Angiology

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Anatomy > Gray's Anatomy of the Human Body > V. Angiology > Introduction

Vascular system

Angiology is the branch of anatomy that deals with the study of blood vessels, lymphatic vessels, and their associated organs, including the heart and lymph nodes. The vascular system is divided into:

• The vascular system, which includes the heart and blood vessels (arteries, veins, and capillaries), and
• The lymphatic system, comprising lymphatic vessels and lymph nodes through which lymph — a clear fluid — circulates.

Though often treated separately, these two systems are developmentally and functionally interconnected.

Circulation of the Blood

The central organ of the circulatory system is the heart, a muscular organ that pumps blood through two main circulatory loops:

• The systemic circulation – from the left ventricle through the body tissues and back to the right atrium
• The pulmonary circulation – from the right ventricle to the lungs for oxygenation and back to the left atrium

Each side of the heart has two chambers:

• An atrium (receiving chamber)
• A ventricle (pumping chamber)

Oxygenated blood is ejected from the left ventricle into the aorta, then distributed to the body's tissues. After exchanging nutrients and gases in the capillaries, deoxygenated blood is returned via veins to the right atrium, passes into the right ventricle, and then is transported via the pulmonary arteries to the lungs. Oxygenated blood returns from the lungs to the left atrium through the pulmonary veins, completing the circuit.

The Portal System

The circulation from the gastrointestinal tract, spleen, and pancreas passes through the liver before returning to the heart. This unique pathway, known as the hepatic portal system, includes:

• Portal vein – carries nutrient-rich blood from digestive organs to the liver
• Hepatic sinusoids – where blood percolates through liver tissue
• Hepatic veins – which drain into the inferior vena cava

This dual capillary system allows the liver to metabolize nutrients and detoxify substances before blood returns to the general circulation.

Arteries

Arteries carry blood away from the heart. Although they generally contain oxygenated blood, the pulmonary arteries are an exception, carrying deoxygenated blood to the lungs.

Structure of Arteries

The walls of arteries are composed of three distinct layers:

• Tunica intima – the innermost layer consisting of:
  • Endothelium – a single layer of flattened cells
  • Subendothelial connective tissue
  • Internal elastic lamina (or fenestrated membrane)

• Tunica media – the middle layer composed of:
  • Smooth muscle arranged in concentric layers
  • Elastic fibers (especially in large elastic arteries such as the aorta)

• Tunica adventitia – the outermost layer made of:
  • Connective tissue
  • Longitudinally arranged elastic fibers
  • In some arteries, a layer called the tunica elastica externa

Transverse section of a small artery and vein from the epiglottis. Note the muscular and elastic tissue of the arterial wall.

Arterial Sheaths and Vasa Vasorum

Arteries are surrounded by loose areolar connective tissue forming their vascular sheath. Larger arteries are supplied by small blood vessels known as vasa vasorum that nourish their outer layers. These vessels typically do not penetrate the inner layers in humans, although some do in other mammals. Arteries also receive autonomic nerve supply, especially sympathetic fibers, which regulate vasoconstriction and vasodilation.

Capillaries

Capillaries are microscopic vessels that connect the terminal arterioles to the initial venules. They are the primary sites of exchange between blood and tissues.

Features of Capillaries

• Diameter: Typically 5–10 μm, just wide enough for erythrocytes to pass in single file.
• Structure: Composed of a single layer of endothelial cells supported by a basement membrane.
• Arrangement: Capillary beds form networks with variable mesh sizes and shapes, adapted to the tissue's metabolic demands.

Capillaries from the mesentery of a guinea pig, after silver nitrate staining.

Types of Capillary Meshes

• Rounded (e.g. lungs, mucosae)
• Elongated (e.g. muscles, nerves)
• Looped (e.g. skin papillae, tongue)

Sinusoids

Certain tissues (e.g., liver, spleen, adrenal gland) possess wide, irregular vessels known as sinusoids. Unlike capillaries, they lack a complete basement membrane and allow more extensive exchange with surrounding cells. Sinusoids are lined by discontinuous endothelium and often come into direct contact with organ-specific parenchymal cells.

Veins

Veins are blood vessels that return blood to the heart. In the systemic circulation, veins carry deoxygenated blood, while in the pulmonary circulation they carry oxygenated blood back to the heart.

