Vasculature Learning Objectives

Describe the variety of blood vessels and their roles. What is angiogenesis?

Blood vessels (vasculature) are a series of tubes that conduct blood throughout the body for distribution and return. Arteries conduct blood away from the heart and distribute blood to beds of exchange vessels called capillaries. Veins conduct blood from capillary beds to the heart. Blood is distributed to capillaries via smaller arteries called arterioles. Blood is conducted from capillary beds to veins via smaller veins called venules. 

Arterioles: Receive blood from arteries, direct blood to capillaries, and have greatest control of blood flow (and pressure) 

• “Resistance vessels” (autonomic control of smooth muscle)

  • Sympathetic = vasoconstriction

  • Walls ~ 50% vessel diameter (ratio of smooth muscle greatest among vessels)

• Thin tunica interna & thin internal elastic lamina

• Feed capillary beds

Arterioles + capillary beds = micro-circulation

Capillaries: Receive blood from arterioles, exchange fluids and materials with interstitial fluid, are drained of blood by post-capillary venules. Blood hydrostatic pressure moves materials out of the capillaries (filtration). There are also osmotic forces (based on concentration differences between substances in the blood in the capillaries vs substances in the interstitial fluid) that favor movement out of the blood (filtration) and into the blood (reabsorption). 

• “Exchange vessels”

  • Simple structure

    • Tunica interna

      • Endothelium

      • Basement membrane

  • Diameter ~ 5-10 µm

Venules: Conduct blood from capillaries to veins

• Drain capillaries → Toward heart 

• Post-capillary venules →  muscular venules →  veins

• Thick tunica externa

• Thin tunica media &  interna

  • Walls ~ 10% diameter

• Tunica interna

  • Valves: infoldings of endothelium to promote unidirectional flow of blood

Veins: Conduct blood from venules to increasing larger veins toward the heart. Venous pressure is quite low when compared to arterial pressure. 

• Tunica externa

  • Elastin & collagen

• Tunica media

  • Typically quite thin

    • Some smooth muscle

• Tunica interna

  • Endothelium & basement membrane

  • Some veins have valves formed from endothelium

    • Unidirectional flow of blood

What are the major forces driving blood flow in veins? What is the significance of venous valves? What are varices? What is a DVT, and what role does DVT play in VTE? 

Venous pressure is lower than arterial pressure, largely because veins lack the largest sources of hydrostatic pressure (the heart and very elastic walls). Blood, therefore, requires additional help in returning to the heart. This help comes from several sources:

• Venous valves, which when present encourage unidirectional flow

• Muscle milking - as muscles contract and relax, they ‘squeeze’ adjacent veins, forcing blood to move

• Respiratory milking - as the processes of ventilation increases and decreases pressures in the thoracic and abdominal cavities, blood in larger veins may be advanced

Venae comitantes (accompanying veins; sing. vena comitans) - frequently veins may be bundled with their associated arteries. The pulsatile nature of arteries exerts a pressure on the accompanying veins, which advances blood.   

Venous valves are an important feature for the return of blood to the heart. Endothelium is relatively delicate tissue and over time with pressure, the valves may become incompetent (fail). When this happens, blood may flood retrograde (in reverse). Retrograde blood flow may cause the walls of veins to enlarge and be more circuitous (twisted), which is known as a venous varicosity (varicose veins). Varicose veins are usually seen in the lower limbs and are typically more of a cosmetic concern, but they may be physically uncomfortable or cause feelings of lower self-esteem. 

Venous Thrombo-Embolisms (VTE):

Due to slower movement, the veins are also an area where blood clots (thrombi) can occur. In particular, the lower limbs are susceptible to thrombosis because: they have to fight against gravity, major veins are deep veins, and long periods of standing or inactivity (without using lower limb muscles, muscle milking is less effective). When a thrombus occurs in the lower limb, it is called a deep vein thrombosis (DVT).

Three factors (known as Virchow’s Triad), signal an increased risk for DVT:

• Stasis

• Hypercoagulopathy (thrombophilia)

• Endothelial injury

A DVT may cause swelling on the affected limb. If the thrombus dislodges, it may move to the heart and/or the vasculature of the lungs. If in the vasculature of the lungs, a thrombosis is known as a pulmonary embolism, which is emergent and life-threatening. 

What is a portal system? What are the portal systems in humans?

