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Veins

Blood from capillaries flows into the venous system as veins. The smallest veins, venules, transition to small veins, medium veins, then to large veins. Venules initially resemble large-diameter capillaries and as they enlarge, additional components are added in their walls. Veins display the most variability of all vessels, and the three tunics are not always well demarcated. Examples of venous structures are shown here.

Venules > <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

Venules >

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

 - Endothelial cells <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

- Endothelial cells

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

Arterioles <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

Arterioles

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

 - Endothelial cells <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

- Endothelial cells

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

 - Tunica media <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

- Tunica media

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

Capillaries <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

Capillaries

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

Nerve <p>The smallest post-capillary venules resemble large-diameter capillaries.  Their thin tunica intima consists of an endothelium and a thin subendothelial layer.  No additional tunics are present.  Accompanying arterioles are also shown. </p>

Nerve

The smallest post-capillary venules resemble large-diameter capillaries. Their thin tunica intima consists of an endothelium and a thin subendothelial layer. No additional tunics are present. Accompanying arterioles are also shown.

Small vein > <p>As venules enlarge, they transition through muscular venules to become small veins.  Compared to arteries, veins are thin-walled vessels with large and irregular lumens.  All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia.  Small veins possess valves that prevent the backflow of blood.  An accompanying small artery is also present.</p>

Small vein >

As venules enlarge, they transition through muscular venules to become small veins. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia. Small veins possess valves that prevent the backflow of blood. An accompanying small artery is also present.

 - Tunica media <p>As venules enlarge, they transition through muscular venules to become small veins.  Compared to arteries, veins are thin-walled vessels with large and irregular lumens.  All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia.  Small veins possess valves that prevent the backflow of blood.  An accompanying small artery is also present.</p>

- Tunica media

As venules enlarge, they transition through muscular venules to become small veins. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia. Small veins possess valves that prevent the backflow of blood. An accompanying small artery is also present.

 - Tunica adventitia <p>As venules enlarge, they transition through muscular venules to become small veins.  Compared to arteries, veins are thin-walled vessels with large and irregular lumens.  All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia.  Small veins possess valves that prevent the backflow of blood.  An accompanying small artery is also present.</p>

- Tunica adventitia

As venules enlarge, they transition through muscular venules to become small veins. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia. Small veins possess valves that prevent the backflow of blood. An accompanying small artery is also present.

 - Valve <p>As venules enlarge, they transition through muscular venules to become small veins.  Compared to arteries, veins are thin-walled vessels with large and irregular lumens.  All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia.  Small veins possess valves that prevent the backflow of blood.  An accompanying small artery is also present.</p>

- Valve

As venules enlarge, they transition through muscular venules to become small veins. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia. Small veins possess valves that prevent the backflow of blood. An accompanying small artery is also present.

Small artery <p>As venules enlarge, they transition through muscular venules to become small veins.  Compared to arteries, veins are thin-walled vessels with large and irregular lumens.  All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia.  Small veins possess valves that prevent the backflow of blood.  An accompanying small artery is also present.</p>

Small artery

As venules enlarge, they transition through muscular venules to become small veins. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. All three tunics are present: a tunica intima, a tunica media containing 2-3 layers of smooth muscle and a prominent tunica adventitia. Small veins possess valves that prevent the backflow of blood. An accompanying small artery is also present.

Medium vein >  <p>Medium veins accompany muscular arteries and are often named, e.g. radial vein.  Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant.  Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities.  Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.     </p>

Medium vein >

Medium veins accompany muscular arteries and are often named, e.g. radial vein. Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant. Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities. Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.

 - Tunica media <p>Medium veins accompany muscular arteries and are often named, e.g. radial vein.  Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant.  Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities.  Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.     </p>

- Tunica media

Medium veins accompany muscular arteries and are often named, e.g. radial vein. Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant. Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities. Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.

 - Tunica adventitia <p>Medium veins accompany muscular arteries and are often named, e.g. radial vein.  Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant.  Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities.  Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.</p>

- Tunica adventitia

Medium veins accompany muscular arteries and are often named, e.g. radial vein. Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant. Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities. Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.

 - Valve <p>Medium veins accompany muscular arteries and are often named, e.g. radial vein.  Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant.  Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities.  Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.</p>

- Valve

Medium veins accompany muscular arteries and are often named, e.g. radial vein. Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant. Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities. Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.

Muscular artery <p>Medium veins accompany muscular arteries and are often named, e.g. radial vein.  Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant.  Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities.  Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.</p>

Muscular artery

Medium veins accompany muscular arteries and are often named, e.g. radial vein. Tunica media and tunica adventitia are thicker than in small veins, with the tunica adventitia dominant. Valves, crescent-shaped folds of the tunica intima that project into the lumen, are particularly prominent in medium veins of the lower extremities. Medium veins transition to large veins, such as the jugular vein and vena cavae, that form the major portals for blood return to the heart.