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Purdue University Globle
NU551 Advanced Physiology and Pathophysiology Across the Lifespan
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The hematological system produces blood cells, transports oxygen and nutrients, protects the body against infection, and prevents excessive bleeding through blood clotting. It includes blood, bone marrow, the spleen, lymph nodes, and other lymphatic tissues. Understanding blood composition, hematopoiesis, erythropoiesis, hemoglobin, and anemia helps nursing students and healthcare professionals recognize normal blood function and identify common hematologic disorders.
The hematological system is the collection of organs, tissues, and circulating blood components responsible for producing, maintaining, and regulating blood cells. It supports several essential physiological processes, including oxygen delivery, immune defense, nutrient transport, waste removal, and hemostasis.
Blood cells are continuously produced and replaced throughout life. The bone marrow generates new blood cells, the spleen removes aging or damaged cells, and the lymphatic system supports immune surveillance. Together, these structures help maintain adequate blood cell levels and normal body function.
The major components of the hematological system include:
Bone marrow: Produces red blood cells, white blood cells, and platelets through hematopoiesis.
Blood: Transports oxygen, nutrients, hormones, and metabolic waste.
Spleen: Filters blood, removes aging red blood cells, stores platelets, and supports immune responses.
Lymph nodes: Filter lymphatic fluid and help activate immune cells.
Lymphatic tissues: Support immune defense and the development and activity of lymphocytes.
Blood is a specialized connective tissue made up of plasma and formed elements. In a typical adult, plasma accounts for approximately 55% of blood volume, while formed elements account for approximately 45%. The exact proportions vary with hydration, health status, and other physiological factors.
Plasma is the liquid portion of blood. It is composed primarily of water, with dissolved proteins, electrolytes, nutrients, hormones, gases, and waste products.
Plasma performs several important functions:
Transports nutrients, hormones, medications, and waste products.
Maintains blood volume and fluid balance.
Supports acid-base and electrolyte regulation.
Carries proteins involved in immunity and coagulation.
The major plasma proteins are albumin, globulins, and fibrinogen.
Albumin is the most abundant plasma protein and is synthesized primarily by the liver. It helps maintain plasma oncotic pressure, which keeps fluid within blood vessels and limits excessive movement into surrounding tissues.
Albumin also transports substances such as fatty acids, hormones, bilirubin, and certain medications.
Low albumin levels are known as hypoalbuminemia. They may occur with liver disease, kidney disease involving protein loss, malnutrition, or severe inflammation. Hypoalbuminemia can contribute to edema, although the cause of edema must be evaluated in the context of the patient’s overall condition.
Globulins are a group of plasma proteins involved in transport, immune defense, and other biological functions. They include immunoglobulins, also called antibodies, which help the immune system recognize and respond to foreign substances.
Globulins may:
Transport lipids, hormones, vitamins, and other substances.
Support antibody-mediated immune responses.
Participate in inflammatory and immune processes.
Fibrinogen is a soluble plasma protein produced by the liver. During coagulation, it is converted into fibrin, which forms a mesh that helps stabilize a blood clot.
Adequate fibrinogen is essential for normal hemostasis and wound healing.
Formed elements include erythrocytes, leukocytes, and platelets. These components perform distinct roles in oxygen transport, immune defense, and blood clotting.
Erythrocytes, or red blood cells (RBCs), are the most abundant formed elements in circulating blood. Their primary function is to transport oxygen from the lungs to body tissues and assist in carrying carbon dioxide back to the lungs.
Mature red blood cells have several distinctive characteristics:
They are biconcave in shape, which increases surface area for gas exchange.
They lack a nucleus and most organelles, providing more space for hemoglobin.
They are flexible, allowing them to pass through small blood vessels.
Their average lifespan is approximately 120 days.
Hemoglobin inside erythrocytes binds oxygen and enables efficient oxygen transport. Red blood cells are essential for maintaining tissue oxygenation and supporting cellular metabolism.
Important distinction: Red blood cell lifespan is relevant to interpreting hemoglobin A1c (HbA1c), but HbA1c measures average blood glucose over approximately two to three months. It is not a direct measurement of red blood cell lifespan.