The caval system is all of the blood returning to systemic circulation, which takes one of two major pathways: the superior vena cava (blood returning to the right atrium from superior to the diaphragm) and the inferior vena cava (from inferior to the diaphragm). We can contrast the caval system with portal systems.

A portal system is a vascular system consisting of a vessel that connects two capillary beds. There are three types of portal systems in humans:

• Renal portal systems (arterial)

  • Glomerulus - afferent arteriole - peritubular capillaries

    • Carries any blood that was not converted to filtrate along the nephron for opportunities for secretion and reabsorption

  • Microscopic

• Hypophyseal portal system (venous)

  • Hypothalamic capillaries - vein - anterior pituitary capillaries

    • Hormones between hypothalamus and anterior pituitary

  • Small

• Hepatic portal system (venous)

  • GI tract capillaries - names veins & hepatic portal vein - Liver capillaries

    • Absorbs nutrients from the GI tract and delivers them to the liver

  • Very large. This is usually what is referred to as the portal system.

Understand the great vessels of the heart. Outline the major distribution networks of the aorta by region (e.g. ascending aorta, aortic arch, etc.). Outline the major arterial branches of the thoracic aorta.

The great vessels of the heart include all the arteries and veins which communicate with chambers of the heart. This will be emphasized in the Heart Session, but these vessels include:

• Superior vena cava & Inferior vena cava (systemic: blood returning from systemic circulation to the right atrium)

• Pulmonary trunk & arteries (pulmonary: blood to the alveoli of lungs from the right ventricle)

• Pulmonary veins (pulmonary: blood from alveoli to left atrium)

Aorta, specifically the ascending aorta (systemic: blood to the systemic loop from left ventricle)

The aorta has three distinct parts, each with their own branches, and one with two subdivisions, including:

• Ascending aorta

  • Most proximal part of aorta

  • Exists the left ventricle of the heart

  • Two branches

    • Left coronary a. (LCA)

    • Right coronary a. (RCA)

Contiguous with the arch of the aorta

• Arch of the aorta

Distal to ascending aorta, Proximal to descending aorta

• Three branches

  • Brachiocephalic trunk

    • Right common carotid a. (right head & neck)

    • Right subclavian a. (right upper limb, neck, and thorax)

  • Left common carotid a. (left head & neck)

    • Within carotid sheath

    • Two branches

      • Internal carotid a. (cranial cavity, brain, orbit, & forehead)

      • External carotid a. (superior neck, face, & scalp)

• Left subclavian a. (left upper limb, neck, and thorax)

• Three parts (as defined by anterior scalene m.)

  • 1st (medial to ASM)

  • 2nd (posterior to ASM)

  • 3rd (lateral to ASM)

    1. 1. • • Transitions to axillary a. at the lateral margin of Rib 1. 

• Descending aorta

  • Thoracic part (thoracic wall & viscera)

    • Branches

      • Parietal brs. (paries=wall; thoracic wall; postero-laterally oriented)

        • Posterior intercostal aa. (3-11)

        • Subcostal aa.

        • Superior phrenic aa.

      • Visceral brs. (anteriorly oriented)

        • Bronchial aa.

        • Pericardial aa.

        • Mediastinal aa.

        • Esophageal aa. 

• Passes through aortic hiatus of diaphragm (@ T12)

• Becomes abdominal part distal to diaphragm

• Abdominal part (abdominal wall and viscera)

  • Branches

    • Inferior phrenic aa. (diaphragm)

    • Celiac trunk (foregut)

    • Superior mesenteric a. (midgut)

    • Gonadal aa. (gonads)

    • Inferior mesenteric a. (hindgut)

  • Bifurcates into common iliac arteries @ L4

  • More details to follow in subsequent LOs

Outline the regions served by the: common, internal, and external carotid aa. Outline the major elements of cerebral and cerebellar circulation.

The common carotid arteries are the major arteries of the neck serving the neck and head. Found within the carotid sheath, the carotid artery bifurcates into the internal carotid a. and external carotid a.

The internal carotid a. has several principal branches, including:

• Ophthalmic a. (orbit)

• Anterior cerebral a. 

• Middle cerebral a.

• Posterior communicating a. (variably present; anastomosis with brs. of vertebral a.) 

