Blood Vessels, Structure & Types · Human Anatomy

Focus: Human Anatomy

Blood Vessels

Blood vessels are the body's plumbing: arteries carry blood away from the heart, capillaries exchange with the tissues, and veins return it. The wall is built three ways for three jobs. Dr. Sharilyn Rennie

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Part 1

Vessel Walls and Types

Three layers, five vessel types, and a wall matched to each job.

Part 1 · Overview

Blood vessels, an overview

Cardiovascular

Artery: carries blood away from the heart. Vein: carries blood toward the heart. Capillary: the smallest vessel, where exchange happens.
The lumen is the hollow channel blood flows through.
The vascular sequence: arteries -> arterioles -> capillaries -> venules -> veins.

Circulation pathway flow chart from the heart through elastic and muscular arteries, arterioles, capillaries, venules, and veins, back to the heart. — The vessels in order, color-coded: arteries (terra), capillaries (gold), veins (blue).

Part 1 · The wall

The three tunics

Cardiovascular

The three tunics of a vessel wall
Tunic	Position	Tissue
Tunica intima	Innermost	Endothelium on a basement membrane (the only layer a capillary has)
Tunica media	Middle	Smooth muscle and elastic fibers (thickest in arteries; sets the diameter)
Tunica externa	Outermost	Mostly collagen; anchors the vessel

OpenStax diagram and micrograph comparing an artery and a vein, showing the tunica intima, tunica media, and tunica externa. — **Artery vs vein, with the three tunics.** Image: OpenStax Anatomy & Physiology, CC BY 4.0.

Two more wall details: large vessels feed their own walls through tiny vasa vasorum (vessels of the vessels) and are controlled by nervi vasorum (nerves of the vessels). Larger arteries also show an internal elastic membrane (between intima and media) and an external elastic membrane (between media and externa), the wavy lines visible on the histology.

Part 1 · Arteries

Arteries, from elastic to arteriole

Cardiovascular

Elastic arteries (the aorta and its big branches): elastic-rich walls stretch with each heartbeat and recoil between beats to keep blood moving.
Muscular arteries: medium vessels that route blood to specific organs; their media is mostly smooth muscle.
Arterioles: the smallest arteries and the gatekeepers of flow into capillary beds.
Vasoconstriction (smooth muscle contracts, lumen narrows) and vasodilation (muscle relaxes, lumen widens) tune blood flow and pressure.

OpenStax diagram comparing the walls of an elastic artery, a muscular artery, and an arteriole. — **Elastic artery, muscular artery, and arteriole.** Image: OpenStax Anatomy & Physiology, CC BY 4.0.

Elastic arteries are also called conducting arteries; muscular arteries are distributing arteries; the smallest are the arterioles.

Part 1 · Capillaries

The three capillary types

Cardiovascular

Capillaries, leakier as the job demands
Type	Wall	Where
Continuous	Unbroken endothelium	Most common: muscle, skin, lungs, brain
Fenestrated	Small pores (fenestrations)	Fast absorption/filtration: kidney, small intestine
Sinusoid	Wide, with large gaps	Whole cells cross: liver, spleen, bone marrow

A capillary bed is fed by one arteriole; precapillary sphincters open or close it, and a vascular shunt lets blood bypass when no exchange is needed.

Diagrams of the three capillary types: continuous, fenestrated, and sinusoid. — **The three capillary types.** Image: OpenStax Anatomy & Physiology, CC BY 4.0.

Part 1 · Microcirculation

The capillary bed and precapillary sphincters

Vascular

A capillary bed is a network of 10 to 100 capillaries fed by a metarteriole that branches from a terminal arteriole.
Precapillary sphincters, rings of smooth muscle at each capillary entrance, open or close to control flow into the bed.
When tissue needs oxygen the sphincters open; when they close, blood runs through the thoroughfare channel straight to the venule, a vascular shunt that bypasses the bed.
An arteriovenous anastomosis connects the arteriole directly to the venule, also bypassing the capillaries.

If every capillary bed opened at once, the body could not fill them all, so the precapillary sphincters ration flow to the tissues that need it.

Diagram of a capillary bed: a metarteriole gives off capillaries guarded by precapillary sphincters, with a thoroughfare channel and an arteriovenous anastomosis bypassing the bed.

The capillary bed. Image: OpenStax Anatomy & Physiology, CC BY 4.0.

Part 1 · Veins

Veins and circulatory routes

Cardiovascular

Venules drain capillaries and merge into veins. Because venous pressure is low, veins have valves and depend on the skeletal-muscle pump to move blood toward the heart.
Anastomoses are connections between vessels that provide alternate routes; an arteriovenous anastomosis links an artery to a vein, bypassing capillaries.
Portal systems send blood through two capillary beds in series before returning to the heart (for example, the hepatic portal system).

OpenStax diagram comparing a large vein, a medium vein, and a venule. — **Large vein, medium vein, and venule.** Image: OpenStax Anatomy & Physiology, CC BY 4.0.