Reticulocytes are immature red blood cells recently released from the bone marrow. They still contain residual ribosomal material and mature into erythrocytes within approximately one to two days in circulation.
The reticulocyte count helps clinicians evaluate bone marrow activity and the body’s response to anemia.
An increased reticulocyte count may occur when the bone marrow responds appropriately to:
Acute blood loss.
Hemolysis, or increased red blood cell destruction.
Successful treatment of certain nutritional deficiencies.
A low or inadequately increased reticulocyte count in a patient with anemia may suggest reduced red blood cell production.
White blood cells (WBCs), or leukocytes, protect the body against infections and other harmful substances. They are essential components of the immune system.
The five major types of white blood cells include:
Neutrophils: Help fight bacterial and fungal infections.
Lymphocytes: Include B cells, T cells, and natural killer cells involved in adaptive and innate immunity.
Monocytes: Develop into macrophages and other cells that participate in phagocytosis and immune regulation.
Eosinophils: Participate in responses to parasites and allergic conditions.
Basophils: Release substances involved in inflammation and allergic reactions.
Changes in white blood cell counts may provide important clues about infection, inflammation, immune disorders, or bone marrow disease.
Platelets are small cell fragments derived from megakaryocytes in the bone marrow. They play a central role in hemostasis, the process that prevents excessive bleeding.
When a blood vessel is injured, platelets adhere to the damaged area, become activated, and aggregate to form a platelet plug. Coagulation proteins then help stabilize the plug with fibrin.
Common platelet disorders include:
Thrombocytopenia: A platelet count below the normal reference range, which may increase bleeding risk.
Thrombocytosis: An elevated platelet count, which may occur as a reactive response or due to a bone marrow disorder.
The clinical significance of either condition depends on the platelet count, underlying cause, and the patient’s symptoms.
The spleen is the largest lymphoid organ in the body and plays important roles in blood filtration, immune defense, and blood cell recycling.
Its major functions include:
Removing aging and damaged red blood cells.
Filtering blood and helping remove certain microorganisms.
Supporting immune responses against blood-borne pathogens.
Storing a portion of the body’s platelets.
Recycling iron from hemoglobin in destroyed red blood cells.
Macrophages in the spleen help identify and remove senescent erythrocytes. The liver and other tissues also participate in the removal of aging blood cells.
The spleen is especially important in defending against certain encapsulated bacteria. Individuals without a functioning spleen may have an increased risk of serious infections and require appropriate preventive healthcare.
Lymph nodes are small, bean-shaped structures located throughout the lymphatic system. They filter lymphatic fluid and provide sites where immune cells encounter foreign antigens.
Lymph nodes contain lymphocytes, macrophages, and other immune cells that help detect and respond to infection.
Their functions include:
Filtering lymph and trapping foreign particles.
Supporting the activation of B cells and T cells.
Helping coordinate immune responses.
Returning filtered lymphatic fluid to the circulation through the lymphatic system.
Lymph node enlargement, known as lymphadenopathy, may occur because of infection, inflammation, or malignancy. Assessment should consider the location, size, tenderness, duration, and associated symptoms.
Bone marrow is the primary site of blood cell production after birth. The process of producing red blood cells, white blood cells, and platelets is called hematopoiesis, or hemopoiesis.
Hematopoiesis begins with hematopoietic stem cells. These cells can self-renew and differentiate into multiple blood cell lineages.
The major blood cell lineages include:
Erythroid lineage: Produces red blood cells.
Myeloid lineage: Produces several types of white blood cells and megakaryocytes, which produce platelets.
Lymphoid lineage: Produces lymphocytes, including B cells, T cells, and natural killer cells.
In adults, active red bone marrow is primarily found in the axial skeleton and selected portions of the proximal long bones. Important locations include:
Pelvis.
Sternum.
Ribs.
Vertebrae.
Skull.
Proximal ends of the femur and humerus.
Bone marrow activity changes with age. Much of the marrow in long bones becomes fatty yellow marrow during adulthood, although the body retains the ability to increase blood cell production when necessary.
Erythropoiesis is the process through which red blood cells develop from hematopoietic stem cells. It occurs primarily in the bone marrow and is regulated by the body’s oxygen requirements.
