The circulatory system is one of the most important organ systems of our body. It is essential for the transport of gases, nutrients, hormones, etc. within the body. It is also important in the transportation of blood and lymph and regulation of body temperature.
The circulatory system consists of
- a) Heart
- b) blood vessels.
The blood vessels include
These Blood vessels are long tubular structures through which blood can flow. The heart pumps as well as receives the blood from different areas of the body.
Blood vessels can be divided into mainly three types
Arteries carry away blood from the heart to the other body parts. They carry oxygenated blood except for the pulmonary artery which carries deoxygenated blood. They have thicker walls (prominent tunica media) with smooth muscle and narrow lumen.
Veins carry blood from other parts of the body back to the heart. They carry deoxygenated blood except for the pulmonary vein which carries oxygenated blood. They have thinner walls (less tunica media) with less smooth muscle and wider lumen.
For more details, read arteries and veins and their differences.
The arteries divide and redivide to give smaller, thin branches like arterioles, meta-arterioles. They further branch out as capillaries.
These capillaries are present in almost every organ and it is in this vessel that gaseous exchange takes place, and the oxygenated blood in the capillaries becomes deoxygenated. From capillaries, this blood goes into venules and then into veins. Thus capillaries act as intermediary vessels between arteries and veins. The circulation of blood from arteries to venules through capillaries is called microcirculation.
Capillaries are also known as microvessels due to their small size and thin walls. Only about 5% of the blood volume circulates in capillaries.
Capillaries are more commonly known as exchange vessels. This is the region of exchange of nutrients, gases, water, and waste products. Physiologically capillaries are of great importance.
The constant supply of oxygen and nutrients is essential for the survival of tissues. Capillaries are present close to all metabolically active cells.
The products of metabolism and the substrate for metabolism can easily be exchanged between the cells and blood through a capillary wall.
Capillaries are present in almost all areas of the body except the lens and cornea of the eye. All other cells in the body are in direct contact with the walls of the capillary.
Though the diameter of individual capillaries is very minimal, the large number of capillaries ensures that the capillaries have the largest cross-sectional surface area among all blood vessels in the body.
Capillaries form an extensive network through branching. This increases the surface area for a more rapid exchange of materials. Capillary walls are made up of a single layer of endothelial tissue
The cells are surrounded by a basement membrane, which is a layer of protein surrounding the capillary. These endothelial cells are joined together by inter-endothelial junctions between two adjacent cells. Unlike other blood vessels like arteries and veins, tunica media is completely absent in capillaries. This absence of tunica media and the gap in the inter-endothelial junctions contribute to the greater exchange of materials and increased permeability of the capillaries.
Capillaries are very thin. Capillaries originate from the arteries and their origin is guarded by a precapillary sphincter, which is a flap of smooth muscle present around the structure to regulate the flow of blood into the capillary.
Capillaries don’t have any smooth muscle. The diameter of the lateral end of the capillary is less than 5 micrometer and the venule end of the capillary has a diameter of about 9 micrometer
The lumen is the empty canal through which the blood passes.
There are about 40 billion capillaries in the average human body. And all the capillaries in a human body together can stretch over 100,000 miles.
Types of capillaries
Capillaries are of three types- Continuous, Fenestrated, and Sinusoidal capillaries. This classification is done according to the size of gaps between the endothelial cells of the capillaries.
These are the most common type of capillary in the body. They are also called non-fenestrated capillaries. Here the endothelial cells form a continuous ring around the lumen of the capillary except at the intercellular clefts or gaps between the endothelial cells. Interoendothelial gaps are usually 4 to 10 nanometres in diameter.
There is no discontinuity in the endothelium of this type of capillaries. They can be seen in skeletal muscles, smooth muscles, connective tissue, lungs, etc. This type of capillaries is also present in the cerebral blood vessels but here, the clefts are absent in the capillaries.
The inter-endothelial junctions in the cells are tight junctions and they form the blood-brain barrier which is important to save the brain from toxic substances, infections, etc.
The endothelial cells in these capillaries are perforated by many fenestrations or pores which are quite large. These pores are 20 to 100 nanometres in diameter.
They can allow the passage of molecules having a molecular weight up to 70000. These capillaries usually surround epithelium like intestinal villi, ciliary processes of eyes, choroid plexuses of ventricles in the brain, kidneys, exocrine glands, etc.
Sinusoids / Discontinuous capillaries
These capillaries have very large gaps or discontinuities between the neighboring endothelial cells. These gaps at the junctions are more than 400 nanometres in diameter.
Some gaps may even range up to 600 to 3000 nanometre in diameter. They also have numerous large fenestrations in the cytoplasm. Thus molecules of large size can easily pass through these gaps into the lumen. Such capillaries can be seen in the liver, spleen, bone marrow.