Capillary in the Human Circulatory System

Blood is a fluid connective tissue that is conducted through the vessels. It circulates all over the body to supply nutrients and oxygen to remove waste products. There are three different types of blood vessels that include veins, arteries, and capillaries where each of them plays a specific role in the process of circulation. The blood vessels that carry blood from the heart and the venous system include the blood vessels which take the blood to the heart, this combined arterial system is connected by capillaries. Hence, capillaries meaning refer to fine blood vessels connecting the arteries and veins. The exchange of oxygen, waste, and nutrients is carried out by the capillaries in between the tissues and blood. 

This exchange happens in two ways:

• If the movement of a particular substance takes place from the region of higher concentration to the lower concentration then this process is called passive diffusion.

• The process through which the body cells take the small molecules such as proteins and fats is called pinocytosis.

Thus the walls of the capillaries consist of a thin layer called the endothelium. This endothelial layer is surrounded by another layer called the basement membrane. The composition of the endothelium layer varies among different types of capillaries. 

Capillary System Structure

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The blood flows through the arteries from the heart that branches narrowly into the arterioles that further branch into capillaries. In the capillaries the exchange of nutrients and wastes takes place. These capillaries join further to become venules. These venules get widened and converge to form veins and through the venae cavae, the blood returns back to the heart. 

The size of the blood capillaries is about three to four micrometers in diameter. They consist of a thin layer made of epithelial cells and a basement membrane or basal lamina called tunica intima. The epithelial cells are partially encircled by an incomplete layer of cells called pericytes. 

The capillary bed is an interweaving network of the capillaries supply tissues and organs, it acts as a part of an individual capillary. When the tissue is more metabolically active then more capillaries are required to remove the waste products of metabolism and to supply the nutrients. 

Types of Capillaries

There are three types of capillaries that perform various functions:

1. Continuous Capillary: These are said to be the most common type. The small gaps are present in between the endothelial cells. These gaps are responsible for the passage of gases, water, sugar, and hormones. But the continuous capillaries present in the brain are a part of the blood-brain barrier. This barrier helps to protect the brain by allowing the essential nutrients to cross the capillary. Thus these capillaries in the brain don’t have gaps in the endothelial cells and the surrounding basement membrane is also thick.

2. Fenestrated Capillary: These are said to be leakier than that of the continuous capillaries. Along with the small gaps these contain small pores in between the cells called fenestrae or fenestra. These pores are present on the walls and help in the exchange of the larger size molecules. The fenestra has a non-membranous and permeable diaphragm-like structure. In the areas where there is a requirement for the exchange of large molecules these fenestrated capillaries are present. Some of the examples include the small intestine which requires the absorption of nutrients from the food. In the kidneys in which the filtration of blood takes place.

3. Sinusoid Capillary: This capillary is the rarest and leakiest type. As they have large gaps in the capillary walls these are responsible for the exchange of larger molecules including the cells. These are found in tissues such as the liver, spleen, and bone marrow. These capillaries are found in between the epithelial cells and the hepatocytes. 

Other Types of Capillaries:

Lymphatic capillaries: The tiny capillaries are located in between the spaces present in the cells are lymph capillaries. It helps to remove the excess fluid. Compared to the blood capillaries these are larger in size and have closed ends. A fluid called lymph flows through these capillaries. When the pressure is more in interstitial fluid than in lymph then the cells present in them separate. 

Systemic Capillaries: The junction point present in between the arteries and veins consists of systemic capillaries. It helps to facilitate internal respiration where the oxygenated blood flows throughout the body. 

Pulmonary Capillary: The exchange of carbon dioxide takes place with the oxygen from the alveoli. The blood which consists of high oxygen levels is transferred to the tissues from the heart. Oxygenated blood is transported through pulmonary veins and deoxygenated blood is carried through pulmonary arteries. Throughout the body pulmonary arteries are the arteries through which the deoxygenated blood flows. 

Diseases Associated With Capillary System

As the capillaries are small any unusual functioning of it can cause severe medical conditions:

• Port-Wine Stains: It is a type of birthmark that is caused due to the widening of the capillaries that are located in the skin. This widening can make the color of the skin look pink or red. They don’t spread to other parts of the body and they don’t disappear on their own. Where laser treatment can make the stains look lighter. 

• Petechiae: The small round spots that are found on the skin whose size is about the size of a pinhead. The color can vary from purple to red. These are found when the capillaries start to leak the blood inside the skin. Petechiae can also occur due to other conditions such as leukemia, low platelet levels, or scurvy. Taking some of the medications such as penicillin can cause petechiae as a side effect.

• Systemic Capillary Leak Syndrome: SCLS is a rare condition that occurs without any proper cause. According to some experts it can be caused by the presence of certain substances in the blood that can harm the walls of the capillaries. Some attacks might also happen due to SCLS that require immediate medications. The warning signs that are associated with the attacks include cough, nausea, headache, fainting, swelling in arms and legs, and abdominal pain.

• Arteriovenous Malformation Syndrome: The abnormal tangle of veins and arteries are connected to each other where the capillaries are found absent. These tangles are found commonly in the brain and spinal cord. This makes it difficult for the supply of blood and oxygen. Some of the symptoms include pain, headache, issues with speech, vision, or movement, and weakness. It is a rare condition that is found during the time of birth. 

• Microcephaly-Capillary Malformation Syndrome: It is a rare condition that is found even before birth. Near the skin surface, the capillaries are found to widen, which increases the flow of blood which causes pinkish-red spots. It is caused due to the mutation in a certain gene called the STAMBP gene.

Conclusion

The capillary blood vessels are the smallest and the numerous blood vessels that provide the connection between arteries and veins. The main function of them is to exchange the materials in between the blood and tissue cells. It acts as a center of circulation where the oxygen is transported to the tissue by liberating the carbon dioxide.