When tissue oxygen levels decrease, the kidneys produce more erythropoietin (EPO). EPO stimulates red blood cell production in the bone marrow.
The simplified maturation sequence is:
Hematopoietic stem cell.
Erythroid progenitor cell.
Proerythroblast.
Erythroblast, or normoblast.
Reticulocyte.
Mature erythrocyte.
During maturation, the developing cell accumulates hemoglobin, becomes smaller, and eventually loses its nucleus. The reticulocyte is released into circulation and develops into a mature red blood cell.
Healthy red blood cell production requires adequate supplies of iron, vitamin B12, folate, protein, and other nutrients.
Iron is necessary for hemoglobin synthesis. Iron deficiency can reduce hemoglobin production and lead to iron-deficiency anemia.
Vitamin B12 and folate are required for normal DNA synthesis during red blood cell development. Deficiency in either nutrient can impair cell maturation and cause megaloblastic anemia.
Red blood cells circulate for approximately 120 days before being removed from circulation. Aging erythrocytes are primarily cleared by macrophages in the spleen, liver, and bone marrow.
This process is called erythrophagocytosis.
During red blood cell breakdown:
Hemoglobin is separated into heme and globin.
Globin is broken down into amino acids that can be reused.
Iron is recovered and recycled for future hemoglobin synthesis.
Heme is converted into bilirubin, which is processed by the liver and excreted through bile.
The balance between red blood cell production and destruction is essential for maintaining normal hemoglobin levels.
When destruction exceeds production, hemolytic anemia may develop.
Hemoglobin is an iron-containing protein located inside red blood cells. It is responsible for carrying oxygen from the lungs to tissues and assisting with carbon dioxide transport back to the lungs.
Each hemoglobin molecule contains four globin chains and four heme groups. Each heme group contains iron that can bind oxygen.
Hemoglobin is important because it:
Delivers oxygen required for cellular metabolism.
Contributes to carbon dioxide transport.
Helps buffer hydrogen ions and support acid-base balance.
Determines much of the blood’s oxygen-carrying capacity.
Iron deficiency can decrease hemoglobin synthesis and reduce oxygen delivery to tissues. This may cause fatigue, weakness, shortness of breath, and other symptoms of anemia.
Aging is associated with changes in bone marrow activity, immune function, and blood cell regulation. These changes do not automatically cause anemia, but they may influence how older adults respond to illness.
Age-related changes may include:
Reduced bone marrow reserve and responsiveness to physiological stress.
Changes in immune cell function.
Increased prevalence of chronic diseases that contribute to anemia.
Greater susceptibility to certain infections.
Reduced ability to tolerate acute blood loss or other physiological stressors.
Anemia in older adults should not be assumed to be a normal consequence of aging. It requires evaluation for causes such as iron deficiency, chronic disease, kidney dysfunction, vitamin deficiency, blood loss, or bone marrow disorders.
Anemia is a condition in which the blood has insufficient oxygen-carrying capacity, usually because hemoglobin concentration is below the expected reference range. It may also result from a reduced number of circulating red blood cells or abnormal red blood cell function.
Anemia develops when the body:
Produces too few red blood cells.
Loses blood through bleeding.
Destroys red blood cells faster than they can be replaced.
The symptoms and severity of anemia depend on its cause, degree, and rate of development. Mild anemia that develops gradually may cause few symptoms, while severe or rapidly developing anemia can produce significant tissue hypoxia.
The major mechanisms of anemia include decreased production, blood loss, and increased destruction.
Decreased red blood cell production may occur with:
Iron deficiency.
Vitamin B12 or folate deficiency.
Chronic kidney disease.
Bone marrow disorders.
Chronic inflammation.
Certain medications or toxic exposures.
Blood loss may result from:
Menstrual bleeding.
Gastrointestinal bleeding.
Trauma or surgery.
Obstetric bleeding.
Other acute or chronic sources of blood loss.
Increased red blood cell destruction, or hemolysis, may occur because of:
Autoimmune disorders.
Inherited red blood cell conditions.
Certain infections.
Medication reactions.
