Blood Vessel Structure

Companion Vessels

• Companion vessels lie next to each other and serve the same body region.

Arteries vs Veins

• Compared to veins, arteries have a thicker tunica media and a narrower lumen.
• Arteries contain more elastic and collagen fibers for resilience and resistance to blood pressure changes.
• Veins have a thicker tunica externa and a larger lumen compared to arteries.
• Veins contain less elastic and collagen fibers, causing their walls to collapse without blood.

Capillaries

• Capillaries consist only of the tunica intima, which lacks a subendothelial layer.
• The tunica intima is composed of endothelium and a basement membrane.
• Thin capillary walls facilitate rapid gas and nutrient exchange.

Arterial Tree

• Arteries branch into smaller vessels as they extend away from the heart.
• As arteries branch, their lumen diameter and elastic fibers decrease.
• Conversely, the relative amount of smooth muscle increases.

Types of Arteries

• Three basic types of arteries: elastic, muscular, and arterioles

Elastic Arteries

• Elastic arteries are the largest, ranging from 2.5 to 1 cm in diameter.
• They conduct blood from the heart to muscular arteries.
• These arteries have a high proportion of elastic fibers, allowing for stretch and recoil.
• They help propel blood through the arteries during diastole.
• Examples: aorta, pulmonary trunk, common carotid, and common iliac arteries

Muscular Arteries

• Muscular arteries are medium-sized, ranging from 0.3 mm to 1 cm in diameter.
• They distribute blood to specific body regions.
• The muscle in these arteries allows for vasoconstriction and vasodilation.
• Elastic tissue is present in the internal and external elastic lamina, located between the tunica intima/media and tunica media/externa, respectively.
• Examples: brachial and coronary arteries

Arterioles

• Arterioles are the smallest arteries, with diameters ranging from 10 micrometers to 0.3 mm.
• Larger arterioles have 3 tunics.
• Smaller arterioles have only a thin endothelium and a single layer of smooth muscle.
• Smooth muscle in arterioles is usually somewhat constricted, known as vasomotor tone.
• Vasomotor tone is regulated by the vasomotor center in the brainstem.
• Arterioles regulate systemic blood pressure and blood flow.

Capillary Characteristics

• They connect arterioles to venules.
• The average capillary is 1 mm long and 8-10 micrometers in diameter.
• Erythrocytes travel through capillaries in single file (rouleau).
• Their walls consist of an endothelial layer on a basement membrane.
• Their thin walls and small diameter are optimal for exchange between blood and tissue fluid.

Capillary Types

• Continuous capillaries have endothelial cells forming a continuous lining with tight junctions that don't completely seal.

• Intercellular clefts are gaps between endothelial cells that allow small molecules like glucose to pass, but restrict large particles.

• Continuous capillaries are common in muscle, skin, lungs, and the central nervous system.

• Fenestrated capillaries have endothelial cells that form a continuous lining but possess fenestrations (pores).

• Fenestrations allow the movement of smaller plasma proteins.

• Fenestrated capillaries are found in areas with high fluid transport, like the intestines and kidneys.

• Sinusoids have endothelial cells forming an incomplete lining with large gaps and an incomplete or absent basement membrane.

• These openings allow the transport of large substances like formed elements and large proteins.

• Sinusoids are found in bone marrow, the spleen, and some endocrine glands.

Capillary Beds

• Capillary beds are groups of capillaries functioning together.
• They are fed by a metarteriole, a vessel branching off an arteriole.
• The proximal part of the metarteriole is encircled by scattered smooth muscle cells.
• The distal part, or thoroughfare channel, lacks smooth muscle cells and connects to a postcapillary venule to drain the bed.
• True capillaries branch from the metarteriole and make up the bulk of the capillary bed.
• A precapillary sphincter, a smooth muscle ring at the origin of a true capillary, regulates blood flow.
• Sphincter relaxation allows blood to flow into true capillaries.
• Sphincter contraction causes blood to bypass the capillary bed.
• Vasomotion is the cycle of contraction and relaxation of precapillary sphincters.
• At any time, only 1/4 of the body's capillary beds are open.

Veins: Venules

• Venules are the smallest veins, ranging from 8-100 micrometers in diameter.
• They are companion vessels with arterioles.
• The smallest venules are postcapillary venules.
• Largest venules have all 3 tunics
• They merge to form veins

Veins: Small/Medium & Largest

• Small and medium veins include companion vessels with muscular arteries.
• Largest veins travel with elastic arteries.
• Most small, medium, and large veins have numerous valves.
• The valves prevent blood from pooling in the limbs and ensure flow toward the heart.
• Valves are made of tunica intima, elastic, and collagen fibers, and have a similar structure to heart's semilunar valves.

