Blood is considered a modified type of connective tissue. Mesodermal is composed of cells and cell frag ments (erythrocytes, leukocytes, platelets), fibrous proteins (fi—brinogen), and an extracellular fluid and proteins (plasma). It also contains cellular elements of the immune system as well as humoral factors.

The formed elements of the blood include erythrocytes, leukocytes, and platelets.

Erythrocytes, or red blood cells, are important in trans porting oxygen from the lungs to tissues and in returning carbon dioxide to the lungs. Oxygen and carbon dioxide carried in the RBC combine with hemoglobin to form ox—yhemoglobin and carbaminohemoglobin, respectively.

Mature erythrocytes are denucleated, biconcave disks with a diameter of 7–8 mm. The biconcave shape results in a 20–30 % increase in sur face area compared to a sphere.

Erythrocytes have a very large surface area: volume ratio that allows for efficient gas transfer. Erythrocyte membranes are remarkably pliable, enabling the cells to squeeze through the narrowest capillaries. In sickle cell anemia, this plasticity is lost, and the subsequent clogging of capillaries leads to sickle crisis. The normal concentration of erythrocytes in blood is 3,5–5,5 million/mm 3 in women and 4,3–5,9 million/mm 3 in men. The packed volume of blood cells per total volume of known as the he—matocrit. Normal hematocrit values are 46 % for women and 41–53 % for men.

When aging RBCs develop subtle changes, macropha—ges in the bone marrow, spleen, and liver engulf and digest them. The iron is carried by transferring in the blood to certain tissues, where it combines with apoferritin to form ferritin. The heme is catabolized into biliver—din, which is converted to bilirubin. The latter is secreted with bile salts.

Platelets (thromboplastids) are 2–3 mm in diameter.

They are a nuclear, membrane—bound cellular fragments derived by cytoplasmic fragmentation of giant cells, called megakaryocytes, in the bone marrow.

They have a short life span of approximately 10 days.

There are normally 150 000–400 000 platelets per mm3 of blood. Ultrastructurally, platelets contain two portions: a peripheral, light—staining hyalomere that sends out fine cytoplasmic processes, and a central, dark—staining granulomere that con tains mitochondria, vacuoles, glycogen granules, and granules. Platelets seal minute breaks in blood vessels and maintain endothelial integrity by adhering to the damaged vessel in a process known as platelet aggregation. Platelets are able to form a plug at the rupture site of a vessel because their mem brane permits them to agglutinate and adhere to surfaces.

Platelets aggregate to set up the cascade of enzymatic reac tions that convert fibrinogen into the fibrin fibers that make up the clot.

New words

mesodermal – мезодермальный

erythrocytes – эритроциты

platelets – тромбоциты

carbon – углерод

dioxid – диоксид

span – промежуток

light—staining – легкое окрашивание

to aggregate – соединяться