10 Cardiovascular system

Blood circulates throughout the body in blood vessels, propelled by the pumping action of the heart.

Blood vessels form a continuous path for blood flow that starts and ends at the heart. Arteries carry blood away from the heart, regardless of the degree of blood oxygenation. Veins carry blood toward the heart. Between arteries and veins, there is a network of capillaries that spread inside all organs, reaching practically every cell in the body. The thin walls of capillaries allow for the passage of substances between blood and interstitial fluid, surrounding the cells bringing nutrients, gases and signaling molecules to the cells and removing waste to the bloodstream. Capillaries are therefore known as the exchange vessels.

Each component of the cardiovascular system, including the heart, has walls composed of three layers, or tunics:

  • tunica intima made of endothelium that is continuous throughout the entire blood path.
  • tunica media made of smooth muscle.
  • tunica externa, or adventitia, made of connective tissue.

The general layout is the same but each part of the cardiovascular system has specific characteristics that optimize it for the function it performs.


The heart is a muscular organ that functions as a pump. It creates the force that starts the movement of the blood within blood vessels.

The heart wall has three layers:

  • endocardium
  • myocardium
  • epicardium

The endocardium is synonymous with endothelium, a single layer of squamous epithelium lying on the basement membrane. Endothelium is continous throughout the entire system, and covers all of the structures within the heart chambers, including the valves.

The myocardium is a layer of cardiac muscle that has already been described in the chapter about muscular tissue. Below is a summary of the cardiac muscle histology.

  • Cardiac muscle is striated
  • Cardiac muscle cells are branched
  • Cardiac muscle cells are connected by intercalated disks
  • Cardiac muscle cells have a single, centrally located nucleus
  • There is a significant amount of pink stained cytoplasm around the nucleus
    Figure 1: Longitudinal section of cardiac muscle. Notice a nucleus in the middle of the muscle cell enveloped by lightly pink cytoplasm.

The epicardium is a layer of connective tissue that lies between the muscle and the neighboring pericardial cavity. From inside, epicardium is attached to myocardium; on the other side it forms a wall of the pericardial cavity and it is a layer of simple squamous epithelium.

Blood vessels

Arteries and veins vary in their build and appearance, but they share the same general structure. Arteries have thicker and more elastic walls than corresponding veins because they are exposed to the higher pressure of the blood that leaves the heart. Arteries appear round and smaller in diameter than veins because of their elasticity. Veins that are exposed to very little pressure have thinner and less elastic walls, have larger diameters and assume an oval, flattened shape when emptied of blood in histological preparations, as seen in Figure 2.

Figure 2: Medium (muscular) artery (top) and vein (bottom)

Structural similarities of the blood vessels

Walls of both arteries and veins have the same three-layered structure. Moving from the inside out, these layers, called tunics (singular tunica), are:

  • tunica intima
  • tunica media
  • tunica externa
Figure 3: Illustration of tunics of the arteries vs veins. The picture adapted from BC OpenStax Anatomy and Physiology book under the CC BY license.

Tunica intima

The tunica intima is composed of a single layer of a specialized simple squamous epithelium called endothelium that lies on a basement membrane. The tunica intima separates the blood from tissue and has anti-clotting properties.

Under the microscope, the endothelial lining is very thin and can be identified by the nuclei of the simple squamous epithelial cells that protrude into the lumen of the blood vessel (Figure 4.)

The tunica intima of arteries appears wavy because of the elastic fibers below it. This “wrinkling” allows for stretching without excessive stress on the endothelial cell layer. The tunica intima of a vein appears smooth as veins have less elastic fibers in their structure.

Figure 4: Arrows indicating the nuclei of the simple squamous epithelium.

In large arteries, there is also an additional layer of elastic fibers at the boundary with the tunica media known as the internal elastic membrane. The internal elastic membrane is not present in veins.

Tunica media

The tunica media is the middle layer of the vessel wall. It is a layer of smooth muscle with some elastic fibers mixed in. Contraction and relaxation of the smooth muscles, controlled by the autonomic nervous system, decrease and increase the diameter of the vessel, leading to changes in blood flow and pressure.

Under the microscope, the tunica media is the thickest layer in arteries, and is much thicker in arteries than it is in veins. The elastic fibers within this layer appear as wavy lines between the smooth muscle cells.

In larger arteries, separating the tunica media from the tunica externa is the external elastic membrane, which appears wavy in slides. This structure is not usually seen in smaller arteries or in veins.

Tunica externa

The tunica externa (also called adventitia) is a layer of connective tissue composed primarily of collagen fibers. The tunica externa is normally the thickest layer in veins and may even be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in place. If the tunica externa did not hold the vessel in place, any movement would likely result in the disruption of blood flow.

Figure 5: Three tunics of the artery.

The table below compares the tunics of the arteries and veins.



general appearance

thick walls with small lumens thin walls with large lumens
appear rounded appear flattened

tunica intima

endothelium appears wavy endothelium appears smooth
internal elastic membrane in large vessels internal elastic membrane absent

tunica media

thickest layer in arteries normally thinner than the tunica externa
smooth muscle cells and elastic fibers (the proportions vary) smooth muscle cells and collagen fibers
external elastic membrane present in larger vessels external elastic membrane absent

tunica externa

normally thinner than the tunica media normally the thickest layer in veins
collagen and elastic fibers collagen and some smooth muscle fibers
vasa vasorum present vasa vasorum present

Table adapted from BC OpenStax Anatomy and Physiology book under the CC BY license.

