The spleen is situated near the stomach. It has a rich blood supply and acts
as a reservoir of red blood cells. When there is a sudden loss of blood, as
happens when a hemorrhage occurs, the spleen contracts to release large numbers
of red blood cells into the circulation. The spleen also destroys old red cells
and makes new lymphocytes but it is not an essential organ because its removal
in adult life seems to cause few problems. In the foetus, the spleen makes both
red and white cells.
Blood is pumped out into the body via the main artery, the aorta. This
takes the blood to the head, the limbs and all the body organs. After passing
through a network of fine capillaries, the blood is returned to the heart in the
largest vein, the vena cava (see diagrams 8.8, 8.12, 8.18 and 8.19).
Arteries and veins to and from many organs often run alongside each other and
have the same name e.g. the renal artery and vein serve the kidney, the
femoral artery and vein serve the hind limbs and the subclavian artery
and vein serve the forelimbs. However, blood to the head passes along the
carotid artery and returns to the cranial vena cava via the jugular vein.
One variation on this arrangement is found in the blood vessels that serve
the digestive tract. A variety of arteries take blood from the aorta to the
intestines but blood from the intestines is carried by the hepatic portal
vein to the liver where the digested food can be processed (see diagram
8.12). This vessel is unlike others in that it transports blood from one organ
to another rather than to or from the heart like arteries or veins.

Diagram 8.18 - The main arteries and veins of the horse
Blood Pressure
The blood pressure is the pressure of the blood against the walls of the main
arteries. The pressure is highest as the pulse produced by the contraction of
the left ventricle passes along the artery. This is known as the systolic
pressure. Pressure is much lower between pulses. This is known as the
diastolic pressure. Blood pressure is measured in millimeters of mercury. A
blood pressure that is higher than expected is known as hypertension
while a pressure lower than expected is known as hypotension.

Diagram 8.19 - The main arteries of the body
*Information gathered from Wikipedia, and is not original content of
Equine Kingdom