Difficulty: Intermediate | Prerequisites: Heart anatomy (Ch. 19), basic tissue types
Tags: blood vessels, arteries, veins, capillaries, tunics, vasa vasorum, vasodilation, vasoconstriction, capillary beds, fenestrated capillaries, sinusoids, hepatic portal system, pulmonary circuit vessels, systemic circuit vessels, venous valves, APK2100c
Now that you know the heart's structure, this chapter maps out the plumbing: the arteries, veins, and capillaries that carry blood to and from every tissue. You need to understand the wall structure of each vessel type, why those structural differences matter for function, and the names and locations of the major vessels in both the pulmonary and systemic circuits. The hepatic portal system is a particular exam favourite because it is an unusual arrangement with two capillary beds in series. If Ch. 19 was the pump, Ch. 20 is the piping.
All blood vessels have walls built from up to three tunics (intima, media, externa), but the proportions differ hugely between arteries, veins, and capillaries. Arteries handle high-pressure flow and have thick muscular or elastic walls. Veins return blood at low pressure and rely on valves and skeletal muscle pumps to fight gravity. Capillaries are the exchange sites, only one cell layer thick, and come in three varieties depending on how permeable the tissue needs to be.
Tunica intima (tunica interna)
The innermost layer of a blood vessel wall, consisting of an endothelial lining, a thin basement membrane, and (in arteries) an internal elastic lamina.
Think of it as the non-stick lining that contacts the blood directly.
Tunica media
The middle layer, composed primarily of smooth muscle and elastic fibres. It is the thickest layer in arteries and is responsible for vasoconstriction and vasodilation.
Tunica externa (tunica adventitia)
The outermost layer of connective tissue (mostly collagen). It anchors the vessel to surrounding structures. It is the thickest layer in veins.
Vasa vasorum
Literally "vessels of the vessels." Small blood vessels in the tunica externa that supply oxygen and nutrients to the walls of large arteries and veins, because the walls are too thick to be nourished by diffusion from the lumen alone.
Vasoconstriction
Contraction of smooth muscle in the tunica media, narrowing the vessel lumen and reducing blood flow.
Vasodilation
Relaxation of smooth muscle in the tunica media, widening the vessel lumen and increasing blood flow.
Metarteriole
A vessel that connects an arteriole to a capillary bed. It has scattered smooth muscle cells, and the precapillary sphincters at the junction of true capillaries regulate whether blood enters the capillary bed or bypasses it through a thoroughfare channel.
Precapillary sphincter
A ring of smooth muscle at the entrance to a true capillary. When it contracts, blood bypasses that capillary; when it relaxes, blood flows through.
Continuous capillary
The most common type. Endothelial cells form a complete, unbroken lining joined by tight junctions. Found in skin, muscle, lungs, and the brain (where they form the blood-brain barrier). The least permeable type.
Fenestrated capillary
Endothelial cells contain small pores (fenestrations) that increase permeability. Found where active absorption or filtration occurs: kidneys, small intestine, endocrine glands, choroid plexus.
Sinusoid (discontinuous capillary)
The most permeable type. Large, irregularly shaped gaps between endothelial cells and an incomplete or absent basement membrane. Found in the liver, spleen, red bone marrow, and some endocrine organs. They allow large molecules and even whole cells to pass.
Venous valve
A fold of the tunica intima that projects into the lumen of a vein to prevent backflow. Found predominantly in the limb veins, where blood must return against gravity.
Vascular anastomosis
A connection (or merger) between two blood vessels, providing alternative routes for blood to reach a tissue if one vessel is blocked.
Hepatic portal system
A special circulatory route in which blood passes through two capillary beds in series: first in the digestive organs (where nutrients are absorbed), then through the portal vein to a second capillary bed (sinusoids) in the liver for processing, before returning to the heart via the hepatic veins.