Veins as Blood Reservoirs

• Systemic veins hold the highest percentage of blood at rest, but the lowest during systemic capillaries.
• Blood can be moved from veins into circulation via vasoconstriction.
  • Example: When more blood is needed during exertion.
• Blood can be shifted back into reservoirs via vasodilation.
  • Example: When less blood is needed during rest.

Simple Pathway

• A simple pathway involves one major artery delivering blood to an organ or region.
  • An end artery provides the only path for blood to reach an organ or region.
• The artery branches into smaller arteries that become arterioles.
• Each arteriole feeds into a capillary bed.
• The capillary bed is drained by a venule.
• Venules merge to one major vein.
  • Example: The splenic artery delivers blood to the spleen, and the splenic vein drains it.

Alternative Pathways

• Arterial anastomosis involves two or more arteries converging to supply the same region.
  • Example: The superior and inferior epigastric arteries supply the abdominal wall.
  • If a junction is small, arteries may function as end arteries.
• Venous anastomosis involves 2 or more veins draining the same body region.
  • Example: The basilic, brachial, and cephalic veins drain the upper limb.
• Arteriovenous anastomosis (shunt) transports blood from an artery directly to a vein.
  • Example: Found in fingers, toes, palms, and ears.
  • Allows areas to be bypassed if the body is hypothermic.
• A portal system involves 2 capillary beds in sequence.
  • The path is: artery → capillary bed → portal vein → capillary bed → vein.
  • Example: The hypothalamo-hypophyseal portal system.

Lymphatic System

• It picks up excess fluid not reabsorbed at the venous capillary end, (about 15%) .
• Filters fluid and returns it to venous circulation.

Tissue Vascularization

• Metabolically active tissues have high vascularity (e.g., brain, skeletal muscle, heart, liver).
• Other structures have little or no vascularity (e.g., tendons, ligaments, epithelia, cartilage, cornea, lens of eye).
• Angiogenesis is the formation of new vessels, occuring over weeks or months to increase potential perfusion.
• Occurs in skeletal muscle in response to aerobic training, in adipose tissue with weight gain, and in coronary vessels to blockage .
• Regression is the return of blood vessels to a previous state.
• Occurs in skeletal muscle after an individual becomes sedentary and in adipose tissue when weight is lost.

Pulmonary Circulation

• The right ventricle pumps deoxygenated blood to the pulmonary trunk.
• The pulmonary trunk splits into the left and right pulmonary arteries, leading to the corresponding lungs.
• Arteries divide into smaller arteries and arterioles leading gas exchange in pulmonary capillaries.
• Oxygen moves from air sacs into blood, while carbon dioxide moves from blood into alveoli.
• Capillaries merge to form venules and then pulmonary veins.
• The 2 left and 2 right pulmonary veins carry oxygenated blood to the left atrium.

Pulmonary vs Systemic Circulation

• Compared to systemic circulation, pulmonary vessels have less elastic connective tissue, wider lumens, and are relatively short.
• Pulmonary vessels have lower blood pressure overall.
• Blood leaves the right ventricle with a systolic pressure of 15-25 mm Hg.
• Pressure drops to 10 mm Hg in pulmonary capillaries, ensuring slow movement and good exchange.
• As pulmonary veins enter the left atrium, blood pressure is almost 0 mm Hg.

General Arterial Flow

• Systemic arteries branch off the aorta.
• Oxygenated blood is pumped from the left ventricle to the ascending aorta.
• The left and right coronary arteries emerge from the ascending aorta to supply the heart wall.
• The ascending aorta curves left and becomes the aortic arch.
• Three main arterial branches stem from the aortic arch.
• The aortic arch curves inferiorly to become the descending thoracic aorta.
• Several branches supply the thoracic wall and viscera.
• As it passes through the diaphragm, it becomes the descending abdominal aorta and supplies to the organ systems in that area.
• At the level of the 4th lumbar vertebra, the abdominal aorta splits into the left and right common iliac arteries, which give rise to the internal iliac artery for the pelvic structures and the external iliac artery for the lower limb.

Aortic Arch: Branches

• The brachiocephalic trunk bifurcates into the right common carotid and right subclavian arteries.
  • The right common carotid supplies the right side of the head and neck.
  • The right subclavian supplies the right upper limb and some thoracic structures.
• The left common carotid artery supplies the left side of the head and neck.
• The left subclavian artery supplies the left upper limb and some thoracic structures.

