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The Circulatory System


Your circulatory system is the body system that 

circulates, or moves around, oxygen and nutrients to every

cell in your body and removes carbon dioxide and waste products from the cells. It is also called the cardiovascular system: cardio (meaning heart) and vascular (meaning vessels).

   The cardiovascular system is made up of the heart, blood vessels and blood. Blood vessels include arteries, capillaries, and veins. The system is a closed system because blood stays inside the vessels and heart as it travels through the body.

Read below to find out about this amazing body system!


Learning Objectives
By the end of this lesson, students should be able to:

  • Compare the structure and function of three types of blood vessels.

  • Describe the structure of the heart.

  • Explain how blood pressure is measured and what the measurements reflect.

  • Explain how pulse is measured and what the measurement reflects.

  • Describe the characteristics and functions of three types of blood cells.



artery—a blood vessel that carries blood traveling away from the heart.


atria—two upper chambers of the human heart.


blood pressure—the pressure exerted on arteries by the force of the heartbeat.


capillary—microscopic blood vessels that connect arteries and veins; nutrients, oxygen and wastes are exchanged through their one-cell thick walls.


diastolic—the blood pressure reading that reflects the phase of heartbeat when the heart muscle relaxes; the lower number of a blood pressure measurement.

heart—the muscular organ that pumps blood through the body.


hemoglobin—the chemical on red blood cells that carries oxygen to tissues and gives blood its red color.

hypertension—high blood pressure.


pericardium—the protective sac surrounding the heart.


plasma—the liquid part of the blood.

platelet—a cell fragment that helps clot blood.

pulse—a measurement of the number of times the heart beats in one minute.

red blood cell—the disc-shaped cell in the blood that carries oxygen.


septum—a thick wall that separates the right and left sides of the heart.


sphygmomanometer—an instrument used to measure blood pressure.


systolic—the measurement that reflects of the phase of the heartbeat when the heart muscle contracts to pump blood to the body; the higher number of a blood pressure measurement.


white blood cell—a cell that is produced in the bone marrow that fights infection.


vein— a blood vessel that carries blood traveling to the heart.


Your Heart

   Your heart is an amazing organ!

About the size of a human fist, it pumps

blood throughout the body. It is behind

the sternum (breastbone) and between

the lungs. It is divided into four cavities,

called chambers: 2 upper chambers

called the right and left atria and 2

lower chambers called the right and

left ventricles.

   The heart is surrounded by the pericardium, a protective sac.

   Atria are receiving chambers and have thin walls; ventricles have more muscular walls and are pumping chambers.

   A thick wall, the septum, separates the right and left sides of the heart. The right atrium receives blood from the body tissues. This blood is low in oxygen and high in carbon dioxide because body cells have used the oxygen to perform normal body functions and have released carbon dioxide as a waste product.

   The blood flows from the atrium into the right ventricle which then pumps it out to the lungs to pick up more oxygen and to release carbon dioxide which you breathe out.


Fast Fact: The thick wall (septum) that separates the right and left sides of the heart prevents the oxygen-rich blood from mixing with the oxygen-poor blood. This is an advantage because the oxygen-rich blood is not “watered down” with oxygen-poor blood and our bodies can work more efficiently.


   The left atrium of the heart receives blood that has traveled to the lungs and is now rich in oxygen. The left ventricle pumps the blood back out to the body. And the circulation continues.


Blood Vessels

The three kinds of blood vessels are arteries, capillaries and veins.

1) Arteries are thick-walled, elastic blood vessels that

give the blood an extra push to help blood travel

further along to body tissues. Arteries carry blood

away from the heart and lungs and therefore, carry

oxygen-rich blood.


                                                2) Capillaries are microscopic, one-cell thick                                                              blood vessels that connect arteries and veins.                                                          These vessels are the sites of transfer of                                                                      nutrients and oxygen into the body cells and                                                              waste materials out of the cell.



3) Veins are thin-walled vessels and carry blood back

to the heart from body tissues. Blood in veins heading

toward the heart have delivered O2 and picked up

CO2 from cells. Veins carry carbon dioxide-rich blood.

   Veins have valves that prevent the blood from

flowing back in the wrong direction.



Arteries carry blood AWAY from the heart. Any time blood is traveling away from the heart, it is in an artery.

