Blood vessels


Blood vessels are responsible for the transportation of blood, made up arteries and veins, they creates pathways for the oxygenated blood to travel to their destination and pathways for the used deoxygenated blood to travel back to the heart or lungs. Capillaries are designed to permit the transfer of gasses within the blood, such as the delivery of oxygen and the return of carbon dioxide. The molecules from the tissues use the oxygenated blood plasma for energy and return the molecules of wastes. Blood vessels form these pathways to reach every living cell within the human body for this gaseous exchange. The network formed by the blood vessels is tubular, extensive, and in many ways fragile to outside influences.


As the blood leaves the heart, they are filled with molecules of necessary oxygen, and traverse a passageway of progressively smaller tubular networks known as (in order) arteries, arterioles, and capillaries. The microscopic capillaries are responsible for the conjoining of arterial flow and venous flow. Capillaries create the environment for the actual gaseous exchange.

As blood returns to the heart for more oxygen it passes through a tubular network of progressively larger diameter known as (in order) venules and veins. Anastomosis is the convergence arteries. While there are several places throughout the body where this process of anastomosis occurs, this includes the necks of the humerus and femur. Anastomosis occurs in areas that require a constant supply of oxygenated blood.


Blood vessels
Image: Blood Vessels

Blood vessels are comprised of three layers which form the tubular network. The outermost layer is comprised of connective tissue. This layer is known as either tunica externa or advetitia. The middle layer is known as tunica media and is comprised of thin muscular tissue. Throughout this stratum there are diverse amounts of elastic fibers. The inner most layer is a combination of simple squamous epithelium and elastin. The layer of squamous epithelium is termed as the endothelium. All blood vessels have this inner layer as their inner lining.


The structure of a capillary is a bit different. They have a basement membrane for support and have only a endothelium layer. Arteries and veins are nearly the same, with the exception of a few vital differences in their structure. The arteries are responsible for the transport of blood away from the heart while veins are responsible for the transport of blood back to the heart. An artery that is compared with the same sized vein is going to have more muscle in their structure. Cross section comparisons show that arteries are more circular than veins. Veins generally do not fill to capacity and therefore have a more relaxed shape. Veins have the capability to expand when filled with additional blood and make up the body’s venous reservoirs. Arteries are also devoid of valves, which veins are equipped with in their structure.


In order for the arteries to expand when the heart fills them with blood and retract when there is absence of blood, the arteries have layers of elastic fibers in between their layers of smooth muscle in the tunica media section. The action of expanding and contracting helps create a more even and less volatile rhythmic pattern of alternating systole and diastole action in the smaller arteries and arterioles. The smaller the artery the less elastic fiber is built into their layers of muscle. This creates an even diameter in the smaller arteries. Larger arteries are designed to expand and contract with the rhythm set forth by the heart.


The small muscular arteries are more rigid, thus they create more resistance through the circulatory system than veins or larger arteries. These small arteries have very narrow lumina, which is also the case with the small arterioles. The smallest of the arteries branch off to form the arterioles. Any artery that is less than 100 micrometers in diameter will create the arterioles which range between 20 and 30 micrometers in diameter.


Image: Arteries

Most often the blood which pulses through the arterioles will then pass through the capillaries for the gaseous exchange. In rare instances, the blood which is pulsating through the arterioles will pass through channels known as metarterioles. This allows the formation of vascular shunts that permit blood to pass directly into the venules.

The smallest members of the tubular network are the capillaries, which measure about 7 to 10 micrometers in diameter. The tiniest of the blood vessels serve as functional nits within the circulatory system by permitting the gaseous exchanges to occur across their walls. The capillary walls provide the necessary environment for the exchange of oxygen and carbon dioxide as well as nutrients and wastes between the blood and the body’s tissue.



The human body contains a highly extensive and intricate network of blood vessels which deliver oxygenated blood to the body’s more than 40 billion capillaries. The capillaries are the exchange site for gases, wastes, and nutrients that allow the body’s tissue to function properly. Without this exchange, the tissues of the body would die. The capillary system is so extensive that there isn’t a single cell much more than a fraction of a millimeter from a capillary site. Over 1,000 square miles of systematic networking of capillaries covers the internal network that nature has designed. Throughout this extensive mileage blood and interstitial fluid creates the necessary functions for each cell’s survival.


