Blood Anatomy and Physiology: Study Guide for Nurses

Every CBC, type and cross, and transfusion reaction you manage comes back to what is in this guide. Know the formed elements, the normal counts, and the clotting cascade, and you will read lab values and recognize trouble faster.

Functions of the Blood

Blood is the only fluid tissue in the body, and it does seven jobs:

1. Carrier of gases, nutrients, and waste. Oxygen loads in the lungs and goes to cells. Cell-made carbon dioxide rides back to the lungs to be expelled. Nutrients, ions, and water move from the digestive tract to cells, and cellular wastes move to the kidneys.
2. Clot formation. Clotting proteins stem blood loss when a vessel is injured.
3. Transport of processed molecules. Substances made in one part of the body are carried in blood to another.
4. Protection against foreign substances. Antibodies help defend against pathogens.
5. Transport of regulatory molecules. Hormones and enzymes that regulate body processes travel in the blood.
6. Maintenance of body temperature. Warm blood carries heat from the core to the surface, where it is released.
7. pH and osmosis regulation. Albumin buffers blood and adds to its osmotic pressure, keeping water in the bloodstream.

Components of Blood

Blood is a complex connective tissue in which living blood cells, the formed elements, are suspended.

Physical Characteristics and Volume

Blood is sticky, opaque, with a metallic taste.

• Color. Varies with oxygen load, from scarlet (oxygen-rich) to dull red (oxygen-poor).
• Weight. Heavier than water and about five times more viscous, mostly because of the formed elements.
• pH. Slightly alkaline, between 7.35 and 7.45.
• Temperature. 38 degrees Celsius (100.4 degrees Fahrenheit), always slightly above core body temperature.

Plasma

Plasma, about 90 percent water, is the liquid part of blood.

• Dissolved substances. Nutrients, salts (electrolytes), respiratory gases, hormones, plasma proteins, and metabolic wastes.
• Plasma proteins. The most abundant solutes. Except for antibodies and protein-based hormones, the liver makes most of them.
• Composition. It shifts constantly as cells add or remove substances, but with a healthy diet homeostatic mechanisms keep it relatively constant.

Formed Elements

A stained blood smear under a light microscope shows disc-shaped red blood cells, brightly stained spherical white blood cells, and scattered platelets that look like debris.

Erythrocytes

Erythrocytes (red blood cells) ferry oxygen to all cells of the body.

• Anucleate. RBCs lack a nucleus and contain few organelles.
• Hemoglobin. This iron-bearing protein carries the bulk of the blood's oxygen.
• Microscopic appearance. Small, flexible biconcave discs with depressed centers, like miniature doughnuts.
• Number of RBCs. Normally about 5 million cells per cubic millimeter of blood. RBCs outnumber WBCs roughly 1000 to 1 and are the main driver of blood viscosity.
• Normal blood. Clinically, normal blood contains 12-18 grams of hemoglobin per 100 milliliters (ml). The content runs slightly higher in men (13-18 g/dl) than in women (12-16 g/dl).

Leukocytes

Leukocytes (white blood cells) are far fewer than RBCs but central to defense against disease.

• Number of WBCs. On average 4,000 to 11,000 WBC/mm3, less than 1 percent of total body volume.
• Body defense. A movable army against bacteria, viruses, parasites, and tumor cells.
• Diapedesis. WBCs slip into and out of blood vessels.
• Positive chemotaxis. They home in on tissue damage and infection by following chemicals diffusing from damaged cells.
• Ameboid motion. Once on the scent, they crawl through tissue spaces using flowing cytoplasmic extensions.
• Leukocytosis. A total WBC count above 11,000 cells/mm3.
• Leukopenia. An abnormally low WBC count.
• Granulocytes. Granule-containing WBCs with lobed nuclei: neutrophils, eosinophils, and basophils.
• Neutrophils. The most numerous WBCs. Multilobed nuclei, fine granules, avid phagocytes at sites of acute infection, partial to bacteria and fungi.
• Eosinophils. Blue-red bilobed nucleus, coarse brick-red lysosome-like granules. Their numbers climb fast during allergies and parasitic worm infections.
• Basophils. The rarest WBCs. Large histamine-containing granules that stain dark blue. Histamine makes vessels leaky and draws other WBCs to the inflammatory site.
• Agranulocytes. WBCs without visible cytoplasmic granules and with spherical, oval, or kidney-shaped nuclei: lymphocytes and monocytes.
• Lymphocytes. Large dark-purple nucleus filling most of the cell. They reside in lymphatic tissue and drive the immune response.
• Monocytes. The largest WBCs. In tissue they become macrophages with huge appetites, key to fighting chronic infections.
• Platelets. Not true cells but fragments of large multinucleate megakaryocytes, which pinch off thousands of anucleate platelet pieces. Needed for clotting when vessels rupture.

