon dioxide to be exchanged for respiration, nutrients
for energy, waste to be removed from the body so that we do not become
intoxicated, and an even transport of hormones.

For the transport of gases, we
have red blood cells which are specialized cells (formed from stem cells in the
bone marrow) that circulate around the body delivering oxygen to other cells
while also removing carbon dioxide from the latter. It does not have a nucleus,
mitochondria or ribosomes which means there is a larger surface area for the
rapid diffusion of oxygen and carbo dioxide, it also has hemoglobin which is a
molecule made of iron that gives red blood cells its red color.    Waste and nutrients are also
effectively transported by the blood. As it flows through the capillaries and
into the lungs, the hemoglobin contained in the red blood cell releases carbon
dioxide which is in turn removed from the lungs when we breathe out. Waste is
also removed from our body tissues and organs and removed from the blood in a
filter like system as it goes through the kidneys. The waste material will then
exit the body in the form of urine. Blood also flows through the intestine’s
capillaries and picks up nutrients (glucose, vitamins and minerals) which are
then delivered to the rest of the body and cells that need it by blood flow.Hormones are chemical
messengers produced by different glands which affect distant organs, such an
example is the hormone insulin which is produced in the pancreas and affects
the sugar levels in the blood. Our blood also transports hormones made by
endocrine systems to organs and tissues. The circulatory system also delivers
hormones such as peptides hormones which are secreted by the heart atrial cells
to help regulate blood volumes and pressures. Estrogen is also transported here
to promote vascular health in women and men.



Heat is absorbed and exchanged throughout
the body by the blood which moves farther and closer from the skin in every
areas of the body. This heat is produced by the tissues when they break down
nutrients for energy, releasing waste and repairing themselves. As the body
temperature rises, the blood vessels closer to the skin will increase in size,
so that heat can be loss through the skin. If body temperature falls, the blood
vessels closer to the skin will become smaller so that they can retain heat. It
also distributes heat generated by muscular contractions. Th circulatory system
regulates blood volume and pressure by regulating fluid loss in the form of
urine, and transports gonadotropic hormones which regulates reproductive



The circulatory system is very important
when it comes to fighting diseases, it makes up most of the immune system and
contains cells known as ” white blood cells”, which patrol the body
in search of foreign bodies. As soon as a foreign body is detected, chemical
signals are sent through the bloodstream, this then alerts the white blood
cells of the situation which then are transported to the site of infection
where they will engulf (phagocytizing) or release chemicals at the bacteria or
body responsible for the infection. Also, during tissue damage or injury,
messenger chemicals are released which starts the activation of platelets or
attract platelets to the site. The platelets in the blood will then stick
together to form a plug which will then cover the wound and coagulate to stop
further blood loss and bacteria to enter the bloodstream through the wound.

The circulatory system transports and makes
tissue fluid not been able to be reabsorbed by the vascular capillaries back to
the lymphatic system for processing. Tissue fluid is formed due to the pressure from the pumping of the heart.
It is composed of water molecules, fatty molecules and carbon dioxide.
They are
found in capillaries; specially adapted one-cell thick tubes which occur after
the arteries.
The pressure
causes, small molecules to be pushed out of the blood plasma through the
capillaries and to form interstitial fluid (tissue fluid). Plasma proteins from
the blood remain in the blood plasma since these are too large to pass through
capillaries. Water is reabsorbed into the blood plasma by osmosis to maintain
blood pressure.

Diffusion is the spreading of the particles
of a gas, or of any substances in a solution, resulting in a net movement from
a region where they are of a higher concentration to a region with a lower
concentration. The greater the difference in concentration, the faster the rate
of diffusion. As organisms get bigger, the surface per volume ratio changes, it
gets smaller and as a result diffusion is no longer enough. There is not a big
enough surface area for diffusion to take place. This is particular concerning
in humans as they are active, this means food and oxygen can’t reach the cells
fast enough to supply cellular respiration. The waste products can’t be removed
fast enough to prevent them damaging the cells. A transport system is needed!