Mechanical destruction of red blood cells.
Anemia reduces the amount of oxygen available to tissues. Common symptoms include:
Fatigue and reduced energy.
Generalized weakness.
Dizziness or lightheadedness.
Shortness of breath, especially during activity.
Pallor of the skin or mucous membranes.
Headache.
Palpitations or increased heart rate.
Cold hands and feet.
Some patients with iron deficiency may develop pagophagia, an unusual craving for ice. This symptom can be associated with iron-deficiency anemia but is not diagnostic by itself.
Severe anemia may cause chest discomfort, fainting, confusion, or signs of cardiovascular strain. These symptoms require prompt medical assessment.
Red blood cell morphology describes the size, shape, and appearance of erythrocytes on a peripheral blood smear. These findings can help clinicians identify the cause of anemia and other hematologic disorders.
Anisocytosis refers to variation in the size of red blood cells.
It may occur in:
Iron-deficiency anemia.
Vitamin B12 or folate deficiency.
Mixed nutritional deficiencies.
Conditions involving abnormal red blood cell production or transfusion.
The red cell distribution width (RDW), included in many CBC reports, helps quantify variation in red blood cell size.
Poikilocytosis refers to variation in the shape of red blood cells.
Abnormally shaped cells may provide clues to underlying conditions, such as sickle cell disease, hereditary spherocytosis, or other hematologic disorders.
A peripheral blood smear can help identify characteristic cell shapes that support further diagnostic testing.
Mean Corpuscular Volume (MCV) measures the average volume of a red blood cell. It is reported as part of the complete blood count and is expressed in femtoliters (fL).
MCV helps classify anemia into three broad categories:
| Anemia classification | MCV | Common causes |
|---|---|---|
| Microcytic anemia | Below approximately 80 fL in adults | Iron deficiency, thalassemia, some chronic inflammatory conditions |
| Normocytic anemia | Approximately 80–100 fL in adults | Acute blood loss, chronic kidney disease, hemolysis, chronic disease |
| Macrocytic anemia | Above approximately 100 fL in adults | Vitamin B12 deficiency, folate deficiency, alcohol-related disease, certain medications |
These ranges are general adult reference values. Laboratory reference intervals may vary.
Microcytic anemia is characterized by smaller-than-normal red blood cells. Iron deficiency is a common cause, particularly when iron stores are depleted.
Other causes include thalassemia and certain disorders of hemoglobin synthesis.
Evaluation may include ferritin, serum iron studies, transferrin saturation, and assessment for possible blood loss.
Normocytic anemia occurs when red blood cells are generally normal in size but hemoglobin levels are reduced.
Common causes include:
Acute blood loss.
Chronic kidney disease.
Anemia of inflammation.
Hemolysis.
Bone marrow disorders.
The reticulocyte count helps determine whether the bone marrow is responding appropriately.
Macrocytic anemia involves larger-than-normal red blood cells. Vitamin B12 and folate deficiencies can impair DNA synthesis and cause megaloblastic anemia.
Other causes of macrocytosis include alcohol use, liver disease, hypothyroidism, and certain medications.
The complete blood count (CBC) is a commonly ordered laboratory test used to assess blood cell levels and identify possible hematologic abnormalities.
A CBC typically includes:
Red blood cell count.
Hemoglobin.
Hematocrit.
Mean corpuscular volume (MCV).
White blood cell count.
Platelet count.
Additional red blood cell indices, such as MCH, MCHC, and RDW.
Hemoglobin measures the concentration of hemoglobin in blood. It is one of the primary laboratory values used to identify anemia.
Hematocrit represents the percentage of blood volume occupied by red blood cells.
It may be affected by anemia, dehydration, fluid overload, and other conditions.
The WBC count measures the number of white blood cells in the blood. Abnormal results may indicate infection, inflammation, immune disorders, or bone marrow disease.
The platelet count helps evaluate the body’s ability to form platelet plugs and maintain hemostasis.
A low platelet count may increase bleeding risk, while a high count may be associated with thrombosis in some conditions.
MCV measures average red blood cell size and helps classify anemia as microcytic, normocytic, or macrocytic.
