Venous Vessels and Veins vs Arteries

Venous Vessels

• Veins carry blood from capillaries to the heart
• Systemic circuit veins carry relatively oxygen-poor blood, except for pulmonary veins, which carry oxygen-rich blood from the lungs
• Venules, the smallest veins, measure 89-100 µm in diameter
• Postcapillary venules consist of an endothelium with pericytes and function like capillaries
• During inflammation, postcapillary venules allow more fluid and leukocytes to leave the circulation than capillaries
• Larger venules have a tunica media with one or two smooth muscle layers and a thin tunica externa

Veins vs Arteries

• Veins form when venules join
• Veins differ structurally from arteries
• Veins have a larger lumen than comparable arteries, holding 65% of the body's blood at any given time
• The tunica externa is thicker than the tunica media in veins, which is the opposite of arteries
• Longitudinal bands of smooth muscle further thicken the tunica externa in the venae cavae
• Veins have less elastin in their walls and don't need to dampen pulsations like arteries do
• Vein walls are thinner than comparable arteries
• Blood pressure is lower in veins due to the pressure drop in arterioles and capillary beds

Systemic Veins

• While veins typically run with corresponding arteries, their distributions differ
• While one large systemic artery (aorta) exits the left ventricle, three major veins enter the heart's right atrium: the superior and inferior venae cavae and the coronary sinus
• Large and medium-sized arteries are deep and accompanied by deep veins of the same name, as well as superficial veins found just beneath the skin
• Superficial veins are important for drawing blood or placing intravenous lines, but are susceptible to cuts or injuries

Venous Patterns and Drainage

• Two or more parallel veins often drain a body region instead of a single larger vein
• Multiple veins can anastomose to form a venous plexus
• The brain and digestive tract have unique venous drainage patterns
• Brain veins drain into dural venous sinuses, which are endothelium-lined channels supported by dura mater
• Venous blood from digestive organs enters the hepatic portal system, passing through liver capillaries before reentering general systemic circulation

Venae Cavae and Tributaries

• Venae Cavae refers to the unpaired superior and inferior venae cavae

• These are the body's two largest veins, emptying directly into the right atrium of the heart

• The name "vena cava" means "cavelike vein"

• Superior Vena Cava receives systemic blood from body regions superior to the diaphragm, excluding the heart wall

• It arises from the union of the left and right brachiocephalic veins behind the manubrium and descends to the right atrium

• The left brachiocephalic vein is longer and more horizontal than the right, which is more vertical

• Each brachiocephalic vein forms from the union of an internal jugular vein and a subclavian vein

• Inferior Vena Cava ascends along the posterior abdominal wall and is the widest vessel

• It returns blood to the heart from regions inferior to the diaphragm

• It begins inferiorly at the union of the two common iliac veins on vertebra L5

• Ascends on the right side of the vertebral bodies, to the right of the abdominal aorta

• Penetrates the central tendon of the diaphragm at T8 before joining the right atrium

Hepatic Portal System

• The hepatic portal system is a specialized vascular circuit for digestion
• It picks up nutrients from the stomach and intestines, delivering them to the liver for processing and storage
• It is a series of vessels with two capillary beds between arterial supply and venous drainage
• Capillaries in the stomach and intestines receive digested nutrients, draining into tributaries of the hepatic portal vein
• The hepatic portal vein delivers nutrient-rich blood to liver sinusoids
• Nutrients reach liver cells, and toxins from the digestive tract are broken down
• Blood then enters hepatic veins and the inferior vena cava, reentering systemic circulation

Hepatic Portal Vein Tributaries

• The hepatic portal system includes multiple tributaries
• The superior mesenteric vein ascends to the right of the superior mesenteric artery
• It drains the entire small intestine, the first half of the large intestine (ascending and transverse colon), and part of the stomach
• Its superior part lies behind the stomach and pancreas
• The splenic vein drains venous blood from the spleen, running horizontally behind the stomach and pancreas
• It joins the superior mesenteric vein to form the hepatic portal vein
• Tributaries from the splenic artery correspond to the splenic vein tributaries
• This process carries spleen microbes to the liver for destruction after immune responses
• The inferior mesenteric vein ascends on the posterior abdominal wall, left of the inferior mesenteric artery
• It drains the distal colon region and the superior rectum, emptying into the splenic vein behind the stomach and pancreas

Hepatic Portal Vein Anatomy

• The hepatic portal vein is a short, vertical vessel inferior to the liver and anterior to the inferior vena cava
• Inferiorly, it starts behind the pancreas from the union of the superior mesenteric and splenic veins
• Superiorly, it enters the liver's underside, dividing into right and left branches that reach liver sinusoids
• The hepatic portal vein also receives the right and left gastric veins from the stomach

Portal Systemic Anastomoses

• Liver cirrhosis from conditions like chronic alcoholism, blocks blood flow through liver sinusoids, raising pressure in the hepatic portal system, leading to portal hypertension

• Portal system veins form anastomoses veins that drain into the venae cavae in order to provide alternate routes for blood to return to the heart

• The main anastomoses are

• Veins in the inferior esophagus

• Hemorrhoidal veins in the anal canal wall

• Superficial veins in the abdominal wall around the navel

• Connecting veins are small and can swell/burst when forced to carry large blood volumes

• In cirrhosis patients, esophageal veins can tear and cause them to vomit blood, hemorrhoids can swell up, and snakelike and distended veins can show through the skin around the navel

• Ruptured esophageal veins can cause mortality in 50% of cases and an additional 30% with recurrence

• Treatment involves injecting a hardening agent/tying them off or implantation of a tube from the inferior vena cava behind the liver to the portal vein to create a new portalcaval shunt, to relieve hypertension