Capillaries CONNECT the arteries and veins in the cells. They are sites where transfer of nutrients and wastes occur in the cells. 

Veins carry blood TO the heart. Any time the blood is on its way back to the heart, it is in a vein.


Blood Pressure and Pulse

  You have probably heard warnings about high blood pressure (hypertension). Maybe someone in your family has high blood pressure. But, how does blood have a pressure and how do they measure it?



   Blood pressure is produced by the force

of the heart pumping blood away from

the heart and through arterial walls.

When we measure blood pressure we

are measuring the pressure of the blood

against the inside of the artery. When the

heart relaxes within the beat, the     Image: Measuring blood pressure at the brachial artery.

pressure against the artery walls decreases, but it is still present.

   Blood pressure is measured by a sphygmomanometer.  This instrument

measures blood pressure by pumping air into a cuff positioned around an artery. The cuff presses against the artery and measures the pressure of the blood as it is pumped by the heart and arteries. A stethoscope is used to listen for sounds made during the heartbeat.  


   Blood pressures have two measurements. One is the systolic pressure—the pressure in the arteries when the ventricles pump blood. The other is diastolic pressure—the pressure of the blood in the arteries when the ventricles are relaxed in the beat.

Normal adult blood pressure is 120/80.

   High blood pressure, or hypertension, can damage heart muscle over time. Heart disease results and the risk of heart attack or stroke increases.


Systolic    =   when ventricles contract and pump blood   =    120
Diastolic   =   when ventricles relax in the beat                  =     80                                    

  Blood pressure is usually measured at the brachial

artery in the arm or the radial artery in the wrist.



  Arteries help push the blood throughout the

body. Arteries pump blood and the pulse that

results can be felt in arteries near the surface of

the skin. Pulse rates are measured in beats per

minute. Normal adult pulse rates are about 60-80

beats per minute

                               Image: Measuring heartbeats per minute (pulse) at the radial artery in the wrist.

  Fast Fact: Humans have a closed circulatory system which means that blood is always inside a vessel (and the heart).  What does it mean to have an open circulatory system?

   In an open circulatory system, blood is not always in a vessel. Organs receive a blood supply when blood is pumped out into the body cavity and sloshes around the organs. Blood then collects in a tube to re-circulate through the body. Insects, spiders, and most mollusks like snails and clams have open circulatory systems.

Can you think of an advantage of having a closed circulatory system? 


The Blood    

   Blood is made up of cells and plasma (the                                                 liquid part of blood). The major types of                                                       cells are red blood cells, white blood cells,                                                 and platelets. Each cell has a different function in

your body.

 Image: Three types of blood cells. Right: a red blood cell. Middle: a platelet. Left: a white blood cell.


   Red blood cells (rbc) are the most numerous in the blood. Just one milliliter (ml) of blood normally contains about 5 million rbc’s. A molecule on the red blood cells called hemoglobin combines with oxygen in the lungs. In this way, oxygen is taken around the body and released into cells. Red blood cells are produced in bone marrow and last about 20-120 days.

   White blood cells (wbc) are cells that fight infections. One milliliter (ml) of blood usually contains about 5,000 to 10,000 white blood cells. There are different types of white blood cells. They all fight infection but in different ways.

   Platelets and proteins in the plasma work together to clot blood when an injury has occurred. Together they make a sticky mesh over the wound and the clot forms.

   Plasma is the liquid part of the blood. It is about 55% of the volume of blood in your body. It is a light, sticky, straw-colored liquid that is mostly water. Salts, sugars, fats, proteins, nutrients, hormones, and waste products are carried in the plasma.


Check for Understanding:

1. What are the major differences among the three types of blood vessels?


2. If someone had a high white blood cell count, what do you think could be wrong with them?

3. Describe what is happening in the diastolic and systolic phases of a heartbeat.


4. Compare and contrast the structure and function and normal numbers of the three major types of blood cells. 

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diagram of heat structure showing chambers and vessels
diagram showing cross section of an artery with blood inside
diagram of capillaries surrounding tissue
diagram of structure of veins showing valves
blood pressure cuff and stethoscope on an arm with person taking blood pressure
person with fingers on pulse at the radial artery
miscroscopic image showing a red blood cell, a platelet, and a white blood cell
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