The venous system is the system of veins in the
systemic and
pulmonary circulations that return blood to the heart. In the systemic circulation the return is of deoxygenated blood from the organs and tissues of the body, and in the pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the heart. Almost 70% of the blood in the body is in the veins, and almost 75% of this blood is in the small veins and venules. All of the systemic veins are tributaries of the largest veins, the
superior and
inferior vena cava, which empty the oxygen-depleted blood into the
right atrium of the heart. Post-capillary venules have a diameter of between 10 and 30
micrometres (μm), and are part of the
microcirculation. Their endothelium is made up of flattened oval or polygon shaped cells surrounded by a
basal lamina. Post-capillary venules are too small to have a smooth muscle layer and are instead supported by
pericytes that wrap around them. Post-capillary venules become
muscular venules when they reach a diameter of 50 μm, and can reach a diameter of 1 mm. Superficial veins are those closer to the surface of the body, and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. Perforator veins drain from the superficial to the deep veins. These are usually referred to in the lower limbs and feet. Superficial veins include the very small
spider veins of between 0.5 and 1 mm diameter, and
reticular or feeder veins. ;Venous plexuses There are a number of
venous plexuses where veins are grouped or sometimes combined in networks at certain body sites. The
Batson venous plexus, runs through the inner vertebral column connecting the thoracic and pelvic veins. These veins are noted for being valveless, believed to be the reason for
metastasis of certain cancers. A subcutaneous venous plexus is continuous, and a high rate of flow is supplied by small
arteriovenous anastomoses. The high rate of flow ensures heat transfer to the vein wall. The venous valves serve to prevent
regurgitation (backflow) due to the low pressure of veins, and the pull of gravity. The endothelial cells in the sinuses are able to stretch twice as much as those in areas without valves. The deep veins of the lower limb include the
common femoral vein,
femoral vein, and the
deep femoral vein; the
popliteal vein, the tibial, and
fibular veins. In the common femoral vein one valve is located above the
saphenofemoral junction called the
suprasaphenic valve. There are sometimes two valves in the same tract. In the femoral vein there are often three valves, the most constantly found valve is just below the joining of the deep femoral vein. The deep femoral vein and its perforators have valves. In the popliteal veins there are between one and three valves; in each
posterior tibial vein there are between 8 and 19 valves, and in the
anterior tibial veins there are between 8 and 11 valves. Their action is supported by the action of
skeletal muscle pumps that contract and compress the veins. A skeletal muscle is confined in its fascia and contraction of the muscle which makes it wider results In compression on the vein that pushes the blood forward. There are more valves in the lower leg, due to increased gravitational pull, with the number decreasing as the veins travel to the hip. There are no valves in the veins of the thorax or abdomen. There is a valve at the junction of the inferior vena cava (one of the
great vessels) and the right atrium known as the
valve of inferior vena cava also known as the
eustachian valve. This valve is an embryological remnant and is insignificant in the adult. However, when persistent it can cause problems.
Circulatory routes s in some specific locations including the
lungs,
liver, and
kidneys There are some separate parallel systemic circulatory routes that supply specific regions, and organs. ;Bronchial circulation In the
bronchial circulation that supplies blood to the lung tissues,
bronchial veins drain
venous blood from the large
main bronchi into the
azygous vein, and ultimately the right atrium. Venous blood from the bronchi inside the lungs drains into the
pulmonary veins and empties into the left atrium; since this blood never went through a capillary bed it was never oxygenated and so provides a small amount of shunted deoxygenated blood into the systemic circulation. ;Cerebral circulation In the
cerebral circulation supplying the
cerebrum the venous drainage can be separated into two subdivisions: superficial and deep. The superficial system is composed of
dural venous sinuses, which have walls composed of dura mater as opposed to a traditional vein. The dural sinuses are therefore located on the surface of the cerebrum. The most prominent of these sinuses is the
superior sagittal sinus which flows in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to the
confluence of sinuses, where the superficial drainage joins with the sinus that primarily drains the deep venous system. From here, two
transverse sinuses bifurcate and travel laterally and inferiorly in an S-shaped curve that forms the
sigmoid sinuses which go on to form the two
jugular veins. In the neck, the
jugular veins parallel the upward course of the
carotid arteries and drain blood into the
superior vena cava. The deep venous drainage is primarily composed of traditional veins inside the deep structures of the brain, which join behind the midbrain to form the
vein of Galen. This vein merges with the
inferior sagittal sinus to form the
straight sinus which then joins the superficial venous system mentioned above at the
confluence of sinuses. ;Portal venous systems A
portal venous system is a series of veins or venules that directly connect two
capillary beds. The two systems in verebrates are the
hepatic portal system, and the
hypophyseal portal system. ;Anastomoses An
anastomosis is a joining of two structures such as blood vessels. In the circulation these are called
circulatory anastomoses, one of which is the join between an artery with a vein known as an
arteriovenous anastomosis. This connection which is highly muscular, enables venous blood to travel directly from an artery into a vein without having passed from a capillary bed. Abnormal connections can be present known as
arteriovenous malformations. These are usually congenital and the connections are made from a tangle of capillaries. A
cerebral arteriovenous malformation is one that is located in the
brain. An irregular connection between an artery and a vein is known as
arteriovenous fistula. A small specialised arteriovenous anastomosis known as a
glomus body or organ serves to transfer heat in the fingers and toes. The small connection is surrounded by a capsule of thickened connective tissue. In the hands and feet there are a great number of glomera. ;Vascular shunt A vascular shunt can also bypass the capillary bed and provide a route for blood supply directly to a collecting venule. This is achieved by a
metarteriole that supplies around a hundred capillaries. At their junctions are precapillary sphincters that tightly regulate the flow of blood into the capillary bed. When all of the sphincters are closed blood can flow from a metarteriole into a thoroughfare channel and into a collecting venule bypassing the capillary bed. ;Other A
communicating vein directly connects two parts of the same system such as the
Giacomini vein that connects the (superficial)
small saphenous vein with the (superficial)
great saphenous vein.
Peripheral veins carry blood from the limbs and
hands and
feet. ==Microanatomy==