Capillaries only contain a maximum of about 250 milliliters of blood which brings the total blood volume to only a mere 5,000 milliliters inside the walls of the capillaries. The lumina of the veins contains the majority of the body’s blood. The specific amount of blood processing at any given moment in controlled by pre-capillary sphincter muscles. These highly specialized muscles permit only at most 10% of the capillary bed they control to remain open during a period of rest. These muscles also control the variation between capillary beds. The capillary bed for the skeletal muscles may differ from the amount of blood permitted into a capillary bed that distributes blood to a vital organ. When the pre-capillary sphincter muscles control a capillary bed for an internal organ, the blood flow is determined by the sphincter muscles as well as the amount of blood flow resistance within the organ’s arteries and arterioles.

While arteries and veins are created by a complex system of layers, capillaries in turn are created by a single layer of cells. This is the endothelium layer, or the simple squamous epithelium layer. The exchange of gases, wastes, and nutrients can only happen in the capillaries because they lack smooth muscle and connective tissue. Otherwise the interstitial fluid and the blood would not be able to pass the necessary molecules through the walls of the capillaries.


Image: Capillaries

The various distinguishing features of the various types of capillaries make them identifiable. The variation of the endothelial lining may be either continuous, discontinuous, or fenestrated. The capillaries with continuously linings are conjoined firmly together with the epithelial cells that they are flanking. The muscles, lungs, central nervous system and the body’s adipose tissue all contain continuous capillaries. The capillaries within the central nervous system require the capillaries to be continuous and devoid of an intercellular channel, as without the intercellular channels the body is able to enforce the design of the blood-brain barrier.


However, other continuous capillaries that reach other organs have these intercellular channels. This is how the various molecules (excluding protein) pass between the blood and the interstitial fluid as well as the body’s circulatory system. While there are still unkowns regarding the human body, studies have confirmed the notion of intercellular channels permitting the exchanges of gases via the study of the endothelial cells. Studies indicate that the endothelial cells have present the pinocytotic cells, which leads researchers to believe intercellular transport happens across the walls of the capillaries. This is the only obvious transport function of molecules within the central nervous system and is likely to account, or at least contribute to, the intense selective operations of the blood-brain barrier.

The kidneys, intestines, and the endocrine glands all contain fenestrated capillaries. The large intercellular pores in conjunction with the mucoprotein layer suggest this acts in a diaphragm function for the transport of nutrients and waste gasses.

The bone marrow, the liver, and the spleen all have discontinuous capillaries. These organs all have space that has the appearance of cavities due the vast voids between the endothelial cells.



After the exchange of gasses and wastes the occurs when blood enters the capillaries, the by-products must be removed from the blood which means the blood must return through the body and back to the heart. The veins are responsible for the transportation of blood back to the heart. The microscopic vessels which begin the blood’s trek back to the heart are called venules. The veins begin to enlarge incrementally in size as the blood gets closer to its destination. Considering that average arterial pressure is about 100 mmHg, veins carry a much lower pressure, averaging 2mmHg. The pressure in the veins and arteries represent the force applied to the side of the vessel wall indicating hydrostatic pressure.


Image: Veins

The pressure within the veins is insufficient for unassisted return of the blood to the heart. This is especially true of the lower extremities. For the necessary assistance required for veins to be able return the blood to the heart, the veins pass by muscle groups which encourage a gentle massaging motion which increases the blood flow. Venous valves ensure that this massaging motion does not provide excessive blood flow into the heart. This assistance provided by the skeletal muscles has been termed skeletal muscle pumps. The skeletal muscle pump contributes significantly to the rate the blood is returned to the heart via the veins. When the body is at rest for a long period of time, such as recovering from significant illness, blood is permitted to accumulate in the veins due to the inactivity of the skeletal muscle pumps. The accumulation of blood can cause protuberances within the veins. The more active the human body becomes the faster the rate of return of venous blood.
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