Cardiovascular System: Blood Vessels – Structure, Types, and Regional Vasculature

Cardiovascular System: Blood Vessels

Overview of Blood Vessels

The cardiovascular system consists of the heart and a network of blood vessels that transport blood throughout the body. Blood vessels are classified based on their structure and function, forming a closed circuit for blood flow.

• Arteries: Carry blood away from the heart.

• Arterioles: The smallest arteries, regulate blood flow into capillary beds.

• Capillaries: The smallest blood vessels, site of exchange between blood and tissues.

• Venules: Receive blood from capillaries.

• Veins: Return blood to the heart from venules.

Structure of Blood Vessels

Blood vessels are composed of three main layers (tunics), each with distinct functions:

• Tunica intima: Innermost layer containing the endothelium (simple squamous epithelium). Vessels larger than 1 mm also have a subendothelial layer of loose connective tissue.

• Tunica media: Middle layer made of smooth muscle.

  • Contraction causes vasoconstriction (narrowing of the vessel).

  • Relaxation causes vasodilation (widening of the vessel).

• Tunica externa (adventitia): Outermost layer of connective tissue.

  • Protects, strengthens, and anchors the vessel to surrounding structures.

• Vasa vasorum: Small vessels that supply and drain large arteries or veins.

Types of Arteries

Elastic Arteries

Elastic arteries are the largest arteries, also called conducting arteries. They have high elastin content in the tunica media, which helps dampen the surge of blood pressure from the heart.

• Diameter: 1 cm to 2.5 cm

• Includes the aorta and its major branches

Muscular (Distributing) Arteries

Muscular arteries lie distal to elastic arteries and distribute blood to specific organs.

• Diameter: 0.3 cm to 1 mm

• Includes most named arteries

• Tunica media contains thick smooth muscle, internal and external elastic membranes

Arterioles

Arterioles are the smallest arteries, controlling blood flow into capillary beds.

• Diameter: 10 μm to 0.3 mm

• Larger arterioles possess all three tunics; smaller arterioles have a single layer of smooth muscle and endothelium

• Diameter regulated by local tissue factors and the sympathetic nervous system

Capillaries

Capillaries are microscopic vessels that facilitate exchange between blood and tissues.

• Diameter: 8 to 10 μm

• Single layer of endothelium

• Renew and refresh surrounding tissue fluid

• Site-specific functions:

  • Lungs: Oxygen enters blood, carbon dioxide leaves

  • Small intestines: Receive digested nutrients

  • Endocrine glands: Pick up hormones

  • Kidneys: Removal of nitrogenous wastes

Types of Capillaries

• Continuous Capillaries: Most common type, found in most organs.

  • Tight junctions and desmosomes join epithelial cells

  • Intercellular clefts (gaps) allow small molecules to pass in and out

• Fenestrated Capillaries: Have pores in their endothelium, allowing high rates of exchange.

  • Found in small intestine, glomeruli of kidneys, endocrine glands

• Sinusoid Capillaries: Wide, leaky, and usually fenestrated with large intercellular clefts.

  • Few cell junctions

  • Found in bone marrow, spleen, liver

Capillary Beds

Capillary beds are networks of capillaries running through tissues, facilitating exchange and distribution of blood.

• Arterioles feed the bed

• Venules drain the bed

• Most tissues and organs have rich capillary supplies, except:

  • Tendons and ligaments (poorly vascularized)

  • Epithelia and cartilage (avascular; receive nutrients from nearby connective tissue)

Veins

Veins return blood to the heart and have structural differences compared to arteries.

• Thinner walls than comparable arteries

• Thicker tunica externa

• Less elastin in walls

• Larger lumen than arteries of the same size

• Contain about 65% of the body's blood at any time

• Valves in some veins (especially in limbs) prevent backflow of blood

• Skeletal muscle pump: Muscles press against thin-walled veins to force blood flow

Varicose Veins and Thrombosis

Veins can be affected by pathological conditions:

• Varicose veins: Abnormally swollen, twisted veins due to loss of elasticity and valve dysfunction

• Thrombosis: Local coagulation or clotting of blood in a part of the circulatory system

• Deep venous thrombosis: Clot formation in deep veins of the lower limb, causing swelling, warmth, and erythema

• Venous stasis: Stagnation of blood flow, a major cause of thrombus formation

Pulmonary and Systemic Circulation

The cardiovascular system is divided into two main circuits:

• Pulmonary circulation: From the right ventricle through the lungs to the left atrium.