Part 2

When the Vessel Wall Fails

Most vessel disorders do one of three things: narrow the lumen, weaken the wall, or let blood pool.

Part 2 · Atherosclerosis

Atherosclerosis

Cardiovascular

Healthy vessel: a smooth endothelial lining and a wall that can stretch and recoil. A vascular disorder narrows the lumen, weakens the wall, or lets blood pool.
Atherosclerosis: fatty plaques build up within the walls of arteries. It is the root of heart attacks and strokes.
Endothelial injury: damage to the smooth tunica intima lining, the starting point of a plaque.
Atheroma (plaque): a deposit of fatty material, cells, and debris within the artery wall.
Narrowed lumen: as the plaque grows it bulges inward, narrowing the channel and reducing flow.
Arteriosclerosis: the general stiffening and thickening of artery walls; atherosclerosis is its most common form.
Thrombus: a clot that can form on a roughened plaque and suddenly block the vessel.

One wall change, many diseases. The same plaque starves the heart (heart attack) in a coronary artery, the brain (stroke) in a cerebral artery, and a leg (peripheral artery disease) in a limb artery.

Part 2 · Arteries

Arterial disorders

Cardiovascular

Common arterial disorders
Disorder	What it is	Why it matters
Aneurysm	a balloon-like bulge in a weakened artery wall	can rupture; common in the aorta and the arteries of the brain
Atherosclerosis	fatty plaque buildup that narrows an artery	the leading cause of coronary artery disease and stroke
Coronary artery disease	narrowing of the coronary arteries that supply the heart wall	can starve the myocardium and cause a heart attack
Peripheral artery disease	narrowing of the arteries of the limbs, usually the legs	causes pain on walking as the muscles are starved of blood
Hypertension	chronically high pressure within the arteries	strains the heart and damages vessel walls over time
Stroke	loss of blood supply to part of the brain, from a blocked or burst artery	brain tissue begins to die within minutes

Part 2 · Veins

Venous disorders

Cardiovascular

Common venous disorders
Disorder	What it is	Why it matters
Varicose veins	superficial veins that become twisted and swollen when their valves fail	blood pools and the veins bulge visibly, most often in the legs
Deep vein thrombosis	a blood clot in a deep vein, usually of the leg	the clot can break free and travel through the bloodstream
Pulmonary embolism	a clot, often from a deep vein, that lodges in an artery of the lung	a life-threatening emergency that blocks blood flow to the lung
Phlebitis	inflammation of a vein	causes pain, redness, and tenderness along the vein
Hemorrhoids	varicose veins of the rectal and anal region	a common cause of rectal discomfort and bleeding
Chronic venous insufficiency	long-term failure of the valves of the leg veins	persistent swelling and skin changes in the lower leg

Follow the clot. A deep vein thrombosis is dangerous for where it can go: break free, up the venae cavae, through the right heart, and into a lung artery as a pulmonary embolism. The route is just the normal venous return.

Part 3

The Fetal Detours

Before the lungs and liver are in use, the fetus gets its oxygen from the placenta and routes blood around the organs not yet working.

Part 3 · Overview

Fetal circulation, an overview

Cardiovascular

Fetal circulation: the circulatory plan of a fetus, which gets its oxygen from the placenta rather than from its own lungs.
The placenta is where fetal blood picks up oxygen and nutrients from the mother and gives up wastes. The umbilical cord carries the umbilical vessels between fetus and placenta.
A shunt is a special vessel or opening that routes blood around an organ. The fetus uses three shunts to bypass the liver and the lungs.
The lungs are bypassed (collapsed, not used for gas exchange). The liver is largely bypassed, since the placenta does the filtering.

Part 3 · Shunts

The fetal vessels and shunts

Cardiovascular

The five fetal vessels and shunts
Structure	What it is	What it does
Umbilical vein	a vein in the umbilical cord	carries oxygen-rich blood from the placenta to the fetus
Ductus venosus	a shunt passing through the fetal liver	lets most blood bypass the liver and flow toward the inferior vena cava
Foramen ovale	an opening in the interatrial septum	lets blood pass from the right atrium to the left atrium, bypassing the lungs
Ductus arteriosus	a short vessel between the pulmonary trunk and the aorta	shunts blood from the pulmonary trunk into the aorta, bypassing the lungs
Umbilical arteries	two arteries in the umbilical cord	carry oxygen-poor blood from the fetus back to the placenta

Part 3 · After birth

The shunts after birth

Cardiovascular

Each fetal structure and its adult remnant
Fetal structure	Adult remnant	Note
Umbilical vein	ligamentum teres (round ligament of the liver)	runs in the free edge of the falciform ligament
Ductus venosus	ligamentum venosum	a fibrous cord on the inferior surface of the liver
Foramen ovale	fossa ovalis	a shallow oval depression in the interatrial septum
Ductus arteriosus	ligamentum arteriosum	a short cord between the pulmonary trunk and the aorta
Umbilical arteries	medial umbilical ligaments	paired cords on the inner surface of the anterior abdominal wall

When a detour stays open. A ductus arteriosus that fails to seal is a patent ductus arteriosus; a foramen ovale that stays open is a patent foramen ovale.

