Cardiovascular | Tunic Layers & Types of Capillaries

Layers of Blood Vessels

In this section, the speaker discusses the layers of blood vessels, focusing on the tunica layers. The speaker explains the composition and function of each layer and mentions some abnormalities related to these layers.

Tunica Intima (Tunica Interna)

• The inner lining of the blood vessel is called the Tunica Intima or Tunica Interna.

• It consists of two distinct layers:

• Endothelial lining: Made up of simple squamous epithelial cells.

• Sub endothelial layer: Composed of collagen and proteoglycans, preventing blowout of the blood vessel.

Internal Elastic Lamina

• The next layer is the internal elastic lamina.

• It is made up of collagen, fibrin, and other proteins.

• Disorders like Marfan syndrome and Ehlers-Danlos syndrome can affect this layer due to a lack of fibrin or specific types of collagen.

Tunica Media

• The tunica media is a thick smooth muscle layer surrounding the blood vessel.

• It plays a crucial role in vasoconstriction (narrowing) or vasodilation (widening) of the blood vessel.

• The sympathetic nervous system innervates this layer to maintain vaso motor tone and prevent neurogenic shock.

External Elastic Lamina

• The external elastic lamina is another layer composed of proteins like collagen and proteoglycans.

• It allows for stretching and recoil of the blood vessel, contributing to its elasticity.

Tunica Externa (Tunica Adventitia)

• The outermost layer is called the Tunica Externa or Tunica Adventitia.

• It provides structural support to the blood vessel.

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Tunica Externa and Vasa Vasorum

The Tunica externa is not highly vascular, but it receives blood supply through a system of tiny blood vessels called vasa vasorum. These blood vessels nourish the Tunica externa and parts of the tunica media. Tertiary syphilis can damage these blood vessels, leading to decreased blood supply and potential complications like thickening of the blood vessel walls and increased risk of aortic aneurysm.

• The Tunica externa is not highly vascular.

• It receives blood supply through a system of tiny blood vessels called vasa vasorum.

• Vasa vasorum nourishes the Tunica externa and some parts of the tunica media.

• Tertiary syphilis can destroy these blood vessels, leading to decreased blood supply to the Tunica externa and potential complications like thickening of the vessel walls and increased risk of aortic aneurysm.

Artery vs Vein Structure

Arteries and veins have similar structures with three main layers - tunica interna (intima), tunica media, and tunica externa. However, there are differences in thicknesses between arteries and veins.

Artery Structure

• Arteries have a thicker tunica media compared to veins.

• The internal elastic lamina is more prominent in arteries than in veins.

• The lumens of arteries are larger than those of veins.

Vein Structure

• Veins have a thinner tunica media compared to arteries.

• The internal elastic lamina may be absent or very minimal in some veins.

• Veins have a much thicker tunica externa compared to arteries.

• Veins often have collapsed lumens when observed histologically.

Recap of Artery and Vein Structure

This section provides a recap of the structure of arteries and veins, highlighting the similarities and differences between them.

• Both arteries and veins have a tunica interna (intima) consisting of an endothelial lining with a subendothelial layer.

• Both arteries and veins have internal and external elastic lamina, but they are more prominent in arteries.

• Arteries have a thicker tunica media compared to veins.

• Veins have a much thicker tunica externa compared to arteries.

• Both arteries and veins are innervated by vasa vasorum.

Types of Capillaries

There are three different types of capillaries, each with distinct morphological differences that help distinguish them.

Continuous Capillaries

• Continuous capillaries have uninterrupted endothelial cells forming a continuous tube.

• They allow for the exchange of small molecules between blood and tissues.

• These capillaries can be found in muscle tissue, lung tissue, and the central nervous system.

Fenestrated Capillaries

• Fenestrated capillaries have pores or fenestrations in their endothelial cells.

• These pores allow for increased permeability to larger molecules like proteins.

• Fenestrated capillaries are found in organs involved in filtration or absorption, such as the kidneys, endocrine glands, and intestines.

