Blood plasma
Red blood cells
White blood cells
Platelets
Coagulation
Blood, the bodily fluid circulating in the blood vessel network, is a type of connective tissue. Its extracellular matrix is the blood plasma. Its cellular elements are the red blood cells, white blood cells and platelets. The cellular elements of blood come from the red bone marrow. Blood accounts for approximately 8% of one’s body weight, its volume is around 5 dm3 on average. Blood plasma makes up 55% of the blood volume, while 45% is made up of cellular elements.
The main component of blood plasma is water. The blood’s cellular elements are transported suspended in this yellowish fluid, which contains a variety of ions and organic substances. Some of its proteins are transport proteins which can bind hormones, fatty acids and different metallic ions. Immunoglobulins are proteins that take part in the defense against foreign substances that enter the body (antigens), while other proteins have a role in coagulation. Carbon dioxide, a gas we breath out, is also transported by the blood plasma. Examining the composition of blood plasma (protein content, ion composition, blood sugar level) gives us important information about our health.
Red blood cells form in the red bone marrow. As they mature they lose their nucleus and other organelles, so they are [E1] not fully functioning cells anymore. Thanks to their donut-like shape their drag is small and they are flexible. They contain a red colored iron-rich metalloprotein called haemoglobin that gives blood its colour. The haemoglobin’s function is to bind and transport oxygen. Various carbohydrate containing membrane proteins can be found in the red blood cells’ membrane – one’s blood type is determined by them. In a healthy person 1mm3 of blood contains 4,5-5 million red blood cells. If the count is much lower than this, we talk about anemia. The life span of red blood cells is approximately 120 days, the old cells are eliminated by the spleen.
White blood cells form in the red bone marrow and in lymphoid organs. They are part of the immune system, their function is to protect the body from foreign substances (antigens). In a healthy human 1 mm3 of blood contains 4-10 thousand of them. The majority of them are phagocytes (granulocytes and monocytes), the rest are are lymphoid cells (lymphocytes). Phagocytes have the ability to leave the blood vessels and attack antigens in tissue spaces. Lymphatic cells can produce antibodies (immunoglobulins).
Platelets are small cell fragments surrounded by a membrane. They are released from the giant cells (megakaryocytes) of the red bone marrow. In a healthy person 1 mm3 of blood contains about 150-300 thousand of them. When the vessel wall gets damaged the platelets provide the initial seal and they release molecules that help to start the process of coagulation and elicit the contraction of the epithelium, decreasing blood loss.
Coagulation happens when the blood vessel wall becomes damaged. The normally fluid plasma changes from liquid to gel at the damaged area and forms a clot together with the cellular elements of blood, to prevent blood loss. As a first measure, the damaged endothelium (inner lining of the vessel) produces chemicals that mediate the adhesion of platelets to the damaged area. Then the platelets release chemicals that activate the plasma proteins taking part in coagulation. A multi-step reaction in the plasma results in one of the plasma proteins becoming water-insoluble, forming a cross-linked mesh. The cellular elements of blood get caught in the mesh, thus forming a clot. Underneath this clot the endothelium is able to regenerate. After this the clot gets broken down when np longer needed.
The blood of a healthy person fully clots in 5-6 minutes. If coagulation takes much longer than this, or doesn’t happen at all, we talk about hemophilia.
