THE CIRCULATORY AND SYSTEM
THE CIRCULATORY SYSTEM
All organisms move substances internally from one place to another. The
circulatory system is responsible for the movement of substances (O2, CO2,
nutrients, & wastes) through the body. The human circulatory system has three
components: the heart, the blood vessels, and the blood.
I. THE HEART
The function of the heart is to keep the blood moving constantly in the body.
The heart is a large organ made of cardiac muscle cells that have a high
number of mitochondria. Humans (plus other mammals and birds) have a 4chambered heart. The heart is divided into a right and left side with two
chambers making up each side. The two sides are divided by a septum.
A. Structure
1. Membranes – There are 3 layers of tissue that surround the heart.
a. Pericardium – Outermost layer; membrane composed of
connective tissue that surrounds and protects the heart.
b. Myocardium – Middle layer; muscle tissue that forms the four
chambers of the heart.
c. Endocardium – Innermost membrane that lines the chambers,
covers the valves, and continues into the vessels. Composed of
smooth, epithelial tissue to prevent blood clotting.
2. Chambers
a. Atria (atrium =singular) Upper chambers of the heart that receive
blood. Walls are thinner and less elastic than ventricles.
b. Ventricles – Lower chambers of the heart that pump blood to the
lungs and body. Have thick, elastic walls, especially the left
ventricle because it pumps blood out to the body.
3. Valves – Flaps of epithelial tissue reinforced with connective tissue
that keep blood flowing in one direction and increase the pumping
efficiency of the heart.
B. Types of Circulation
1. Systemic – The flow of blood from the left side of the heart to the body
and back to the heart. The main artery involved is the aorta; main veins
involved are the superior and inferior vena cava.
2. Pulmonary – The flow of blood from the right side of the heart to the
lungs and back to the heart. Pulmonary arteries carry O2 poor blood
from the heart to the lungs; pulmonary veins carry O2 rich blood from
the lungs to the heart.
3. Coronary – since the heart is a major organ, it needs its own supply of O2
rich blood. This is referred to as coronary circulation. The coronary
arteries are the first two branches of the aorta. These arteries branch
into capillaries that circulate oxygenated blood to the myocardium for the
heart to use, then the coronary veins return the deoxygenated blood to the
right atrium.
C. Pathway of Blood Flow Through the Heart.
Superior/Inferior Vena Cava  Right Atrium Right Ventricle 
Pulmonary Artery  Lungs  Pulmonary Veins 
Left Atrium  Left Ventricle  Ascending/Descending Aorta
THE HEART
II. THE BLOOD VESSELS
Blood vessels are composed of connective tissue for strength, smooth muscle
tissue to facilitate blood flow, and they are lined with epithelial tissue for
smoothness.
Types – There are three types of blood vessels:
A. Arteries – Carry blood Away from the heart. Arteries have thick, elastic
walls to withstand the pressure of spurts of blood from contraction of the
heart. Arteries branch into arterioles and then to capillaries. Arteries
carry blood that is high in oxygen and low in carbon dioxide, except for
the pulmonary artery.
B. Veins – Carry blood to the heart. Veins have thinner, less elastic walls.
They contain valves to keep blood flowing in one direction.
Veins branch into venules which lead to the other side of the capillary
bed. Veins carry blood low in oxygen and high in carbon dioxide
except for the pulmonary vein.
C. Capillaries – Network in which O2 , glucose, & other nutrients are
exchanged with CO2 and other wastes. Capillaries are only one-cell
thick, so red blood cells must flow through one-by-one. This allows time
for diffusion to occur.
CAPILLARY NETWORK
III. THE BLOOD
Blood is classified as connective tissue with a matrix of plasma. Plasma is
the liquid that makes up approximately 55% of blood. Plasma is 90% water,
the other 10% is made up of proteins, ions, vitamin K, lipids, hormones, etc.
A. Erythrocytes (RBC’s)
Function is to deliver O2 to all body cells. Most numerous cells in blood,
they are doughnut-shaped cells that lose their nuclei as they mature in the
red marrow. This adaptation allows them to squeeze through tiny
capillaries and provides more area for oxygen transport. Red blood cells
contain hemoglobin, an iron- containing protein that binds oxygen.
Erythrocytes circulate every 120 days & are destroyed in the spleen.
B. Leukocytes (WBC’s)
Larger and fewer in number than RBC’s. Their primary function is to fight
infection by engulfing pathogens or marking them for destruction. Some
WBC’s are able to leave the blood vessels.
C. Thrombocytes (platelets)
Cell fragments that break off from large cells produced in the red marrow.
Like RBC’s, they lack a nucleus, but platelets are smaller than red blood
cells. Platelets begin the clotting process by sticking together to form a plug
when they come in contact with a rough surface. Thrombocytes live
about 10 days.