Platelets

Platelets, or thrombocytes, are minute fragments of cells consisting of a small amount of cytoplasm surrounded by a cell membrane. They are roughly disk shaped and average approximately 3 m in diameter.

The life expectancy of platelets is approximately 5 to 9 days. Platelets are produced within the marrow and are derived from megakaryocytes, which are extremely large cells with diameters up to 100 m. Small fragments of these cells break off and enter the circulation as platelets.

Platelets play an important role in preventing blood loss. This prevention is accomplished in two ways: the formation of platelet plugs, which seal holes in small vessels; and the formation of clots, which help seal off larger wounds in the vessels.

If the number of platelets is too low, excessive bleeding can occur. However, if the number of platelets is too high, blood clots can form (thrombosis), which may obstruct blood vessels and result in such events as stroke, heart attack, pulmonary embolism or blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs.

Ex. 23. Answer the following questions:

1. What is platelet? 2. What are the shape of platelet? 3. What is the life expectancy of platelets? 4. Where are the platelets produced? 5. What is the major function of platelets?

Ex. 24. Read and retell the following text:

Leukocytes

Leukocytes, or white blood cells, are nucleated blood cells that lack hemoglobin. They are clear or whitish in color and are larger than erythrocytes, ranging from 8 to 19 m in diameter. In stained preparations leukocytes attract stain, whereas erythrocytes remain relatively unstained.

Leukocytes protect the body against invading microorganisms and remove dead cells and debris from the body. Leukocytes leave the circulation by the process of diapedesis (movement through vessel walls) and move through the tissues in which they ingest foreign material or dead cells. At the site of an infection leukocytes accumulate and phagocytize bacteria, dirt, and dead cells; then they die. This accumulation of dead leukocytes, along with fluid and cell debris, is called pus.

Leukocytes are named according to their appearance in stained preparations. Leukocytes containing large cytoplasmic granules are granulocytes, and those with very small granules that cannot be seen easily with the light microscope are agranulocytes. The three types of granulocytes are named according to the staining characteristics of their cytoplasm: neutrophils, eosinophils, and basophils. There are two types of agranulocytes: monocytes and lymphocytes.

Ex. 25. Translate the following sentences into English.

1. Основними життєво важливими функціями крові є транспортувальна та захисна функції, а також функція підтримання стабільного клітинного середовища. 2. Кров переносить поживні речовини з шлунково-кишкового тракту та кисень з дихальних органів до усіх клітин організму. 3. Продукти розпаду транспортуються кров’ю до дихальних та видільних органів. 4. Загальний обсяг крові поділяється на циркулюючий (circulating) обсяг крові та резервний (reserve) обсяг крові. 5. Резервна кров зберігається в певних органах, наприклад, в печінці. 6. Плазма крові містить амінокислоти, прості цукри, різноманітні ліпіди. 7. Функції плазми полягають у перенесенні газів, поживних речовин та вітамінів, у регулюванні рідинного та електролітичного (electrolyte) балансу, у підтриманні відповідного pH.