While the internal carotid a. supplies much of the brain with blood, the other major branch participating in the (cerebral) circle of Willis is the vertebral a. (a branch of the first part of the subclavian a.). The vertebral a. supplies the following major brs. to the brain and spinal cord:

• Posterior inferior cerebellar a. (PICA)

• Anterior spinal a.

• Basilar a. (confluence of vertebral aa.)

  • Superior cerebellar aa.

  • Posterior cerebral aa.

The external carotid a. supplies the neck, superficial face, deep face, and head, with the following principal brs.:

• Superior thyroid a. (thyroid gland & larynx)

• Lingual a. (tongue & sublingual space)

• Facial a. (face)

• Ascending pharyngeal a. (pharynx, etc.)

• Occipital a. (posterior superficial head)

• Posterior auricular a. (head posterior to ear)

• Maxillary a.* (terminal br.; deep face)

• Superficial temporal a.* (terminal br.; superficial head)

Outline the major branches of the subclavian, axillary, brachial, radial, & ulnar aa.

The right subclavian a. originates from the brachiocephalic trunk, whereas the left subclavian a. is a branch of the arch of the aorta. 

The subclavian a. is conceptually divided into three parts, with respect to the vessel’s relationship to the anterior scalene m. The first part of the subclavian a. is found medial to the anterior scalene, the second part of the subclavian a. is posterior to the anterior scalene m., and the third part is lateral to the anterior scalene m.

The first part of the subclavian a. typically hosts three major branches:

• Vertebral a. (supplies the brain & spinal cord)

• Thyrocervical trunk,

  • Inferior thyroid a. (supplies the thyroid, parathyroid glands, and larynx)

  • Ascending cervical a. (supplies deep muscles of the neck)

  • Suprascapular a. (supplies supra- and infraspinatus mm.)

  • Transverse cervical a. (supplies trapezius m. & often gives rise to the dorsal scapular a.)

  • Internal thoracic a. (supplies anterior thoracic & abdominal walls & the diaphragm).

The second part of the subclavian a. typically hosts one major branch, the:

• Costocervical trunk

  • Deep cervical a. (supplies deep neck muscles)

  • Highest (supreme) intercostal a. (supplies 1st & 2nd intercostal spaces).

The third part of the subclavian a. typically hosts one major branch, the:

Dorsal scapular a. (supplies rhomboid mm. and levator scapulae m.). The dorsal scapular a. may also be a branch of the transverse cervical a.

Beyond the lateral border of the first rib, the subclavian a. transitions into the axillary a. at the lateral margin of the 1st rib.

The axillary a. will supply shoulder muscles, and will continue as the brachial a. at the inferior margin of the teres major m. The axillary a. also has three parts, defined by their relationship to pectoralis minor m. The 1st part is medial to pec. minor, the 2nd part posterior to pec. minor, and the 3rd part is lateral to pec. minor and proximal to teres major m. where it transitions to the brachial a. 

The axillary a. has the principal brs.:

• 1st part

  • Superior thoracic a. (thoracic wall)

• 2nd part (Pectoralis minor m.)

  • Thoraco-acromial a. (pectoral mm.)

  • Lateral thoracic a. (thoracic wall)

• 3rd part

  • Subscapular a.

    • Circumflex scapular a. (shoulder)

    • Thoracodorsal a. (latissimus dorsi m.)

  • Anterior circumflex humeral a. (neck of humerus)

  • Posterior circumflex humeral a. (neck of humerus)

The brachial a. will supply the arm compartments, and will bifurcate into the radial a. & ulnar a. in the cubital fossa.

The radial a. will supply lateral portions of the anterior forearm compartment and portions of the hand, being the major contributor to the deep palmar arch but also a portion to the superficial palmar arch.

The ulnar a. will supply medial & deep portions of the anterior forearm compartment, the posterior compartment of the forearm, and portions of the hand, being the major contributor to the superficial palmar arch but also a portion to the deep palmar arch. There are variable anastomoses with the arches.

Outline the major arterial branches of the abdominal aorta. 

The thoracic aorta (thoracic part of descending aorta) becomes the abdominal aorta (abdominal part of descending aorta) at the aortic hiatus (of the diaphragm), approximately at the T12 vertebral level. The abdominal aorta descends in the retroperitoneum, left lateral to the inferior vena cava, and anterior to the bodies of the vertebrae, sending branches to viscera and the body wall along its length, until bifurcating (approximately at L4) into the common iliac aa.