General Venous Return

• Blood returns to the right atrium via three vessels: the superior vena cava, the inferior vena cava, and the coronary sinus.
• The superior vena cava is formed by the merger of the left and right brachiocephalic veins.
  • Drains the head, neck, upper limbs, thoracic, and abdominal walls.
• The inferior vena cava is formed from veins below the diaphragm.
  • Carries blood from the lower limbs, pelvis, perineum, and abdominal structures.
  • The IVC lies to the right of the descending abdominal aorta.
• The coronary sinus drains into the right atrium and carries deoxygenated blood from the heart myocardium.

Head and Neck Blood Supply

• The common carotid arteries supply most of the blood in the head and neck.
• They travel parallel to the trachea, one on each side.
• Each divides into the external carotid artery and the internal carotid artery.
• The external carotid artery supplies structures external to the skull.
• The internal carotid artery supplies internal skull structures. Additional blood is supplied by branches of the subclavian artery.
• The vertebral artery supplies the brain.
• The thyrocervical trunk supplies the thyroid gland, part of the neck, and the shoulder.

Head and Neck Branches

• The costocervical trunk supplies the deep neck and upper intercostals.
• The external carotid artery has the following branches:
  • Superior thyroid artery: Thyroid gland and larynx
  • Ascending pharyngeal artery: Pharynx
  • Lingual artery: Tongue
  • Facial artery: Face
  • Occipital artery: Posterior scalp
  • Posterior auricular artery: Ears and part of the scalp
  • Maxillary artery: Teeth, gums, nasal cavity, mastication muscles, meninges
  • Superficial temporal artery: Parotid gland, part of the scalp
• After entering the skull, the internal carotid artery branches into:
  • Anterior and middle cerebral arteries: Brain
  • Ophthalmic artery: Eyes and surrounding structures
• Vertebral arteries come off subclavian arteries
• Travel in transverse foramina of cervical vertebrae
• Enter skull through foramen magnum
• Merge to form unpaired basilar artery
• Basilar artery divides into posterior cerebral arteries (posterior cerebrum)

Cerebral Arterial Circle

• The cerebral arterial circle (circle of Willis) is an important arterial anastomosis around the sella turcica.
• It is formed from the posterior cerebral, posterior communicating internal carotid arteries, anterior cerebral, and anterior communicating artery.
• Cerebral aterial circle equalizes blood pressure in the brain.
• It provides collateral channels if one vessel becomes blocked.

Head and Neck Veins

• There are 3 primary veins:
  • The vertebral vein empties into the subclavian vein.
  • The external jugular vein drains the superficial head and neck and empties into the subclavian vein.
  • The internal jugular vein drains blood from the cranial cavity and joins the subclavian vein to form the brachiocephalic vein.
• In the cranial cavity, some blood drains to vertebral veins.
• Most blood drains through dural venous sinuses, which are large modified veins between 2 layers of dura mater.
  • These sinuses also receive excess cerebrospinal fluid, and primarily drain into internal jugular veins.

Thoracic & Abdominal Walls: Arteries

• The internal thoracic artery supplies the anterior thoracic wall and mammary gland and emerges from the subclavian artery.
  • Branches include 6 anterior intercostal arteries and the musculophrenic artery. -Musculophrenic artery gives rise to the anterior intercostal arteries 7-9.
  • The internal thoracic artery becomes the superior epigastric artery (superior abdominal wall).
• The inferior epigastric artery supplies the inferior abdominal wall and is a branch of the external iliac artery, which anastomoses with the superior epigastric artery.
• The supreme intercostal artery is a branch of the costocervical trunk and branches into the first and second posterior intercostal arteries.
• Posterior intercostal arteries 3-11 branch off the thoracic aorta. All posterior intercostal arteries anastomose with anterior intercostal arteries.
• Lumbar arteries supply the posterolateral abdominal wall, and 5-Pairs branch off the abdominal aorta.
• The median sacral artery supplies the sacrum and coccyx and is an unpaired artery extending from the bifurcation of the aorta in the pelvis.

Thoracic & Abdominal Walls: Veins

• The internal thoracic vein receives blood from anterior intercostal veins,
  • The internal thoracic vein receives blood from the musculophrenic and superior epigastric veins and drains into the brachiocephalic vein.
• The inferior epigastric vein merges with the external iliac vein.
• The first and second posterior intercostal veins merge with the supreme intercostal vein and drain into the brachiocephalic vein.
• The azygos system of veins drains blood from lumbar and posterior intercostal veins.
• The hemiazygos and accessory hemiazygos veins drain the left side and the azygos veins drains the right side veins, also draining the hemiazygos veins and draining them into the superior vena cava.

Thoracic Organs: Lungs

• The bronchial arteries supply the bronchi and bronchioles and are tiny branches off the descending thoracic aorta.
• The bronchial veins drain into the azygos and pulmonary veins.

