Download File

The Human Circulatory System
Overview:
The circulatory system interacts with almost every other body system, therefore it is important in maintaining
homeostasis. It transports gases, nutrients, wastes, electrolytes, and hormones throughout the body. The
circulatory system is highly organized and consists of blood, blood vessels, and the heart.
Blood:

The blood is a type of connective tissue that is composed of red blood cells, white blood cells,
platelets, and plasma. These components all mix together to form a mixture.

Plasma makes up about 55% of blood and is composed of about 90% water and 10% dissolved proteins,
lipids, salts, and glucose. Plasma is the medium used for transporting substances such as hormones,
nutrients, and waste products. Some proteins in plasma help with clotting and some help with the
immune system.
All blood cells originate in the bone marrow. Different types of blood cells are made based on what the body
needs.
Red Blood Cells:





Most of the cells in the body are red blood cells or erythrocytes. Did you know that in a single drop of
blood there are millions of red blood cells?
Red blood cells get their color from hemoglobin which is composed of an iron molecule that attracts
oxygen .
The function of red blood cells is to carry oxygen to the cells and carbon dioxide away from cells.
(Oxygen and carbon dioxide move into and out of cells through diffusion ).
These cells are unique because as they mature they lose their nucleus and other organelles. This gives
them the depression in the center. Because they lose these structures they are unable to reproduce.
Eyrthrocytes have proteins on their surface that help them navigate through blood vessels. Red blood
cells have an average life span of about 120 days after which they are broken down in the liver and
spleen.
White Blood Cells:






White blood cells are responsible for defending our body against infection and can help in removing
dead cells. White blood cells are larger than red blood cells.
Unlike red blood cells, white blood cells do not contain hemoglobin and therefore are not red, but
colorless.
There are five different types of white blood cells, also called leukocytes, each with their own function
in defending the body. Some cells surround and engulf foreign objects such as bacteria, while others
known as lymphocytes release antibodies.
Antibodies are proteins that destroy bacteria and other pathogens.
White blood cells do contain a nucleus but it has an irregular shape. Their shape is what allows them
to move their cytoplasm to engulf pathogens.
There is a fewer number of white blood cells compared to red blood cells. For instance, there are only
3,700 to 10, 500 white blood cells in a drop of blood compared to the 4 million red blood cells in a drop
of blood. White blood cells have a short life span of about a day or two.
Platelets or thrombocytes are cell fragments of a larger cell that stay in the bone marrow after the larger cell
differentiates. They are responsible for clotting blood and repairing injuries.
Blood Types:





There are four blood types: type A, type B, type AB, and type O.
Blood type is determined by the genes you inherit from your parents.
Each blood type is named for the antigen or protein present on the outside of each red blood cell.
Type A blood has antigen A, type B blood has antigen B, type AB blood has both antigen A and
antigen B, type O blood has neither antigen.
Antigens can cause an immune response if an individual is exposed to a different blood type than their
own. The body responds with antibodies. For instance, type A blood produces antibodies against type
B blood.
The Rh factor is another blood protein that can affect transfusions. Individuals who inherit the Rh factor are
Rh positive. Individuals who do not inherit the Rh factor are Rh negative. Anyone can receive Rh negative
however, and Rh negative person will produce antibodies against Rh positive blood.


Type O-negative blood is considered the universal donor because it does not have any antigens on the
surface for antibodies to react to. Therefore it is compatiable with all blood types.
Type AB-positive is considered the universal recipient because it does not have any antibodies in the
plasma against the other blood types. It can receive all blood types.
Transport of Blood:
Blood vessels are necessary for blood to flow because blood is fluid. There are three major types of blood
vessels including arteries, veins, and capillaries.

Arteries carry blood away from the heart. They are thick-walled and very muscular. There special
structure allows them to withstand the pressure produced by the heart when it contracts and pushes
blood into the vessels.

Think of arteries like major highways that carry blood throughout the circulatory system. Like
highways, the connector road gets smaller and smaller until eventually it turns into a one lane road.



In the body, these one lane roads are called capillaries.
Capillaries are the smallest type of blood vessel.
Red blood cells must line up in a single file in order to pass through them.Capillaries are very thinwalled and are just one cell layer thick.
This allows for oxygen and nutrients to pass out of the blood and into the tissues.
 As oxygen and nutrients enter the cells from the capillaries, carbon dioxide and waste
diffuse out of the cells into the capillaries, and the blood continues its way back to the
heart. Capillaries are very extensive and move a lot of blood into and out of every organ
in each organ system.




Veins are blood vessels that carry blood to the heart. These are large vessels with little to no muscle.
They are not as thick as arteries because blood pressure is not as high inside their walls. Veins in many
cases must carry blood against gravity.
Veins in the legs contain valves on the inner walls that prevents blood from flowing backwards.
Skeletal muscles also help move blood back to the heart.
The Heart


The heart is the star of the show, and is only about the size of your fist. It pumps blood throughout your
body.
The heart is composed of cardiac tissue, which have a lot of mitochondria that constantly produce
energy so the heart can contract. The average heart beats 72 times per minute!

The heart is a hollow organ with four chambers.
o
The top two chambers are called atria and the bottom two chambers are called ventricles.
When the heart beats the atria contract first, squeezing blood down to the ventricles, then the
ventricles contract and pump the blood out of the heart to the body.
There are a set of valves between the atria and ventricles that keep blood from flowing backwards. The heart is
divided into two halves separated by the septum, this keeps oxygenated blood separated from deoxygenated
blood.
The right side of the heart pumps blood form the heart to the lungs during pulmonary circulation. In this
pathway, blood leaves the heart and flows to the lungs where carbon dioxide is released from the blood and
oxygen is absorbed. The oxygenated blood flows from the lungs back to the heart and enters the left side.
The left side pumps oxygenated blood from the heart to the rest of the body which is called systematic
circulation. The blood delivers oxygen to the body tissues then comes back to the heart deoxygenated and
carry wastes such as carbon dioxide. It enters the right side and the pathways repeat.
Let’s look at more detail of how a single drop of blood travels through the heart.
When blood enters the heart from the body it travels through a large vein called the venae cavae. The venae
cavae empties blood into the right atrium. The right atrium sends blood down to the right ventricle. The right
ventricle pumps blood out to the pulmonary artery which carries blood to the lungs. The blood exchanges
carbon dioxide for oxygen and returns to heart through the pulmonary vein which empties into the left atrium.
The left atrium sends blood down the left ventricle which pumps blood into the aorta. The aorta carries blood
to the rest of the body.
Blood flows only one-way in the heart. Valves located between the atria and ventricles close when the
ventricles contract and there are valves at the exit points that prevent blood from leaking back into the ventricles
when they relax.
Blood Pressure:
Blood pressure is the force which blood pushes against the wall of an artery. When the heart contracts
pressure is greatest, this is the systolic pressure. When the heart relaxes the pressure is reduced, this is the
diastolic pressure. When measuring blood pressure, the systolic pressure is the number on top and the diastolic
pressure is the number on bottom. A healthy blood pressure is around 120/75 mm Hg.
Factors affecting blood pressure include, exercise, stress, strong emotion, excess weight, drugs, alcohol, and
diet.