Structure of Veins

Veins are composed of three layers, analogous to those of arteries, though generally thinner and less muscular:

• Tunica intima:
  • Inner endothelial lining
  • Subendothelial connective tissue layer
  • Internal elastic membrane (less distinct than in arteries)

• Tunica media:
  • Composed primarily of connective tissue and a modest amount of smooth muscle
  • Much thinner than in arteries
  • Elastic fibers arranged in loose networks

• Tunica adventitia:
  • The thickest layer in veins
  • Contains collagen and elastic fibers
  • Often includes longitudinal bundles of smooth muscle in large veins

Small artery and vein from pia mater (sheep). Note thinner vein wall and collapsed lumen compared to artery.

Regional Variations

The structure of veins varies with size and location:

• In the smallest veins (venules), the three coats are not clearly defined.
• In medium-sized veins, elastic and muscular elements become more distinct.
• In large veins (e.g., inferior vena cava, renal vein, hepatic veins), the tunica adventitia is particularly thick and contains bundles of longitudinal smooth muscle.

Some veins are devoid of muscular tissue altogether, such as:

• Veins of the placenta
• Venous sinuses of the dura mater
• Veins of the retina
• Veins of spongy bone (diploë)
• Venous sinuses of the corpora cavernosa

These vessels consist mainly of endothelial lining supported by fibrous connective tissue.

Valves in Veins

Most veins, particularly those of the limbs, are equipped with venous valves. These structures:

• Prevent the backward flow (reflux) of blood
• Are formed by folds of the tunica intima reinforced with collagen and elastic tissue
• Are covered by endothelial cells, aligned longitudinally on the flow-facing surface

Section of a medium-sized vein. Note the valve structure and thick adventitia.

Venous valves are:

• Most numerous in the lower limbs
• Typically arranged in pairs
• Oriented toward the heart
• Located just distal to vein branches

Absent in:

• Superior vena cava and inferior vena cava
• Pulmonary veins
• Cerebral veins
• Portal vein and its larger tributaries
• Uterine and renal veins

Vasa Vasorum and Nervous Supply

Large veins and arteries are nourished by their own small vessels, known as the vasa vasorum. These:

• Originate from nearby arteries
• Supply the outer and middle layers of the vessel wall
• Are more extensive in veins than arteries

Nervous innervation to veins, like arteries, comes from the autonomic nervous system. Sympathetic fibers control contraction and tone of the smooth muscle in the vessel walls, regulating venous return and capacity.

Capillaries and Sinusoids (Review)

The capillary is the functional unit of exchange between blood and tissue. Sinusoids are specialized capillaries found in:

• Liver
• Spleen
• Adrenal glands
• Bone marrow
• Pituitary gland

They are lined by discontinuous or fenestrated endothelial cells and lack a basal lamina, allowing close interaction with parenchymal cells.

Functional Summary

Comparative Features of Blood Vessels

Feature	Arteries	Veins	Capillaries
Direction of flow	Away from heart	Toward heart	Between arterioles and venules
Wall thickness	Thick, muscular	Thin, less muscular	Single endothelial layer
Pressure	High	Low	Intermediate
Valves	Absent (except aorta and pulmonary trunk)	Present in many veins	Absent
Lumen shape	Circular (maintains shape)	Collapsible, irregular	Narrow, uniform

Clinical Relevance

• **Varicose veins** – dilated, tortuous veins caused by valve failure, common in lower limbs.
• **Thrombophlebitis** – inflammation of a vein associated with clot formation.
• **Deep vein thrombosis (DVT)** – dangerous clot formation, often in the femoral or popliteal veins; can lead to pulmonary embolism.
• **Aneurysm** – localized arterial dilation due to wall weakness; common in the aorta.

See Also

• Heart
• Blood vessels
• Systemic circulation
• Pulmonary circulation
• Vasa vasorum
• Endothelium
• Veins of the head and neck
• Arterial system
• Capillary bed

Additional Images

• 

  Medium-sized artery showing layered structure

• 

  Vein with venous valve

• 

  Capillary network of guinea pig mesentery

External Links

• Anatomy of the Cardiovascular System – NCBI
• Arteries vs. Veins – Visible Body
• Anatomy photo:25:04-0000 at the SUNY Downstate Medical Center – "General Structure of Arteries and Veins"

Gray's Anatomy

• Gray's Anatomy Contents
• Gray's Anatomy Subject Index
• About Classic Gray's Anatomy
• Glossary of anatomy terms

Anatomy atlases (external)

[1] - Anatomy Atlases

Adapted from the Classic Grays Anatomy of the Human Body 1918 edition (public domain)