Part 4

Same Name, Three Jobs

Comparing artery, capillary, and vein side by side shows how wall structure follows function.

Part 4 · Compare & contrast

Artery vs capillary vs vein

Synthesis

Comparing the three vessel types
Feature	Artery	Capillary	Vein
Direction	Away from the heart	Between artery and vein	Toward the heart
Wall	Thick; muscular/elastic media	One layer (endothelium only)	Thin; wider lumen
Pressure	High	Low and falling	Low
Valves?	No	No	Yes (against backflow)
Job	Carry blood under pressure	Exchange with tissue	Return blood; act as a reservoir

Blood is drawn from veins, not arteries: a vein is superficial, thin-walled, wide, and low-pressure, so it is easy to enter and slow to bleed.

Part 5

Regional Vessels: Head & Neck, Upper Limb, Thorax

Trace the major arteries and veins of the upper body as branching maps. Tap any chart to enlarge.

Part 5 · Arteries

The aortic arch and its branches

Vascular

Aortic arch branching into the brachiocephalic trunk, left common carotid, and left subclavian arteries. — **Aortic arch:** brachiocephalic trunk (to the right side), left common carotid, and left subclavian. Carotids feed the head; subclavians feed the arms.

Part 5 · Arteries

The external carotid artery

Vascular

Part 5 · Arteries

Arteries of the upper limb

Vascular

Subclavian artery continuing as axillary, brachial, then radial and ulnar arteries to the palmar arches. — **Upper limb:** subclavian to axillary to brachial, then radial and ulnar to the deep and superficial palmar arches.

Part 5 · Veins

Veins draining to the superior vena cava

Vascular

Veins of the head, neck, and upper limb draining through the brachiocephalic veins to the superior vena cava. — **SVC:** internal jugular (head) and subclavian (arm) join as the brachiocephalic veins; the azygos drains the thoracic wall.

Part 6

Regional Vessels: Abdomen and Lower Limb

The abdominal aorta, the gut and portal drainage, and the vessels of the leg.

Part 6 · Arteries

The abdominal aorta and its branches

Vascular

Abdominal aorta giving the celiac trunk, superior mesenteric, renal, gonadal, and inferior mesenteric arteries, then the common iliacs. — **Abdominal aorta:** celiac trunk (foregut), SMA (midgut), IMA (hindgut), plus paired renal and gonadal arteries, then splits into the common iliacs.

Part 6 · Arteries

Arteries of the lower limb

Vascular

External iliac continuing as femoral, popliteal, then anterior and posterior tibial arteries to the foot. — **Lower limb:** external iliac to femoral to popliteal, then anterior tibial (dorsalis pedis) and posterior tibial (fibular and plantar arch).

Part 6 · Veins

The hepatic portal system

Vascular

Splenic and superior mesenteric veins forming the hepatic portal vein that carries gut blood to the liver. — **Portal system:** the splenic and superior mesenteric veins form the hepatic portal vein, carrying nutrient-rich gut blood through the liver before it reaches the IVC.

Part 6 · Veins

The inferior vena cava and lower limb veins

Vascular

Inferior vena cava receiving hepatic, renal, and common iliac veins, with the lower limb veins draining upward. — **IVC:** collects hepatic, renal, and common iliac blood; the leg drains tibial to popliteal to femoral, with the great saphenous as the superficial route.

Part 7

Regional Vessels: Pelvis, Renal, and Reproductive

The internal iliac branches and the urogenital blood supply and drainage.

Part 7 · Arteries

The internal iliac artery

Vascular

Internal iliac artery dividing into anterior and posterior divisions supplying the pelvic organs, perineum, and gluteal region. — **Internal iliac:** the anterior division supplies the bladder, rectum, genitals, and uterus; the posterior division supplies the pelvic wall and gluteal region.

Part 7 · Arteries

Renal and gonadal arteries

Vascular

Renal arteries to the kidneys and gonadal arteries to the testis or ovary, both arising from the abdominal aorta. — **Urogenital supply:** the renal arteries feed the kidneys; the gonadal arteries (testicular or ovarian) arise directly from the aorta.

Part 7 · Veins

Renal and gonadal venous drainage

Vascular

Venous drainage showing the right gonadal vein to the IVC and the left gonadal vein to the left renal vein. — **Key asymmetry:** the right gonadal vein drains straight into the IVC, while the left gonadal vein drains into the left renal vein first.

Wrap-up

Key takeaways

Summary

Three tunics: intima (endothelium, the only capillary layer), media (smooth muscle/elastic, sets diameter), externa (collagen anchor).
Sequence: artery -> arteriole -> capillary -> venule -> vein.
Arteries: elastic, muscular, arteriole; capillaries: continuous, fenestrated, sinusoid.
Veins run low-pressure, so they have valves and use the muscle pump.
Same vessel name, built three ways, does three different jobs.

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