Capillaries

Capillaries are fine and thin-walled blood vessels that form a network to transport blood, oxygen and nutrients to cells throughout the body. Oxygen and other nutrients are released and carbon dioxide and other waste products are passed into the blood, which then enters veins.   

Capillaries connect arteries and veins in addition to facilitating the exchange of certain elements between our blood and tissues. This is why tissues that are very active, such as our muscles and liver, have a dense web of capillaries. While on the other hand, less metabolically active tissues, such as certain types of connective tissue, don’t have as many.

Structure of a Capillary 

Capillaries are 5–10 micrometres (a micron is 0.001 mm) in diameter. They’re so minute that red blood cells have to pass through in a single file line.There are 40 billion capillaries in the average human body.The wall of a capillary consists of the endothelial layer. This layer is surrounded by a basement membrane with smooth muscle fibres. 

There is again a distinction depending on the wall structure. In a wide capillary, several cells bordering each other may line the lumen. In a thin capillary, there may be only a single cell layer that wraps around. This layer makes capillaries a bit more permeable than other blood vessels. This allows oxygen and other molecules to reach your body’s cells with greater ease. 

Hence the Capillaries contain two layers of cells:

• Endothelial cells: They form the inside of the capillary. They control the flow of nutrients, fluids and gasses.
• Epithelial cells: They Form a protective layer around the endothelial cells.

The function of A Capillary 

The primary function of capillaries is to connect the arterial system to our venous system. The arterial system includes the blood vessels that carry blood away from our heart while the venous system includes the blood vessels that carry blood back to our heart. The Goal of capillaries is also to play a central role in circulation. It delivers oxygen in the blood to the tissues and picks up carbon dioxide which is set to be eliminated. Thus the exchange of oxygen, nutrients, and waste between our blood and tissues also happens in our capillaries. This happens through two processes:

• Passive diffusion:  The movement of a substance from an area of higher concentration to an area of lower concentration is called passive diffusion 
• Pinocytosis: This is the process through which our body’s cells take in small molecules, such as proteins and fats.

Gas Exchange: Oxygen in the lungs diffuses from the alveoli into capillaries to be attached to haemoglobin which is then carried throughout the body. Carbon dioxide from the blood in turn flows from the capillaries back into alveoli to be exhaled out of the body.

Nutrient and Fluid Exchange: Similarly, fluids and nutrients diffuse through permeable capillaries into the tissues of the body. Likewise, waste products are picked up in the capillaries to be transported through veins to the kidneys and liver where they are then processed and eliminated from the body.

Ensuring regulated Blood Flow: Regulated blood flow is extremely important for a healthy body. If not regulated, we might suffer from high or low blood pressure. Capillary beds are regulated automatically so that if blood pressure would drop, flow through the capillaries will continue to provide oxygen and nutrients to body. Blood flow in the capillaries is regulated by precapillary sphincters. A precapillary sphincter is a muscular fibre that controls the movement of blood in the blood vessels by obstructing the flow of blood in the capillaries.

Types of Capillaries 

Continuous Capillaries: such Capillaries are the most commonly found capillaries. It is found in almost all vascularized tissues.  They contain small spaces or gaps in between their endothelial cells which allow gasses, water, glucose, and some hormones to pass through. But continuous capillaries present in the brain are an exception. Continuous capillaries not associated with the brain are rich in transport vesicles while those in the brain are part of the blood-brain barrier. This helps to protect the brain by only allowing the most essential nutrients to cross.

Fenestrated Capillaries: Fenestrated capillaries are the ones that have pores (or fenestrations) They also contain small gaps between cells and in their walls that allow for the exchange of larger molecules. Fenestrated capillaries are commonly found in: 

Small intestine: Since it is a site for nutrient absorption from foodFenestrated Capillaries are found here

Kidneys: Since the impure blood is filtered out in the kidney, it is a hub of this type of capillaries.

 Sinusoid capillaries: These are the least common type of capillaries. They allow the exchange of quite large molecules, even cells. They can do this because of their structure.  Sinusoid capillaries are flattened, and they have too many intercellular gaps. In addition to this, they are porous and have small gaps. These capillaries are found in our liver, spleen, and bone marrow.

Conclusion

It can thus be concluded that capillaries are a very important part of the circulatory system which ensures the functioning of a healthy body. It plays a huge role in facilitating the exchange of various substances between our bloodstream and tissues. If our capillaries do not function properly, our bodies will be prone to several severe disorders.