  • Carbon dioxide is exchanged for oxygen in the capillaries of the lungs

• Systemic circulation: From the left ventricle to the right atrium.

  • Oxygen and nutrients are exchanged for carbon dioxide in the body's capillaries

Pulmonary Circulation

• Pulmonary trunk: Exits from the right ventricle, divides into right and left pulmonary arteries

• Vessels have thinner walls than systemic vessels

• Maximum arterial pressure is lower in the pulmonary circuit

• Left and right pairs of superior and inferior pulmonary veins carry oxygenated blood into the left atrium

Systemic Circulation

• Blood supply and drainage to/from:

  • Great vessels

  • Head and neck

  • Thoracic and abdominal wall

  • Pelvis

  • Upper and lower limbs

  • Abdominal and thoracic viscera

Great Vessels

Aorta

• Ascending aorta: Arises from the left ventricle; branches into right and left coronary arteries

• Aortic arch: Lies posterior to the manubrium; branches into brachiocephalic trunk, left common carotid, and left subclavian artery

• Descending aorta: Continues from the aortic arch

  • Thoracic aorta: T5–T12 region

  • Abdominal aorta: Ends at L4, divides into left and right common iliac arteries

Abdominal Aortic Aneurysm

• Aneurysm: Bulging, weakened area in the wall of a blood vessel

• Acute rupture of an abdominal aortic aneurysm has a mortality rate of nearly 90% due to heavy blood loss

Venae Cavae

• Superior vena cava: Returns blood from body regions superior to the diaphragm

• Inferior vena cava: Returns blood from body regions inferior to the diaphragm

Major Veins

• Brachiocephalic veins: Branch into internal jugular and subclavian veins

• Common iliac veins: Branch into right/left external and internal iliac veins

Regional Vasculature

Head and Neck

• Common carotid arteries: Located in the anterior triangle of the neck; split at the level of the "Adam's apple" into external and internal carotid arteries

• Internal carotid artery: Enters cranial cavity via carotid canal; supplies orbits and most of the cerebrum

  • Branches: Ophthalmic, supraorbital, anterior cerebral, middle cerebral arteries

• Subclavian artery branches:

  • Vertebral arteries: Ascend through transverse foramina of C1–C6, enter skull via foramen magnum, supply vertebrae, spinal cord, brainstem, occipital lobes

  • Thyrocervical trunk: Supplies cervical vertebrae, spinal cord, thyroid gland

  • Costocervical trunk: Supplies deep neck muscles, intercostal spaces

• Veins:

  • Internal jugular veins: Drain blood from brain, face, scalp

  • External jugular veins: Drain scalp, empty into subclavian veins

Upper Limb

• Arteries:

  • Axillary artery: Gives rise to thoracoacromial, lateral thoracic, subscapular, anterior/posterior circumflex humeral arteries

  • Brachial artery: Divides into radial and ulnar arteries, which form palmar arches and digital arteries

• Veins:

  • Deep veins: Axillary, brachial, radial, ulnar veins (follow companion arteries)

  • Superficial veins: Cephalic (drains into subclavian), basilic (drains into axillary), median cubital (used for blood draw), median vein of forearm

Thoracic Wall

• Arteries:

  • Internal thoracic arteries: Supply anterior thoracic wall

  • Anterior intercostal arteries: Branches of internal thoracic arteries

  • Posterior intercostal arteries: Superior pairs from costocervical trunk, inferior pairs from thoracic aorta

• Veins:

  • Posterior intercostal veins: Drain into azygos (right), hemiazygos (left inferior), accessory hemiazygos (left superior)

