Blood vessels

Aorta and branches

The aorta is the largest artery in the body. It has four major segments - ascending, arch, thoracic or descending and abdominal aorta.

1. Ascending Aorta: It arises from the left ventricle. The right and left coronary arteries originate from the right and left aortic sinuses respectively.
2. Arch of Aorta: The arch gives rise to many critical branches. There are three direct branches, left common carotid and left subclavian arteries and the brachiocephalic trunk (also called as innominate artery). The brachiocephalic trunk will further divide into right common carotid and right subclavian arteries. The subclavian artery gives rise to four major branches, vertebral artery, internal thoracic artery, thyrocervical trunk and costocervical trunk which becomes the axillary artery. Axillary artery will continue as the brachial artery which divides into radial and ulnar arteries which anastomose in the hand as the superficial and deep palmar arches. The vertebral artery ascends to the base of the brain through the foramen magnum and joins with the contralateral vertebral artery to form the basilar artery. The common carotid artery will divide into external and internal carotid arteries. The external carotid artery ends by branching into superficial temporal and maxillary branches. The facial artery is a branch of the external carotid artery.
3. Thoracic Aorta: It gives off visceral and parietal branches. Visceral branches are pericardial, mediastinal, esophageal and bronchial. There are two left bronchial and one right bronchial artery. Left bronchial arises from the thoracic aorta while right bronchial may arise from the thoracic aorta or right third posterior intercostal artery. Parietal branches are posterior intercostal, subcostal and superior phrenic arteries.
4. Abdominal Aorta: The thoracic aorta becomes abdominal aorta after passing through the aortic hiatus of the diaphragm. It gives rise to paired and unpaired branches as follows:
   1. Paired branches: Inferior phrenic, suprarenal, renal, gonadal and lumbar arteries.
   2. Unpaired branches: Celiac trunk, superior mesenteric artery, inferior mesenteric and median sacral artery.

The abdominal aorta bifurcates into two common iliac arteries which further divide into internal and external iliac arteries. The femoral artery is a continuation of the external iliac artery.

Subclavian steal syndrome

It is characterized by retrograde flow in the vertebral artery resulting from subclavian artery stenosis or occlusion proximal to the origin of the vertebral artery, commonly due to atherosclerosis and rarely also from Takayasu arteritis or giant cell arteritis. It presents as upper limb claudication after exercising the upper limb and/or neurologic symptoms from relative cerebral ischemia especially in the posterior circulation. This may manifest as pre-syncope, dizziness, vertigo, blurry vision, diplopia etc. characteristically following upper extremity exercise. On examination, there is a blood pressure differential between both arms of more than 20 mmhg, affected arm will have weak or absent pulses and a bruit may be auscultated over the supraclavicular fossa.

Blood vessels histology

Blood vessel walls are made of three layers from outside to inside are tunica adventitia, tunica media and tunica intima. Adventitia has type I collagen and is very tough while media has smooth muscle cells and type III collagen. Tunica intima consists of the endothelial layer and it secretes types I and IV collagen among other things.

Capillaries are made of a single layer of endothelial cells surrounded by a basal lamina. They can be divided as follows -

1. Continuous Capillaries: They have zona occludens. Pores are absent. Seen in lung, muscle, brain (BB barrier).
2. Fenestrated with diaphragms: They are made of endothelial cells joined together by fascia occludens creating slit like intercellular spaces and contain fenestrae with diaphragms. Seen in endocrine glands, intestine and kidney.
3. Fenestrated without diaphragms: Seen in kidney glomerulus only.
4. Discontinuous / Sinusoids: They have a single layer of endothelial cells separated by wide gaps (no zona occludens). Seen in liver, spleen, bone marrow.