1.6 - Transfusion and Blood Replacement

Formed element lifespan and blood donation

Most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks (memory cells, a type of leukocyte, can survive for years); when a person donates a unit of blood (about 0.5 L), the body replaces the plasma within 24 hours but takes about 4 to 6 weeks to replace the formed elements

Hematopoiesis (hemopoiesis)

The process by which blood cell replacement occurs; following birth, most hematopoiesis occurs in red bone marrow

Red marrow location (hematopoiesis)

A connective tissue within the spaces of spongy (cancellous) bone tissue; in children, hemopoiesis can occur in the medullary cavity of long bones, while in adults it is largely restricted to the cranial and pelvic bones, vertebrae, sternum, and proximal epiphyses of the femur and humerus

Hematopoietic stem cell (hemocytoblast)

The specific type of cell from which all formed elements of blood originate

Hemopoietic growth factors

Chemical stimuli that prompt the hemocytoblast to divide and differentiate; one daughter cell remains a hematopoietic stem cell (allowing hemopoiesis to continue), while the other becomes a more specialized stem cell

Lymphoid stem cells

Give rise to lymphocytes (T cells, B cells, and natural killer cells), which function in immunity; quickly migrate from bone marrow to lymphatic tissues (lymph nodes, spleen, thymus) where production and differentiation continue; B cells mature in the bone marrow, while T cells mature in the thymus

Myeloid stem cells

Give rise to all other formed elements, including erythrocytes, megakaryocytes (which produce platelets), and a myeloblast (which gives rise to neutrophils, eosinophils, and basophils)

Erythropoiesis

The formation of new red blood cells; occurs over the course of several days in three general steps: ribosome synthesis, hemoglobin accumulation, and ejection of the nucleus/formation of reticulocytes

Ribosome synthesis (erythropoiesis step 1)

Proerythroblasts generate ribosomes to prepare for the synthesis of hemoglobin and other vital proteins

Hemoglobin accumulation (erythropoiesis step 2)

Erythroblasts fill with hemoglobin, which enables the future red blood cells to transport oxygen

Ejection of the nucleus / reticulocyte formation (erythropoiesis step 3)

Normoblasts expel their nuclei to form reticulocytes, which shortly mature into biconcave erythrocytes circulating in the blood; reticulocytes maintain a few organelles, but within a few days the remaining organelles are ejected, becoming a mature red blood cell

Kidney filtration and oxygen monitoring

The kidneys filter about 180 liters of blood in an average adult each day, about 20 percent of total resting volume, making them ideal sites for receptors that determine oxygen saturation in the blood

Hypoxia

Low oxygen concentration in the tissues, which occurs as red blood cells die; too many red blood cells, by contrast, causes increased blood viscosity, so there must be a balance in RBC production and destruction

Hypoxemia

A lower than normal level of oxygen specifically in the blood (as distinct from hypoxia, a deficiency in oxygen reaching the tissues)

Erythropoietin (EPO)

A hormone secreted by interstitial fibroblasts within the kidney in response to hypoxemia/hypoxia in kidney tissue fluid; acts to increase the rate of erythrocyte production, restoring oxygen levels over 1-2 days

EPO negative feedback loop

As oxygen saturation rises, EPO secretion falls, and vice versa, maintaining homeostasis; testosterone helps enhance EPO production, causing males to have a higher RBC count than females

Blood transfusion

A medical procedure where a type of donor blood is infused into another person; risky until the discovery of major human blood groups in 1900, since incompatible donor blood could cause death

Blood groups (basis)

Determined by the presence or absence of specific marker molecules on the plasma membranes of erythrocytes; their discovery made it possible to match patient-donor blood types and prevent transfusion reactions and deaths

Whole-blood transfusion

Used when there is major blood loss and the person needs a large volume of blood to be replaced

Packed red blood cell transfusion

Removes the plasma before transfusion; used when a person needs an increase in oxygen-carrying capacity, such as in various forms of anemia