  • Anterior intercostal veins: Tributaries of internal thoracic veins

Diaphragm

• Thoracic/superior surface: Supplied by superior phrenic arteries (drained by superior phrenic veins)

• Abdominal/inferior surface: Supplied by inferior phrenic arteries (drained by inferior phrenic veins)

Abdominal Wall

• Superior epigastric artery: Continuation of internal thoracic artery

• Inferior epigastric artery: Arises from external iliac artery

• Lumbar arteries

• Deeper veins accompany arteries and bear the same names

Pelvis

• Internal iliac arteries:

  • Superior gluteal artery: Supplies gluteal muscles

  • Inferior gluteal artery: Supplies muscles and skin of buttocks, posterior thigh

  • Internal pudendal artery: Supplies perineum and external genitalia

  • Umbilical artery

• Internal iliac veins: Share names with arteries (except no vein for umbilical arteries)

Lower Limbs

• External iliac artery passes through inguinal canal, becomes femoral artery

• Femoral artery: Relatively superficial, used to take pulse and apply pressure to stop bleeding

• Popliteal artery: Supplies knee joint

• Anterior tibial artery: Supplies anterior compartment of leg

• Posterior tibial artery: Supplies posterior compartment of leg

• Deep veins: Anterior tibial, popliteal, femoral veins

• Superficial veins: Great saphenous (empties into femoral), small saphenous (empties into popliteal)

Thoracic Viscera

• Heart:

  • Coronary arteries: Arise from base of aorta, supply heart wall

  • Cardiac veins: Drain deoxygenated blood from heart wall to right atrium via coronary sinus

• Lungs:

  • Pulmonary arteries: Carry deoxygenated blood to lungs

  • Pulmonary veins: Carry oxygenated blood to left atrium

  • Bronchial arteries/veins: Supply and drain supporting tissues of lungs

Abdominal Viscera

• Posterior body wall: Inferior phrenic, suprarenal, renal, gonadal arteries

• Gut organs:

  • Celiac trunk: Supplies foregut (left gastric, splenic, common hepatic arteries)

  • Superior mesenteric artery: Supplies midgut

  • Inferior mesenteric artery: Supplies hindgut

• Veins:

  • Common iliac veins, left gonadal vein drains into left renal vein

• Capillary beds regulate blood flow to tissues

Hepatic Portal System

The hepatic portal system is a specialized circuit for digestion, transporting nutrients from the gastrointestinal tract to the liver for processing.

• Hepatic portal vein: Formed by superior mesenteric and splenic veins, receives blood from stomach, intestines, spleen, pancreas

• Blood passes through liver sinusoids before entering hepatic veins and returning to systemic circulation

Pelvic Viscera

• Blood supply mainly by internal iliac arteries

• Blood drainage mainly by internal iliac veins

Summary Table: Types of Blood Vessels

Type	Structure	Function	Location
Elastic Arteries	Thick tunica media, high elastin	Conduct blood, dampen pressure	Aorta, major branches
Muscular Arteries	Thick smooth muscle, elastic membranes	Distribute blood to organs	Named arteries
Arterioles	Small, variable tunics	Regulate blood flow to capillaries	Connect arteries to capillaries
Capillaries	Single layer endothelium	Exchange of gases, nutrients, wastes	All tissues except cartilage, epithelia
Venules	Thin walls	Collect blood from capillaries	Connect capillaries to veins
Veins	Thin walls, large lumen, valves	Return blood to heart	Throughout body

Key Equations

• Blood Flow Equation: $Q = \frac{\Delta P}{R}$ Where $Q$ = blood flow, $\Delta P$ = pressure difference, $R$ = resistance

• Poiseuille's Law (for laminar flow): $Q = \frac{\pi r^4 \Delta P}{8 \eta l}$ Where $r$ = vessel radius, $\eta$ = blood viscosity, $l$ = vessel length

Example

Clinical Application: The femoral artery is commonly used to take a pulse and can be compressed to control bleeding in the lower limb.

Additional info: The hepatic portal system is unique in that it contains two capillary beds in series, allowing for nutrient processing before blood returns to the heart.