The branches of the abdominal aorta may be conceptualized as: 1) anteriorly oriented, unpaired branches to viscera, 2) laterally oriented, paired branches to viscera, and 3) posterolaterally oriented, paired branches to the body wall.

Anteriorly oriented, unpaired brs. to viscera:

• Celiac trunk (~T12) - serves the foregut (abdominal esophagus - proximal duodenum @ Major duodenal papilla)

• Superior mesenteric a. / SMA (~L1) - serves the midgut (distal duodenum @ Major duodenal papilla - proximal two-thirds of transverse colon)

• Inferior mesenteric a. / IMA (~L3) - serves hindgut (distal third of transverse colon - rectum)

Laterally oriented, paired brs. to viscera:

• Middle Suprarenal aa. (~L1) - serve the suprarenal (adrenal) glands

• Renal aa. (~L1) - serve the kidneys

• Inferior suprarenal aa.

• Gonadal aa. (~L2) - serve the gonads

Posterolaterally oriented, paired parietal brs. to body wall:

• Inferior phrenic aa. (~T12)

• Superior suprarenal aa.

• Lumbar aa. (~L1-L4)

The abdominal aorta bifurcates into the common iliac aa. typically around L4. 

Anastomoses among abdominal arteries are quite common, and generally there will be anastomoses between some branches of the celiac trunk and the SMA, and between some branches of the SMA with the IMA (e.g. via the marginal a.).

Outline the major branches of the celiac trunk, SMA, & IMA. 

The celiac trunk is the main supply to the abdominal viscera derived from the foregut. The trunk branches from the abdominal aorta anteriorly at approximately the level of T12, or just inferior to the aortic hiatus of the diaphragm. The trunk is very short and immediately branches into: L. gastric a., common hepatic a., and splenic a.

The L. gastric a. supplies small portions of the distal esophagus and liver and the lesser curvature of the stomach, where it openly anastomoses with the R. gastric a. (br. of the common hepatic a.).

The common hepatic a. serves the liver, gallbladder, head of pancreas, and duodenum.

The large and tortuous splenic a. runs posterior to the superior portion of the pancreas. There are multiple branches that supply the pancreas. It also has branches that supply the stomach and spleen. 

The superior mesenteric a. (SMA) is the principal blood supply to the midgut. The SMA branches from the abdominal aorta anteriorly at L1 and accompanies the superior mesenteric v (SMV) posterior to the neck of the pancreas. 

• Inferior pancreaticoduodenal a.

• Intestinal aa.

  • Jejunal aa. (jejunum)

  • Ileal aa. (ileum)

• Middle colic a. (transverse colon)

• Right colic a. (ascending colon)

• Ileocolic a. (distal ileum, cecum, appendix, and proximal ascending colon)

Marginal a. - large artery that runs along the margin of the colon, fed into by the colic brs. of the SMA and IMA

The inferior mesenteric a. (IMA) is the principal blood supply to the hindgut. The IMA exits the abdominal aorta anteriorly at the level of L3, and gives rise to the follow brs.:

• Left colic a. (descending colon)

• Sigmoid aa. (sigmoid colon)

• Superior rectal a. (rectum and anal canal)

The abdominal aorta typically bifurcates into the common iliac aa. at L4. 

Outline the major branches of the common, internal, & external iliac aa.

The common iliac artery is the continuation of the abdominal aorta @L4, it moves inferolaterally along the pelvic brim, and bifurcates into two brs.:

• Internal iliac a., serving pelvic viscera, perineum, & external genitalia

• External iliac a. serving the inferior anterior abdominal wall, hip, & lower limb

Becomes the femoral a. @ inguinal ligament

Outline the major branches of the femoral, popliteal, & anterior and posterior tibial aa. 

The femoral a. is the continuation of the external iliac a. distal to the inguinal ligament and is the principal blood supply to the lower limb. Its branches include:

• Deep artery of thigh

• Becomes the popliteal a. @ adductor hiatus

  • Genicular aa.

  • Anterior tibial a.

    • Dorsalis pedis a.

    • Deep plantar a. → Plantar arterial arch

  • Posterior tibial a.

    • Fibular a.

    • Medial plantar a.

    • Lateral plantar a. → Plantar arterial arch

Understand the pathways of blood between the heart and lungs.

