Physiology Topics
Blood is made of 55% plasma, 45% erythrocytes (red blood cells) and <1% white blood cells and platelets ("buffy coat" in centrifuged blood). Platelets (thrombocytes) are not whole cells but cell fragments. White blood cells include neutrophils (60-70%, lymphocytes (20-25%, monocytes (3-8%), eosinophils (2-4% and basophils (0.5-1%).
An hematocrit is a red blood cell count. A patient with anemia will have an hematocrit below 25%, one wit polycythemia will have a hematocrit as high as 70%.
Plasma is 91.5% water and 8.5% solutes, mostly proteins like albumin, globulins and fibrinogen. Albumin is synthesized in the liver and do not cross blood vessels, contributing to maintain proper blood osmotic pressure. Globulins include the proteins in HDL, VLDL and LDL and others associated with immune responses. Fibrinogen is involved in blood clotting. Other functions of plasma proteins include energy reserves (especially albumin) and serving as carriers for other molecules and ions. Plasma also contains clotting factors, while serum (a plasma derivative) does not.
Blood is more viscous than water, and the more viscous it become, the harder it is to pump and te higher the blood pressure.
All blood cells arise from mesenchyme-derived pluripotent cells, which differentiated into either myeloid or lymphoid stem cells. Lymphoid cells arise in the red bone marrow and mature into B and T lymphocytes in lymphatic tissue. Myeloid cells arise and mature in the red bone marrow into all other blood cell types.
Several hemopoietic growth factors regulate development of particular blood cells. Erythropoietin is produced by the kidneys and increases the number of red blood cell precursors. Hydroxyurea and arginine butyrate increase fetal hemoglobin. Thrombopoietin is produced by the liver and stimulates platelet formation. Cytokines are small glycoproteins produced by red bone marrow, leukocytes, macrophages, fibroblasts and endothelial cells, that regulates the development of white blood cells. Two important cytokine families are the colony stimulating factors (CSFs) and interleukins.
A blood transfusion will be needed to survive after >80% blood loss. If blood loss is minor, only the volume needs to be replenished by infusion of saline or plasma. Whole blood transfusions must be from the same blood type as the patient. The surface of red blood cells contain genetically determined glycoprotein and glycolipid antigens that are recognized as foreign if given to a patient with a different blood type, triggering an immune response.
The ABO blood grouping is based on the red blood cell antigens A and B. For example, people with type A blood have A antigens and anti-B antibodies, and will have an immune reaction to a blood transfusion containing the B antigen. All the ABO types are described below:
| Blood Type | Antigens | Antibodies | Donor to | Can receive |
| A | A | B | A and AB | A or O |
| B | B | A | B and AB | B or O |
| AB | A and B | none | none | any |
| O | none | A and B | All | O |
The Rh blood grouping is based in the presence of the Rh antigen on red blood cells. People with the antigen are Rh+, people without it are Rh-. Plasma does not have anti-Rh antibodies until an Rh- person receives an Rh+ transfusion, or is pregnant with an Rh+ fetus AND maternal and fetus bloods somehow mix (does not always happens). If a second transfusion or pregnancy put the Rh patient's blood in contact with Rh+ blood, the previously formed Rh antibodies will cause hemolysis of the donated or fetal blood.
Continue to "Red Blood Cells" or take a quiz: [Q1] [Q2].
Need more practice? Answer the review questions below.
Question coming soon