Vasa vasorum

The walls of arteries and veins are composed of living cells that require nourishment and produce waste. Since blood passes through the larger vessels relatively quickly, there is limited opportunity for blood in the lumen of the vessel to provide nourishment to or remove waste from the blood vessel’s walls. Further, the walls of larger vessels are too thick for nutrients to diffuse through to all of the cells. Larger arteries and veins contain small blood vessels within their walls known as the vasa vasorum—literally “vessels of the vessel”—to provide them with this vital exchange.


An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart.

Based on their distance from the heart, and subsequently the pressure they experience, arteries can be divided into three types:

  • elastic arteries
  • muscular arteries
  • arterioles

Elastic arteries

Elastic arteries, known also under the name of conducting arteries, comprise large arteries that leave the heart. Elastic arteries are usually larger than 10 millimeters (mm) in diameter. The aorta and the major first branches from the heart, such as the brachiocephalic artery, common carotid, and subclavian belong to this category.

The walls of these blood vessels are exposed to high pressures and contain a large number of elastic fibers in all three of their tunics. The presence of elastic fibers allows them to expand as blood is pumped from the ventricles, and recoil when the heart is in diastole. Elastic arteries have thick walls, and their endothelium has a wavy appearance.

Muscular arteries

Muscular arteries are located farther from the heart, where the blood pressure is lower. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Most of the arteries in the body belong to this category.

In muscular arteries the intima has less elastic fibers because it is no longer exposed to very high pressures. The tunica media has more smooth muscle, allowing the muscular arteries to constrict and dilate in response to environmental signals such as, hormones or temperature, and thus control the distribution of blood between organs. Intima has less elastic fibers but still appears wavy under the microscope.


Arterioles are the smallest arteries just before they turn into capillaries. Arterioles are 30 micrometers or less in diameter. They have all three tunics but the thickness of each is greatly diminished. The arterioles usually have only one (and not more than two) layer of smooth muscle.

Under the microscope, arterioles can be identified by a distinctive arrangement of endothelial cells and smooth muscle in their walls. The endothelial cells of intima lie longitudinally to the length of the blood vessel, while the smooth muscle cells in the tunica media are circular. This creates a regular pattern of cell nuclei that is different from the seemingly random pattern in veins, and can be used for identification.

Arterioles are referred to as resistance vessels, as they pose resistance to blood flow because of their small size. By constricting and dilating they are responsible for the regulation of blood pressure.

Figure 6: Arteriole. Notice nuclei of the endothelial cells and characteristic pattern of nuclei in the muscular layer.


Capillaries are the smallest blood vessels. The diameter of a capillary lumen ranges from 5–10 micrometers. They only have one-layered walls made of endothelium (indicated by the arrows in Figure 7) on top of the basement membrane underneath. In a large capillary, several endothelial cells wrap anound the lumen; in a small capillary, there may be only a single cell that bends around to form a cylinder. Endothelial cells are connected by tight junctions to prevent the leakage of proteins or blood cells out of circulation.

Figure 7: A capillary. Endothelial cells (indicated by arrows) are the only layer of the wall.
Capillaries are classified into three types, based on their “leakiness”, and are either:
  • continuous
  • fenestrated
  • sinusoidal (e.g., spleen, bone marrow, liver)


Figure 8: Illustration of the three types of capillaries. The picture adapted from BC OpenStax Anatomy and Physiology book under the CC BY license.

Continuous capillaries are the most common type, and are found in muscle, the central nervous system, and the lungs. Cells are tightly sealed and permeable only to fluid and the smallest of molecules, such as ions.
Fenestrated capillaries have small holes and look like a kitchen strainer. They are located in endocrine glands, intestines, and the kidney glomeruli, where their structure allows for the transport of larger molecules such as hormones or fat particles. 
Sinusoidal capillaries, or sinusoids, can be found in the liver, spleen, and bone marrow. They have large gaps between cells that allow larger particles, and even blood cells, to enter or exit the blood.
Under the microscope, capillaries are hard to find as they have only one layer of cells, endothelium, and are usually collapsed in histological sections. Capillaries are exchange vessels, as the interchange of nutrients/waste between blood and organs takes place across their walls.


A vein is a blood vessel that conducts blood toward the heart. Veins have thin-walls, and compared to arteries, large and irregular lumens. Venules are the smallest veins that form when capillaries merge together. Veins are categorized differently than arteries and capillaries, but can be broken down into four other subtypes:

  • Pulmonary
  • Systemic
  • Superficial
  • Deep

Pulmonary veins are responsible for carrying oxygen rich blood from the lungs to the heart. Systemic veins carry deoxygenated blood from the rest of the body back to the heart. Superficial veins lie near the skin’s surface and lack corresponding arteries, while deep veins are found within muscle tissue and do have a corresponding artery.

The blood pressure in veins is very low, resulting in the blood needing to be lifted toward the heart by neighboring muscle. Veins also have valves that prevent the backflow of blood toward the capillaries.


Figure 9: Small vein (left) and aretery (right). Notice the difference in the structure of the walls.

The tunica intima of veins has less elastic fibers than in a corresponding artery. The tunica media is thinner, and the tunica externa is the thickest layer of the wall.


Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

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