Pulmonary arteries are of pulmonary circulation, and carry relatively de-oxygenated blood to the lungs for oxygenation. The typical pattern of branching is as follows:

Pulmonary trunk exits the right ventricle of the heart and bifurcates near the level of the sternal angle → R. & L. pulmonary aa. (part of root of lung; enters hilum of lung) → secondary lobar aa. (branch for each lobe of the lung) → tertiary segmental aa. (branch for each bronchopulmonary segment)

The pulmonary aa. branches travel in close association with the various branches of the tracheobronchial tree (the ventilation pathway of air from larynx to alveoli).

Pulmonary veins carry relatively oxygenated blood to the left atrium of the heart, destined for systemic circulation. 

Outline the major systemic veins. What is venipuncture? What is the significance of the median cubital vein? What is the significance of the great saphenous vein?

Systemic veins return blood from metabolically active tissues toward and to the right atrium of the heart. Blood returning from superior to the diaphragm is returned to the heart via the superior vena cava (SVC), whereas blood returning from inferior to the diaphragm is returned to the heart via the inferior vena cava (IVC). 

The SVC is formed from the confluence of the brachiocephalic veins. Each brachiocephalic v. is the confluence of an internal jugular v. (IJV; drains the head & neck) and subclavian v. (drains the neck, thorax, and upper limb). Where IJVs and subclavian veins meet is known as the venous angles, and these are important locations for the return of lymph to venous circulation.

The upper limbs are drained of blood by the subclavian v. which is the continuation of the axillary v. The axillary v. is a vena comitans (accompanying vein) of the axillary a. as are most deep veins of the upper limb are venae comitantes of their named arterial counterparts. There are several superficial veins which drain the upper limb and flow into the axillary v., including:

• Cephalic v., which drains into axillary v.

• Basilic v., which becomes the axillary v. (with brachial vv.)

• Median cubital v., which variably connects cephalic and basilic veins.

  • Common target for venipuncture (blood draw)

Draining the thoracic wall is the responsibility of the azygos system. The azygos system consists of three major veins, each receiving blood from the intercostal spaces and one another, including:

• Azygos v.

  • Drains right posterior IC vv.

  • Drains into Superior vena cava 

• Accessory hemi-azygos v.

  • Drains left posterior IC (5-8) vv.

  • Drains into the azygos v.

• Hemi-azygos v.

  • Drains left posterior IC (9-11) vv. (inferior)

  • Drains into the azygos v.

The inferior vena cava (IVC) drains blood from inferior to the diaphragm. Like the aorta, the IVC begins as the confluence of common iliac vv. @L4, ascends the abdomen to the right of the abdominal aorta, and travels through the hiatus for the vena cava at the T8 level before entering the left atrium of the heart. The IVC receives blood from nearly all the targets of the abdominal aorta with the exception of the GI tract - those organs are drained by the hepatic portal system, which is drained into the IVC via hepatic veins. The pattern of return of blood to the IVC is not bilaterally symmetrical, with many organs on the left side (e.g. inferior diaphragm, suprarenal gland, and gonad) returning to the left renal v.

The major veins of the lower limb are deep veins, and these are typically venae comitantes of their arterial counterparts. Additionally, there are two major superficial veins which drain the lower limb, including:

• Great saphenous v.

  • Medial lower limb

  • Reliably found anterior to the medial malleolus

    • Great site for vascular access

  • Drains into femoral v.

  • May be harvested for a vascular graft (e.g. for CABG)

• Small saphenous v.

  • Lateral lower limb

Drains into popliteal v.

Outline (general) the hepatic portal system.

Portal systems are vessels that link sets of capillary beds. The hepatic portal system is a venous portal system consisting of all veins that drain the gastrointestinal tract (GIT) from the abdominal esophagus to the superior rectum. These veins coalesce into the hepatic portal v. which delivers blood from the capillary beds of the GIT to capillary beds of the hepatic sinusoidal cells. As the hepatic portal system connects capillary beds, it is a separate system from the caval system (those veins which drain into the vena cavae). Blood from the hepatic sinusoidal cells is delivered back into the caval system via hepatic vv. draining into the inferior vena cava.

The hepatic portal v. typically originates as a union of the superior mesenteric v. with the splenic v. in the transpyloric plane (approximately at L1), posterior to the neck of the pancreas.