Sinusoidal Capillaries

• Sinusoidal capillaries have large gaps between their endothelial cells.

• These gaps allow for the passage of large cells like red blood cells through the capillary walls.

• Sinusoidal capillaries are found in organs involved in hematopoiesis (blood cell formation), such as the bone marrow and liver.

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Continuous Capillaries

This section discusses the characteristics and locations of continuous capillaries.

Characteristics of Continuous Capillaries

• Continuous capillaries have large intercellular clefts, which are gaps between endothelial cells.

• These capillaries are highly permeable, allowing red blood cells, plasma, and large molecular weight proteins to leak out.

• They can be found in locations where there is a need for easy movement of cells in and out.

Locations of Continuous Capillaries

• Common locations for continuous capillaries include bone marrow (specifically red bone marrow), spleen, and liver.

• The blood-brain barrier also contains continuous capillaries, but with small intercellular clefts replaced by tight junctions between endothelial cells.

Fenestrated Capillaries

This section explains the characteristics and locations of fenestrated capillaries.

Characteristics of Fenestrated Capillaries

• Fenestrated capillaries have small intercellular clefts and contain fenestrations or pores on their endothelial cells.

• These capillaries are highly permeable, allowing for the leakage of small solutes, plasma, and even larger molecular weight proteins.

• Pericytes are present in fenestrated capillaries and play a role in controlling endothelial cell growth.

Locations of Fenestrated Capillaries

• Fenestrated capillaries can be found in areas that require rapid exchange of materials such as the kidneys (glomeruli), endocrine glands (e.g., pancreas), intestines (villi), and choroid plexus.

Sinusoidal Capillaries

This section discusses the characteristics and locations of sinusoidal capillaries.

Characteristics of Sinusoidal Capillaries

• Sinusoidal capillaries have large intercellular gaps and irregularly shaped endothelial cells.

• They are the most permeable type of capillaries, allowing for the leakage of blood cells, plasma, solutes, and even large molecular weight proteins.

• Pericytes are present in sinusoidal capillaries and are believed to control endothelial cell growth.

Locations of Sinusoidal Capillaries

• Sinusoidal capillaries are commonly found in organs that require extensive exchange with the bloodstream, such as the liver, spleen, and bone marrow.

Summary

This section provides a summary of the different types of capillaries discussed.

• Continuous capillaries have large intercellular clefts and are highly permeable. They can be found in bone marrow, spleen, liver, and the blood-brain barrier.

• Fenestrated capillaries have small intercellular clefts and contain fenestrations or pores. They allow for rapid exchange of materials and can be found in kidneys, endocrine glands, intestines, and choroid plexus.

• Sinusoidal capillaries have large intercellular gaps and irregularly shaped endothelial cells. They are highly permeable and commonly found in organs like the liver, spleen, and bone marrow.

Finished Rate of Capillaries

In this section, the speaker discusses the characteristics and importance of finished rate of capillaries.

Characteristics of Finished Rate of Capillaries

• Finished rate of capillaries have medium-sized intercellular cleft.

• They have fenestration pores between endothelial cells, allowing for the leakage of plasma and larger molecules.

• These capillaries can also perform pinocytosis.

Locations where Finished Rate of Capillaries are Found

• Finished rate of capillaries are commonly found in the kidneys and glands.

• Glands can be classified as exocrine (with ducts) or endocrine (ductless).

Recap

Sinusoidal Capillary

• Largest intercellular cleft

• Most permeable

• Found in red bone marrow, spleen, liver

Continuous Capillary

• Least permeable

• Tiny intercellular cleft

• Blood-brain barrier lacks tight junctions

• Can perform pinocytosis

• Found in muscles, skin, blood-brain barrier, lungs

Finished Rate of Capillaries

• Medium-sized intercellular cleft

• Fenestration pores between endothelial cells

• Allows for plasma and solutes to leak out

• Can perform pinocytosis

• Found in kidneys, glands