Arteries:
Thick tunica media (the dominant layer) with abundant smooth muscle and elastic fibres
Well-defined internal and external elastic laminae
Small, round lumen relative to wall thickness
Elastic arteries (aorta, pulmonary trunk, common carotids, subclavians) have more elastin for absorbing pressure surges
Muscular arteries (most named arteries) have more smooth muscle for regulating flow distribution
Veins:
Thinner walls overall; the tunica externa is often the thickest layer
Larger, more irregular lumen
Less smooth muscle and elastin; they are distensible and act as blood reservoirs (roughly 60% of blood volume is in the veins at any time)
Many contain valves
Capillaries:
Walls are just a single layer of endothelium on a basement membrane (essentially tunica intima only)
No tunica media or tunica externa
Diameter is so small that red blood cells pass through single file
This thinness is what makes exchange of gases, nutrients, and wastes possible
Smooth muscle in the tunica media of arterioles is the primary controller of blood flow into a capillary bed
Sympathetic stimulation causes vasoconstriction, diverting blood away from that bed
Local metabolic signals (low O₂, high CO₂, histamine) cause vasodilation, increasing blood flow to active tissue
Precapillary sphincters open and close to regulate flow through individual capillaries within a bed
The vessel feeding into a capillary bed is an arteriole (the terminal branch of an artery)
The vessel draining out of a capillary bed is a venule (which merges into progressively larger veins)
Permeability depends on how tightly the endothelial cells are joined and whether fenestrations or gaps are present
Continuous capillaries: tight junctions; least permeable; found in muscle, skin, lungs, CNS
In the brain, these form the blood-brain barrier with especially tight junctions and astrocyte reinforcement
Fenestrated capillaries: pores in endothelial cells; moderately permeable; found in kidneys (glomeruli), intestinal villi, endocrine glands
Sinusoids: large gaps, incomplete basement membrane; most permeable; found in liver, spleen, red bone marrow
Liver sinusoids allow hepatocytes direct access to blood contents for detoxification and metabolism
Splenic and bone marrow sinusoids allow blood cells to enter and exit the bloodstream
Veins that contain valves: most medium and large veins of the limbs (great and small saphenous veins, femoral vein, popliteal vein, veins of the upper limb)
Veins that do NOT contain valves: venae cavae (SVC and IVC), veins of the thorax, most veins of the head and neck, and veins of the abdominal viscera
These either benefit from gravity, respiratory pressure changes, or smooth muscle tone instead
Skeletal muscle pump: contraction of surrounding skeletal muscles squeezes the veins, pushing blood upward past one-way valves. When the muscles relax, the valves prevent backflow.
Respiratory pump: during inhalation, decreased thoracic pressure and increased abdominal pressure push blood from abdominal veins toward the heart. During exhalation, the gradient reverses slightly, but venous valves keep blood from falling back.
Anastomoses are merging points where two or more vessels supply the same region, creating redundancy
Common locations:
Around joints (e.g., knee, elbow) where movement could kink a vessel
In the brain (Circle of Willis)
Coronary circulation (limited, which is why coronary artery blockages are so dangerous)
Mesenteric circulation (supplying the gut)
Palmar and plantar arches of the hand and foot
Arteries: pulmonary trunk → splits into right and left pulmonary arteries → carry deoxygenated blood to each lung
Capillaries: pulmonary capillaries surrounding the alveoli (continuous capillaries, very thin for gas exchange)
Veins: four pulmonary veins (two from each lung) → carry oxygenated blood back to the left atrium
This list is extensive and follows what is typically covered in lecture. Key vessels include:
Arterial (from aorta):
Ascending aorta → aortic arch (brachiocephalic trunk, left common carotid, left subclavian) → descending aorta (thoracic then abdominal)
Brachiocephalic trunk → right common carotid + right subclavian
Common carotids → internal and external carotids (head and neck)
Subclavians → axillary → brachial → radial and ulnar (upper limb)
Abdominal aorta → celiac trunk, superior mesenteric, renal, gonadal, inferior mesenteric (abdominal organs)
Abdominal aorta → common iliacs → internal and external iliacs → femoral → popliteal → anterior and posterior tibial (lower limb)
Venous (returning to heart):
Head: internal jugular veins (join subclavian to form brachiocephalic veins → SVC)
Upper limb: radial/ulnar → brachial → axillary → subclavian
Superficial upper limb: cephalic vein, basilic vein, median cubital vein (common blood draw site)
Lower limb: anterior/posterior tibial → popliteal → femoral → external iliac → common iliac → IVC
Superficial lower limb: great saphenous vein (longest vein in the body), small saphenous vein
Abdominal organs: hepatic veins → IVC; renal veins → IVC; gonadal veins → IVC (right) or left renal vein (left)
This is a portal system, meaning blood passes through two capillary beds before returning to the heart:
First capillary bed: in the walls of the GI tract (stomach, intestines, spleen, pancreas), where nutrients, toxins, and other absorbed substances enter the blood
Hepatic portal vein: collects this nutrient-rich, deoxygenated blood and carries it to the liver
Major tributaries: superior mesenteric vein, splenic vein (these two join to form the portal vein), inferior mesenteric vein (drains into the splenic vein)
Second capillary bed: liver sinusoids, where hepatocytes process nutrients, detoxify substances, and produce bile
Hepatic veins: drain processed blood from the liver into the IVC
The capillary types involved are fenestrated capillaries in the intestinal villi and sinusoids in the liver.
The name of the vessel often tells you its territory:
Renal artery → kidney; gonadal artery → ovary or testis
Superior mesenteric → most of the small intestine and proximal colon
Celiac trunk → stomach, liver, spleen, proximal duodenum
Internal iliac → pelvic organs; external iliac → lower limb
If a named artery is blocked, the tissue it supplies is at risk of ischaemia unless an anastomosis provides an alternative route
Varicose veins develop when venous valves in the lower limbs fail, causing blood to pool and the vein walls to stretch. This is the same valve mechanism tested in the exam. The hepatic portal system is clinically important because drugs absorbed in the intestine are processed by the liver before reaching the general circulation, a phenomenon called "first-pass metabolism" that pharmacologists must account for when dosing oral medications.
