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The circulatory system of the horse is comprised of the heart, blood vessels, blood,
lymph glands, lymph vessels, and lymph. Its main purpose is to transport nutrients
and oxygen and to collect waste formed in the tissues throughout the body.
The Heart
The heart has four chambers, divided into left and right and upper and lower.
The upper chambers, known as the atrium, are smaller and receive the blood flowing
into the heart. The lower chambers, or ventricles, pump the blood out of the heart
and though the body. The atria and the ventricles are separated by the
atrioventricular valves (A-V valves). The left valve has two flaps and is called the
bicuspid valve. The tricuspid valve is on the right and has three valves.
These flaps fall open when the ventricles relax, allowing blood to enter the lower
chambers when the atria are full. As the ventricles contract, the valves are closed,
pushing the blood out through the arteries. The arteries are separated from the
ventricles by the semilunar valves, so named for their half-moon shape. The valve on
the left side is known as the aortic semilunar valve because it separates the left
ventricle and the aorta. The pulmonary semilunar valve on the right side separates
the right ventricle from the pulmonary artery. These valves are opened and closed by
the pressure of the blood. TOP
Systemic and
Pulmonary Circulation
| A. Pulmonary Artery |
K. Caudal Vena Cava |
 |
| B. Lungs |
L. Hepatic Artery |
| C. Pulmonary Veins |
M. Hepatic Vein |
| D. Right Atrium |
N. Stomach, Spleen,
Intestine |
| E. Left Atrium |
O. Portal Vein |
| F. Right Ventricle |
P. Liver |
| G. Left Ventricle |
Q. Renal Artery |
| H. Aorta |
R. Renal Vein |
| I. Head and Forelegs |
S. Kidney |
| J. Cranial Vena Cava |
T. Abdomen and Hindlegs |
TOP
The Blood Vessels
Blood vessels carry blood to and from the tissues of the body, creating a
circular path. The dorsal aorta is a large artery, which transports blood away from
the left ventricle. It branches out into smaller and smaller arteries. These
arteries and arterioles then carry the blood to the capillaries. The blood continues
through the capillaries to the veins and veinules. The veins merge to form two large
veins, the cranial and caudal vena cava. The blood enters the heart from the vena
cava at the right atrium and falls to the ventricles. From there it is pushed out
the pulmonary artery, which branches out to the capillaries in the lungs where oxygen and
carbon dioxide are exchanged. From these capillaries the blood goes to the veins as
they get larger and larger, before finally forming the two pulmonary veins. The
blood then empties into the left atrium and down to the ventricle, completing the path.
Arteries are made from thick walls of elastic connective tissue mixed with some muscle
fibers. The muscle fibers allow the arteries to help control blood pressure.
The contracting and relaxing action of the arteries can be felt as the pulse, which in a
normal adult horse is between 32 and 44 beats per minute.
Capillaries are microscopic vessels with very thin walls. These walls are
semipermeable (only allow certain materials to pass through). Capillaries form
networks of tubes between arteries and veins. They allow the exchange of nutrients
and wastes between blood vessels and the tissues of the body.
Veins drain the blood from the capillary networks back to the heart. They have
thinner walls and are larger than arteries. The veins have one-way valves in them,
that only allow blood to flow towards the heart. TOP
Blood
The blood is comprised of cellular components and the liquid the components float
in. The liquid is called plasma and is about 90% water. Blood has many vital
functions that it carries out as it flows through the body. It carries nutrients
from the digestive tract, as well as oxygen from the lungs, to all the body tissues.
It also takes carbon dioxide from the tissues back to the lungs. The waste
products from cells are transported to the kidneys. Blood also carries hormones and
various disease fighting agents throughout the body. Body temperature is controlled
by the blood as it transports heat from the inner body cavity out towards the skin.
There are three types of cellular components in the blood: erythrocytes, or red blood
cells; leukocytes, or white blood cells; and thrombocytes, or platelets.
Erythrocytes contain an agent called hemoglobin which allows the blood the carry oxygen
from the lungs. The leukocytes combat infection by surrounding the invading bacteria
and eating them. Platelets form clots when a blood vessel in ruptured, preventing an
excessive loss of blood. TOP
Lymphatic Circulation
Lymphatic circulation consists of the lymph, lymph vessels, and lymph glands including
the spleen. The lymph vessels and glands create a series of channels throughout the
body to aid in the filtering out and destruction of harmful substances. This system
also collects nutrients from the intestines and waste products in the body in order to
properly direct them for elimination. Lymph is the excess fluid
that escapes from the capillaries. Lymph glands are masses of
encapsulated lymphocytes spaced along the path of blood vessels. These lymph glands
are the first barriers to infection because they produce lymphocytes and antibodies.
They also filter the lymph fraction of the blood going through them. The
spleen is a ductless gland that acts like a sponge by storing blood, and destroys worn-out
erythrocytes. Lymph vessels from one-way channels, like veins,
allowing fluid to move only toward the heart. The lymph vessels converge to from
larger vessels before finally joining the cranial vena cava. TOP
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