capillaries

• Capillaries are the minute connections between the smallest arteries and the smallest veins within the tissues, where molecular exchange takes place
• The lumen of a capillary measures about 5-15 am in diameter, arranged in a three-dimensional network, forming a so-called capillary bed
• Capillaries are present in all tissues other than the cornea, lens, enamel, and cartilage
• In general veins return blood from the periphery to the heart, while arteries lead blood from the heart to the tissues
• PULMONARY VEINS: The pulmonary veins, usually seven or eight in number, return the aerated blood from the lungs and open into the left atrium of the heart.
• CARDIAC VEINS: The coronary sinus is a very short, bulbous trunk which

  receives most of the blood from the wall of the heart , situated just below the termination of the posterior vena cava and is covered in part by a thin layer of atrial muscle fibers, opens into the right atrium just below the posterior vena cava.
• cardiac is formed by the union of two tributaries (great cardiac or left coronary vein and middle cardiac or right coronary vein).
• ANTERIOR VENACAVA: It returns to the heart the blood from the head, neck, thoracic limbs, and the greater part of the thoracic wall by the confluence on the two jugular and two brachial veins and opens into the right atrium opposite to the fourth rib
• AV receives, 1. internal thoracic vein, 2. vertebral vein, 3. deep cervical vein and 4. dorsal vein.
• POSTERIOR VENA CAVA: It returns almost all the blood from the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right and left common iliac veins at the fifith lumber vertebra, above the terminal part of the aorta,
• pvc's tributaries.

  1.phrenic veins, two or three in number

  2. lumbar veins, five pairs

  3. internal spermatic veins

  4. renal veins

  5. hepatic veins
• PORTAL VEIN: It is a large trunk which returns the blood carried to the viscera by the gastric, splenic, and mesenteric arteries, formed behind the pancreas and below the posterior vena cava.
• Major tributaries of PV are:

  Anterior mesenteric vein

  2. Posterior mesenteric vein

  3. Splenic vein

  Collateral tributaries of PV are:

  1. Pancreatic veins

  2. Gastro-duodenal vein,

  3. Anterior gastric vein