Blood Vessel Anatomy, Circulation, and Major Artery Pathways

General structure of a blood vessel

A tube that carries blood. The lumen is the open space containing blood, and the tunics make up the wall.

Three tunics of a blood vessel

Outside → inside: adventitia → media → intima. Capillaries only have the intima.

Tunica adventitia

Outermost connective tissue layer. Protects and anchors the vessel to nearby structures.

Tunica media

Middle smooth muscle layer. Changes vessel diameter to control blood flow and pressure.

Vasodilation

Smooth muscle relaxes → vessel gets wider → lumen increases.

Vasoconstriction

Smooth muscle contracts → vessel gets narrower → lumen decreases.

Tunica intima

Innermost layer touching the blood. Made of simple squamous epithelium and continuous with the endocardium.

Tunica intima functions

Lines vessels, allows exchange in capillaries, and helps with clotting, vessel repair, and vessel diameter.

Artery

Carries blood away from the heart. Has 3 tunics and thick, elastic walls for high pressure.

Why arteries contain elastin

Lets them stretch when high-pressure blood enters and recoil afterward.

Capillary

Tiny, one-layer vessel where substances are exchanged between blood and tissues.

Why capillaries are good for exchange

Thin + permeable + large surface area + slow blood flow = easy exchange.

Vein

Carries blood toward the heart. Thinner wall, larger lumen, less muscle and elastin. Lower-limb veins have valves.

Artery vs. vein vs. capillary

Artery: away, high pressure, thick. Vein: toward, low pressure, thin. Capillary: exchange, one layer.

What the vascular system transports

Gases, nutrients, wastes, and hormones throughout the body.

Pulmonary circuit

Heart → lungs → heart for O₂ and CO₂ exchange.

Systemic circuit

Heart → body tissues → heart.

Blood vessel flow sequence

Arteries → arterioles → capillaries → venules → veins.

Arteriole

Small vessel between an artery and capillaries. Controls how much blood enters tissues.

Precapillary sphincter

Smooth muscle at the entrance to a capillary bed that opens or closes to control blood flow.

Arteriole diameter regulation

Local factors like NO, O₂, and CO₂ plus sympathetic nerves control the smooth muscle.

Sympathetic effect on blood vessels

Constricts vessels to abdominal organs while blood flow to the heart and skeletal muscle increases.

Vasa vasorum

Small vessels that supply the walls of large and medium arteries.

Capillary size

About 8 μm wide, so RBCs pass through about one at a time.

Continuous capillary

Tightest/least permeable type. Found mainly in muscle, connective tissue, and nervous system.

Fenestrated capillary

Has small pores for faster exchange. Found in kidneys, intestines, and endocrine glands.

Sinusoidal capillary

Has large gaps that allow large proteins and cells through. Found in red marrow, liver, and spleen.

Capillary types by increasing permeability

Continuous → fenestrated → sinusoidal.

Venule

Small vessel that collects blood from capillaries and carries it toward veins.

Venous system as a blood reservoir

Veins stretch easily and hold about 60% of the body's blood.

Venoconstriction

Sympathetic stimulation narrows veins, pushing stored blood toward other parts of the body.

Venous valve

Flap that prevents backflow and helps blood move toward the heart.

Varicose veins

Dilated, deformed veins associated with poor valves and increased venous pressure.

Subclavian artery

is the major upper-limb artery located under the clavicle. As it enters the axilla, it becomes the axillary artery.

Axillary artery

is the continuation of the subclavian artery as it passes through the axilla, or armpit. It continues into the arm as the brachial artery.

Brachial artery

is the major artery of the arm. Just past the elbow, it divides into the radial and ulnar arteries.

Common iliac arteries

The right and left common iliac arteries are formed when the descending aorta bifurcates at L4. Each common iliac then divides into an internal and external iliac artery.

Femoral artery

is the major artery of the thigh and is the continuation of the external iliac artery into the lower limb. It continues toward the back of the knee.

Popliteal artery

is the continuation of the femoral artery as it passes through the popliteal fossa behind the knee. It later divides into the anterior and posterior tibial arteries.

Anterior and posterior tibial arteries

are the major arteries of the leg that branch from the popliteal artery.

Lower-limb vein order

Venous blood from the lower limb returns toward the heart in this order:Plantar arches → tibial veins → popliteal → femoral → external iliac → common iliac → IVC.

Aorta

Main systemic vessel leaving the heart. Gives rise to the body's systemic arteries.

Coronary arteries

First branches of the aorta; supply the heart itself.

Aortic arch branches

Brachiocephalic trunk → left common carotid → left subclavian.

Brachiocephalic trunk

First aortic arch branch. Splits into right common carotid + right subclavian.

Brain arterial supply

The brain receives arterial blood from two paired routes: the right and left internal carotid arteries and the right and left vertebral arteries.

Descending aorta

Continuation of the aorta through the thorax and abdomen.

Major branches of the abdominal aorta

Major abdominal aorta branches emphasized by Costa include the celiac trunk, superior mesenteric artery, inferior mesenteric artery, renal arteries, and gonadal arteries.

Celiac trunk branches

The celiac trunk is a branch of the abdominal aorta that divides into the left gastric, splenic, and common hepatic arteries.

Deep upper-limb veins

Deep upper-limb veins generally have the same names as the arteries they accompany. Examples include the radial, ulnar, and brachial veins.

Superficial upper-limb veins

The cephalic and basilic veins are major superficial veins of the upper limb. Costa notes that superficial veins are larger and fewer in number than deep veins.

Median cubital vein

Superficial vein at the elbow connecting cephalic + basilic. Common site for venipuncture.

Anastomosis

Connection between vessels that can provide an alternate route for blood.

Dural venous sinuses

Collect blood from cerebral veins → eventually drain into the internal jugular. No valves.

Head and neck venous return

The internal jugular and subclavian veins join to form the brachiocephalic veins. The right and left brachiocephalic veins then drain into the superior vena cava.

Azygos vein

Provides an alternate venous route back to the heart for blood from the trunk/lower limb.

Great saphenous vein

Superficial lower-limb vein that can be used as a graft for coronary bypass surgery.

Pulmonary circuit chamber and vessel order

RA → RV → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → LA.

Portal system

Blood passes through 2 capillary beds in a row before returning to the heart.

Three portal systems Costa lists

Costa lists the hepatic portal system from digestive organs to liver, hypophyseal portal system from hypothalamus to pituitary, and renal system from glomerulus to peritubular capillaries.

Systolic vs. diastolic pressure

Systolic pressure is arterial pressure when the ventricles contract. Diastolic pressure is arterial pressure when the ventricles relax. Blood pressure is written systolic/diastolic in mm Hg.

Where a pulse can be felt

A pulse can be felt where an artery runs near the body surface or over bone or firm tissue, making the pressure wave easier to detect.

Major limb pulse points

Major limb pulse points include the radial artery at the wrist, brachial at the elbow, popliteal behind the knee, posterior tibial behind the medial malleolus, and dorsalis pedis on the dorsum of the foot.

Arterial pressure points

Arterial pressure points are superficial arterial locations where an artery can be compressed against bone or other firm tissue to help stop arterial bleeding.