Hematopoiesis

Blood cell formation (hematopoiesis) happens in red bone marrow (myeloid tissue).

• Hemocytoblast. The common stem cell all formed elements arise from.
• Descendants of hemocytoblasts. Two lines: the lymphoid stem cell, which makes lymphocytes, and the myeloid stem cell, which makes all other formed elements.

Formation of Red Blood Cells

Because they are anucleate, RBCs cannot synthesize proteins, grow, or divide.

• Life span. As they age, RBCs stiffen and fragment in 100 to 120 days.
• Lost RBCs. Replaced continuously by hemocytoblast division in red bone marrow.
• Immature RBCs. Developing RBCs divide repeatedly, then synthesize huge amounts of hemoglobin.
• Reticulocyte. Once enough hemoglobin accumulates, the nucleus and most organelles are ejected and the cell collapses inward. The young RBC is a reticulocyte because it still holds some rough endoplasmic reticulum (ER).
• Mature erythrocytes. Within 2 days of release they shed the remaining ER and become fully functioning. The full path from hemocytoblast to mature RBC takes 3 to 5 days.
• Erythropoietin. This hormone controls the rate of RBC production. A small amount always circulates, so RBCs form at a fairly constant rate.
• Control of RBC production. Production is not driven by the number of RBCs but by their ability to deliver enough oxygen to meet the body's demands.

Formation of White Blood Cells and Platelets

Like RBC production, leukocyte and platelet formation is hormone-driven.

• Colony stimulating factors and interleukins. They prompt red bone marrow to release leukocytes and boost mature leukocytes' ability to protect the body.
• Thrombopoietin. This hormone speeds platelet production, though its regulation is not well understood.

Hemostasis

Hemostasis is a multistep process that starts when a vessel is damaged and its wall connective tissue is exposed to blood.

• Vascular spasms occur. The immediate response is vasoconstriction. The vessel goes into spasm, narrowing and cutting blood loss until clotting can occur.
• Platelet plug forms. Injury exposes collagen fibers, platelets adhere to the site, and a platelet plug forms.
• Coagulation events occur. Injured tissue releases tissue factor (TF). PF3, a phospholipid coating the platelets, interacts with TF, vitamin K, and other clotting factors. This prothrombin activator converts prothrombin in the plasma to the enzyme thrombin. Thrombin links soluble fibrinogen into long insoluble fibrin strands, which mesh and trap RBCs to form the clot. Within the hour the clot retracts, squeezing out serum and pulling the ruptured vessel edges together.

Blood Groups and Transfusions

Blood transports substances throughout the body. When blood is lost, vessels constrict and marrow ramps up cell formation to keep circulation going.

Human Blood Groups

Whole blood transfusions save lives, but people have different blood groups, and transfusing mismatched blood can be fatal.

• Antigen. A substance the body recognizes as foreign, triggering antibodies or other defenses.
• Antibodies. One person's RBC proteins read as foreign when transfused into someone with different RBC antigens. The recognizers are plasma antibodies that attach to RBCs bearing surface antigens different from the recipient's.
• Agglutination. Antibody binding clumps the foreign RBCs, clogging small vessels throughout the body.
• ABO blood groups. Based on which of two antigens (type A or type B) a person inherits. Neither antigen gives type O, both give type AB, and either A or B alone gives type A or type B.
• Rh blood groups. Named because one of the eight Rh antigens (agglutinogen D) was first identified in Rhesus monkeys, then found in humans. Most Americans are Rh+ (Rh positive), carrying the Rh antigen.
• Anti-Rh antibodies. Unlike ABO antibodies, anti-Rh antibodies are not automatically present in Rh- (Rh-negative) individuals.
• Hemolysis. Rupture of RBCs does not occur on the first transfusion because the body needs time to react and make antibodies.

Blood Typing

Determining the blood group of both donor and recipient before transfusion is essential.

• Blood typing of ABO groups. Adding serum with anti-A or anti-B antibodies to a saline-diluted blood sample causes agglutination between antibody and matching antigen.
• Cross matching. Tests for agglutination of donor RBCs by recipient serum, and recipient RBCs by donor serum.
• Blood typing for Rh factors. Done the same way as ABO typing.