Nurses play an important role in recognizing signs of hematologic disorders, monitoring laboratory results, and identifying changes that require further evaluation.
During a nursing assessment, healthcare professionals should consider:
Fatigue, weakness, dizziness, and activity intolerance.
Pallor, shortness of breath, and increased heart rate.
Bleeding, bruising, petechiae, or prolonged bleeding.
Nutritional intake and possible vitamin or iron deficiency.
Menstrual, gastrointestinal, surgical, or other blood loss.
Medication history and chronic health conditions.
CBC results and trends in hemoglobin, hematocrit, MCV, and platelets.
A nurse should report significant changes in vital signs, active bleeding, severe shortness of breath, chest pain, or other signs of clinical deterioration according to the patient’s condition and facility protocols.
| Term | Definition |
|---|---|
| Hematopoiesis | Production of blood cells, primarily in the bone marrow |
| Erythropoiesis | Formation of red blood cells |
| Erythrocyte | Mature red blood cell |
| Reticulocyte | Immature red blood cell recently released from the bone marrow |
| Hemoglobin | Iron-containing protein responsible for oxygen transport |
| Albumin | Plasma protein that helps maintain oncotic pressure |
| Globulins | Plasma proteins involved in transport and immune defense |
| Fibrinogen | Plasma protein converted to fibrin during clotting |
| Leukocyte | White blood cell involved in immune defense |
| Thrombocyte | Platelet involved in hemostasis |
| Thrombocytopenia | Low platelet count |
| Thrombocytosis | Elevated platelet count |
| MCV | Average red blood cell volume |
| Anisocytosis | Variation in red blood cell size |
| Poikilocytosis | Variation in red blood cell shape |
| Anemia | Reduced blood oxygen-carrying capacity, commonly due to low hemoglobin |
The hematological system is the body’s blood-forming and blood-regulating system. It includes blood, bone marrow, the spleen, lymph nodes, and other lymphatic tissues. Its main functions are producing blood cells, transporting oxygen, supporting immunity, and maintaining hemostasis.
Erythrocytes, or red blood cells, transport oxygen from the lungs to body tissues and help carry carbon dioxide back to the lungs. Hemoglobin inside red blood cells binds oxygen and supports tissue oxygenation.
Reticulocytes are immature red blood cells released from the bone marrow. They mature into erythrocytes within approximately one to two days and help indicate the body’s response to anemia or blood loss.
Hemoglobin is important because it binds oxygen in the lungs and transports it to tissues. Low hemoglobin can reduce oxygen delivery and cause symptoms such as fatigue, weakness, dizziness, and shortness of breath.
Hypoalbuminemia means a low blood albumin level. Causes include liver disease, kidney disease with protein loss, malnutrition, and severe inflammation. Low albumin can contribute to edema and should be interpreted alongside other clinical findings.
Hematopoiesis is the process of producing red blood cells, white blood cells, and platelets. After birth, it occurs primarily in the bone marrow and is regulated by growth factors and the body’s physiological needs.
MCV measures the average size of red blood cells. It helps classify anemia as microcytic, normocytic, or macrocytic, which guides further evaluation of possible causes.
Hematopoiesis refers to the production of all blood cells, including red blood cells, white blood cells, and platelets. Erythropoiesis specifically refers to the formation of red blood cells.
The spleen filters blood, removes aging red blood cells, recycles iron, stores platelets, and supports immune responses against blood-borne microorganisms.
The three main types are microcytic anemia, normocytic anemia, and macrocytic anemia. They are classified according to the average size of red blood cells.
The hematological system is essential for maintaining oxygen delivery, immune protection, nutrient transport, and normal blood clotting. The bone marrow produces blood cells through hematopoiesis, while erythropoiesis specifically generates red blood cells. Plasma proteins, erythrocytes, leukocytes, platelets, the spleen, and lymphatic tissues work together to maintain healthy blood function.
Understanding anemia, red blood cell morphology, hemoglobin, MCV, and CBC findings provides nursing students and healthcare professionals with a foundation for recognizing blood disorders. By connecting normal hematological physiology with clinical assessment and laboratory interpretation, nurses can identify potential problems early and support appropriate patient care.
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