overview of the anatomy and physiology of blood vessels · blood vessels •blood vessels are the...

Overview of the anatomy and physiology of blood vessels

Miss Adebayo O.E

PST 515 – Physiotherapy in the disorders of blood and lymph vessels

Blood vessels

• Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart while the vein returns blood to the heart.

• The heart consists of two muscular pumps that act in series, dividing the circulation into two components: the pulmonary and systemic circulations

• De-oxygenated blood is propelled into the lungs by the right ventricles via the pulmonary arteries for oxygenation

• Oxygenated blood is returned to the left atrium via the pulmonary veins


Blood vessels• Blood vessels are the arteries, veins and capillaries

• Blood under high pressure leaves the heart and is distributed to the body by the arteries. The final distributing vessels, arterioles, deliver oxygen-rich blood to capillaries.

• Capillaries form a capillary bed, where the interchange of oxygen, nutrients, waste products, and other substances with the extracellular fluid occurs.

• Blood from the capillary bed passes into thin-walled venules, which resemble wide capillaries.

• Venules drain into small veins that open into larger veins.

• The largest veins, the superior and inferior venae cavae, return low-oxygen blood to the heart.



Keith L. Moore, Arthur F. Dalley(2010): Clinically Oriented Anatomy sixth edition p 38. Lippincott Williams &Wilkins

Blood Vessels

Most blood vessels consist of three coats

Tunica intima

Tunica media

Tunica adventitia

• Tunica intima: composed of epithelial and connective tissue layers. The tunica intima is lined by a specialized simple squamous epithelium called the endothelium. The endothelium is continuous throughout the entire vascular system, including the lining of the chambers of the heart.


Tunica intima

• The next layer after the endothelium is the basement membrane(basal lamina). The basal lamina effectively binds the endothelium to the connective tissue. It also provides strength while maintaining flexibility. It is permeable, allowing materials to pass through it.

• The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibres to provide the vessel with additional flexibility. The outer layer also contains some collagenous fibres to provide additional strength.


Tunica Media

• Tunica Media

• A middle layer consisting primarily of smooth muscle. The tunica media is the substantial middle layer of the vessel wall

• It is the thickest layer in arteries. it is much thicker in arteries than it is in veins.

• This coat (tunic) is made up of layers of smooth muscles supported by connective that is primarily made up of elastic fibres, most of which are arranged in circular sheets.

• The outer portion of the tunic consist of layers of longitudinal muscle.


Tunica Media

• Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.

• Vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure.

• Vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.


Tunica Adventitia

• This is a substantial sheath of connective tissue composed primarily of collagenous fibres. The tunica adventitia in veins also contains groups of smooth muscle fibres. This is normally the thickest tunic in veins.

• The outer layers of the tunica adventitia are not distinct but rather blend with the surrounding connective tissue outside the vessel



Keith L. Moore, Arthur F. Dalley (2010): Clinically Oriented Anatomy sixth edition p 39. Lippincott Williams &Wilkins

Structure of Blood Vessels (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them.PST 515 – Physiotherapy in the disorders of blood and lymph

vessels

Arteries

• Arteries are blood vessels that carry blood under relatively high pressure (compared to the corresponding veins) from the heart and distribute it to the body.

• All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls

Types of Arteries

• Large elastic arteries

• Medium Muscular arteries

• Arterioles.


Large Elastic Artery

• The abundant elastic fibres in the large elastic allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed.

• If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump (the stroke volume) and maintain adequate pressure and flow.


Large Elastic Artery

• The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system.

• An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.


Medium muscular artery

• The medium muscular artery are also called distributing arteries

• They have lesser quantity of elastic fibres than conducting arteries, this limits their ability to expand

• The muscular arteries play a leading role in vasoconstriction

• There is no line of demarcation where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches.

• Muscular arteries then branch to distribute blood to the vast network of arterioles.


Arterioles

• An arteriole is a very small artery that leads to a capillary.

• Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica adventitia remains but is very thin

• Arterioles are critical in slowing down (and resisting) blood flow causing a substantial drop in blood pressure. They are therefore sometimes referred to as resistance vessels.


Types of Arteries and Arterioles


Veins

• Systemic veins are more variable than arteries, and venous anastomoses—natural communications, direct or indirect, between two veins—occur more often between them.

• Venules: A capillaries exiting from a capillary bed. Multiple venulesjoin to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibres, plus an outer layer of connective tissue fibres that constitute a very thin tunica externa


Veins

• Veins: A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens. Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity.


Veins and venules



Structure of blood vessels

Capillaries

• A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion.

• Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid).

• The capillary wall is lined by surrounded by a basement membrane with occasional smooth muscle fibres.

• The walls of the capillaries walls are permeable, allowing substances to pass through (however the walls are relatively impermeable to plasma proteins)


Capillaries

• There are three major types of capillaries, which differ according to their degree of Permeablility: •Continuous, • Fenestrated and • sinusoid capillaries


Capillaries

• Continuous Capillaries: found in almost all vascularized tissues. Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells.

• Fenestrated Capillaries: consists of pores (or fenestrations) in addition to tight junctions in the endothelial lining. These make the capillary permeable to larger molecules. Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood.


Capillaries

• Sinusoid capillary (or sinusoid) is the least common type of capillary.

• They are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells.

• Blood flow through sinusoids is very slow to allow more time for exchange of gases, nutrients, and wastes.

• They are found in the liver and spleen, bone marrow, lymph nodes (where they carry lymph, not blood), and many endocrine glands including the pituitary and adrenal glands. These organs needs these specialized capillaries because of the functions they perform.


Capillaries

• When bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary (they cannot pass through the small openings of continuous or fenestrated capillaries.

• The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation.


Types of capillaries


Venous pump(muscle pump)

• The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart.

• One factor that helps maintain this pressure gradient is physiologic “pumps” that increase pressure in the venous system.

• In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle.

• This helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart.


Venous pump(muscle pump)

• As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves.

• This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through.

• Simultaneously, valves inferior to the contracting muscles close so that blood will not seep back downward toward the feet.



Musculovenous pump. Muscular contractions in the limbsfunction with the venous valves to move blood toward the heart. The outwardexpansion of the bellies of contracting muscles is limited by deep fasciaand becomes a compressive force, propelling the blood against gravity.Red arrows indicate the direction of blood flow.

Keith L. Moore, Arthur F. Dalley(2010): Clinically Oriented Anatomy sixth edition p 42. Lippincott Williams &Wilkins

The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area. This action forces blood closer to the heart where venous pressure is lower.

Venous pump


Venous pump

• When the valves become incompetent or when an individual stay still for a prolonged period of time and also due to the pull of gravity some blood will inevitably pull in the lower limb

• When blood accumulates in a vein, the pressure within it is increased, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries.

• Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid.

• The presence of excess tissue fluid around the cells leads oedema.


References

• Keith L. Moore and Arthur F. Dalley (2010): Clinically Oriented Anatomy sixth edition. Lippincott Williams &Wilkins

• Guyton and Hall (2006): Textbook of medical Physiology. 11th edition. Philadephia PA: Saunders Elsevier.

• OpenStax College (2013): Anatomy & Physiology OpenStax College. http://cnx.org/content/col11496/latest/


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