Students often assume arteries always carry oxygenated blood and veins always carry deoxygenated blood. The pulmonary arteries carry deoxygenated blood, and the pulmonary veins carry oxygenated blood. Arteries are defined by direction (away from the heart), not oxygen content.
The tunica externa is not always the thinnest layer. In veins, the externa is often the thickest layer, whereas in arteries the media dominates.
Capillaries do not have a tunica media. Students sometimes describe capillary "smooth muscle"; what they are thinking of is the precapillary sphincter, which is on the arteriole side, not the capillary itself.
The hepatic portal vein is a vein, not an artery, even though it carries blood toward an organ (the liver). Portal veins connect two capillary beds.
⚠️ Be able to name all three tunics and state which is thickest in arteries vs. veins.
⚠️ Know the three types of capillaries, their structural differences, and where each is found.
⚠️ The hepatic portal system: name the vessels, the two capillary beds, and the capillary types involved (fenestrated in the intestine, sinusoids in the liver).
⚠️ Veins with valves vs. veins without valves.
⚠️ Two mechanisms for venous return against gravity: skeletal muscle pump and respiratory pump.
⚠️ Be ready to trace blood flow through named vessels from the heart to a specific organ and back.
True or False: Vasa vasorum supply nutrients to the walls of large blood vessels.
Fill in the blank: The type of capillary found in the liver is a __________.
True or False: Pulmonary arteries carry oxygenated blood.
Fill in the blank: Blood from the GI tract travels through the hepatic portal vein to a second capillary bed in the __________.
True or False: The venae cavae contain valves.
Answers: 1. True. 2. Sinusoid. 3. False (deoxygenated). 4. Liver. 5. False (they do not).
Q: Name the three tunics of a blood vessel wall, from innermost to outermost.
A: Tunica intima (interna), tunica media, tunica externa (adventitia).
Q: What is the key structural difference between continuous, fenestrated, and sinusoidal capillaries?
A: Continuous capillaries have unbroken endothelium joined by tight junctions (least permeable). Fenestrated capillaries have small pores (fenestrations) through the endothelial cells (moderately permeable). Sinusoids have large gaps between endothelial cells and an incomplete or absent basement membrane (most permeable).
Q: Define vasa vasorum and explain why they are necessary.
A: Vasa vasorum are tiny blood vessels within the tunica externa of large arteries and veins. They supply oxygen and nutrients to the vessel wall, which is too thick for diffusion from the lumen alone.
Q: Explain the two ways veins return blood against gravity.
A: The skeletal muscle pump compresses veins during limb movement, pushing blood upward past one-way valves. The respiratory pump uses pressure changes during breathing: inhalation lowers thoracic pressure and raises abdominal pressure, drawing blood toward the heart.
Q: Trace the blood flow through the hepatic portal system, naming the vessels and capillary types.
A: Arterial blood reaches the GI tract → exchanges occur in fenestrated capillaries in the intestinal walls → blood drains into the superior and inferior mesenteric veins and splenic vein → these merge to form the hepatic portal vein → blood enters sinusoid capillaries in the liver → hepatocytes process the blood → blood drains via hepatic veins into the IVC.
Q: Which veins in the body lack valves?
A: The superior and inferior venae cavae, thoracic veins, most veins of the head and neck, and the abdominal visceral veins generally lack valves.
This connects to the Heart (Ch. 19) because the great vessels leaving and entering the heart are the roots of everything covered here. It also connects forward to the Respiratory System (Ch. 22) through the pulmonary capillaries where gas exchange occurs, and to the Digestive System (Ch. 23) through the hepatic portal system. The capillary types discussed here (especially fenestrated and sinusoidal) reappear in the Urinary System (Ch. 24) when you study the glomerular capillaries and peritubular capillaries of the nephron.
blood vessel tunics, tunica intima, tunica media, tunica externa, vasa vasorum, elastic arteries, muscular arteries, arterioles, venules, capillary beds, precapillary sphincter, metarteriole, thoroughfare channel, continuous capillaries, fenestrated capillaries, sinusoids, discontinuous capillaries, blood-brain barrier, venous valves, skeletal muscle pump, respiratory pump, vascular anastomosis, Circle of Willis, hepatic portal vein, hepatic portal system, portal circulation, first-pass metabolism, pulmonary arteries, pulmonary veins, aorta, vena cava, common carotid, subclavian, brachial, radial, ulnar, femoral, popliteal, tibial, great saphenous vein, celiac trunk, superior mesenteric artery, renal artery, common iliac, internal jugular vein