Thoracic Organs: Esophagus

• Esophageal arteries are several small branches off the descending thoracic aorta, and some esophageal branches emerge off the left gastric artery.
• Esophageal veins drain into the azygos or left gastric vein.

Thoracic Organs: Diaphragm

• Superior phrenic arteries emerge from the descending thoracic aorta.
• Inferior phrenic arteries emerge from the descending abdominal aorta.
• The musculophrenic and pericardiacophrenic arteries arise from the internal thoracic artery.
  • The superior and inferior phrenic arteries drain to the inferior vena cava.
  • The musculophrenic vein drains to internal thoracic veins.

Gastrointestinal Tract: Arterial Supply

• The abdominal aorta provides arterial supply to the Gastrointestinal tract
• 3-Unpaired arteries serve the gut
  • Celiac trunk
  • Located just inferior to the diaphragm
  • Has 3 branches
  • Left gastric artery
  • part of stomach and esophagus
  • Splenic artery
  • Spleen and part of stomach / pancreas
  • Common hepatic artery
  • 2 branches - Hepatic artery proper
    • Liver, gallbladder and part of stomach - Gastroduodenal artery
    • Parts of stomach, duodenum and pancreas
  • Superior mesenteric artery
  • Immediately inferior to the celiac trunk
  • Branches
  • Intestinal arteries
  • Jejunum and ileum
  • Middle colic artery
  • Most of transverse colon
  • Right colic artery
  • Ascending colon
  • Ileocolic artery
  • Ileum, cecum and appendix
  • Inferior mesenteric artery
  • located just above bifurcation of the aorta
  • Branches
  • Left colic artery
  • Distal transverse, and descending colon
  • Sigmoid arteries
  • Descending and sigmoid colon
  • Superior rectal artery
  • Rectum

Gastrointestinal Tract: Venous Return

• Hepatic Portal system
• Blood from digestive organs sent to liver
• Digested nutrients are processed and harmful agents absorbed
• Blood comes from 3 main veins that drain to hepatic portal vein
• Splenic vein - Horizontally positioned
• Inferior Mesenteric vein - Vertically positioned
• Superior mesenteric vein - Vertically positioned on the right side of the body
• Hepatic portal vein flows into liver sinusoids
• Blood Leaves liver by Hepatic Veins draining to IVC.

Posterior Abdominal Organs

• Adrenal gland supply
• Middle suprarenal artery
• Kidney
• Renal artery
• Gonads
• Gonadal artery
• All 3 above branch off descending aorta and are drained by same named veins

Pelvis & Perineum: Arterial Supply

• R/L Common iliacs divide to internal and external iliac arteries
• Internal iliac primary arterial to pelvic region w/branches
• Superior and inferior Gluteal artery
• Superior vesical artery
• Middle rectal and Obturator artery
• Internal pudendal artery
• Vaginal and uterine artery
• Medial umbilical ligaments are Remnants of fetal umbilical arteries

Pelvis & Perineum: Venous Drainage

Pelvic and perennial veins have same names as arteries, merging with internal iliac, which merges w common iliac vein.

Upper Limb

• Subclavian Artery
• L Subclavian from Aortic arch
• R Subclavian off Brachiocephalic trunk.
• Axillary artery becomes Brachial after passing teres major Muscle
• Deep Brachial artery
• Branch off Brachial supplies most upper arm Muscles
• Radial and ulnar arteries division
• Anastomose and form 2 arterial arches
• Deep (radial), superficial(ulnar)
• Digital Arteries emerging from Arches to supply Fingers

Upper Limb

• Superficial venous drainage
• highly variable by individual
• Dorsal arch network drains through medial Basilic and lateral Cephalic vein
• drains into Axillary vein
• Median cubital joins Cephalic and Basilic(common venipuncture site)
• Deep Venous Drainage from Digital vessels > superficial palmar > Ulnar / radial, traveling parallel with named arteries.
  • Brachial travel with artery
  • merging ulnar and radial vv and merging to axillary.

Lower Limb

• Extern iliac artery = main supply, transitioning to Femoral a when passing under inguinal ligament, branching a Deep Femoral supplying many thigh m.
• transitions to popliteal a, supplying knee/muscles, then Anterior and posterior Tibial a"
• Anterior tibial a > Dorsalis Pedis, crossing ankle
  • posterior > fibular artery
  • Medial and lateral plantar a > plantar arch + digital arteries (toes)

Lower Limb: Superficial Veins

• the Dorsal Arch network Drains to great/ small saphenous, medial and lateral in origin.

• great saphenous extends whole surface and drains to femoral vein.

• small drains adjacent and into popliteal .

• Deeper vein system drains digital> Plantar> tibula

• popliteal vein formed from tibials, becomes Femoral when passing through thigh then, External illiac .