Contents:
Composition of Blood
Blood consists of both cellular components and liquid plasma. In a typical adult, blood makes up about 7-8% of total body weight, with an average volume of 5-6 liters.
Blood Plasma
Plasma is the yellowish liquid component of blood, making up about 55% of total blood volume. It serves as the medium in which blood cells are suspended.
| Component | Percentage | Description |
|---|---|---|
| Water | 91-92% | Main constituent that dissolves various substances |
| Proteins | 7-8% | Including albumin, globulins, and fibrinogen |
| Other substances | 1-2% | Electrolytes, nutrients, hormones, waste products, gases |
Cellular Components
The cellular components make up the remaining 45% of blood volume and include:
| Cell Type | Percentage of Blood | Number per μL | Function |
|---|---|---|---|
| Red Blood Cells (Erythrocytes) | 44-45% | 4.5-5.5 million | Oxygen and carbon dioxide transport |
| White Blood Cells (Leukocytes) | <1% | 4,500-11,000 | Immune defense |
| Platelets (Thrombocytes) | <1% | 150,000-450,000 | Blood clotting |
Types of White Blood Cells
White blood cells can be further classified into several types, each with specific functions in the immune system:
| Type | Percentage of WBCs | Primary Function |
|---|---|---|
| Neutrophils | 55-70% | First responders to infection, particularly bacterial |
| Lymphocytes | 20-40% | B and T cells for specific immune responses |
| Monocytes | 2-8% | Develop into macrophages; clean up debris and pathogens |
| Eosinophils | 1-4% | Combat parasitic infections and involved in allergic responses |
| Basophils | 0.5-1% | Release histamine in allergic reactions |
Functions of Blood
Blood performs numerous vital functions essential for survival:
1. Transportation
Blood serves as the body’s primary transport system, carrying:
- Oxygen from the lungs to tissues
- Carbon dioxide from tissues to the lungs
- Nutrients from the digestive system to cells
- Waste products to excretory organs
- Hormones from endocrine glands to target tissues
- Enzymes and other proteins to sites where they’re needed
2. Defense Against Disease
Blood plays a crucial role in the body’s immune system:
- White blood cells identify and destroy pathogens
- Antibodies in plasma neutralize foreign substances
- Complement proteins enhance the body’s immune response
- Clotting factors prevent excessive blood loss by sealing wounds
3. Regulation
Blood helps maintain homeostasis by:
- Regulating body temperature through heat distribution
- Maintaining pH balance through buffer systems
- Preserving fluid balance between blood and tissues
- Controlling electrolyte levels for proper cellular function
Blood Types and Compatibility
Human blood is classified into different types based on the presence or absence of antigens on red blood cells and antibodies in plasma.
ABO Blood Group System
| Blood Type | Antigens on RBCs | Antibodies in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB only | All types (Universal recipient) |
| O | None | Anti-A and Anti-B | All types (Universal donor) | O only |
Rh Factor
The Rh factor is another important blood classification:
- Rh-positive (Rh+): Has the Rh antigen on RBCs
- Rh-negative (Rh-): Lacks the Rh antigen on RBCs
Rh factor is particularly important during pregnancy, as Rh incompatibility between mother and fetus can lead to hemolytic disease of the newborn.
Blood Formation (Hematopoiesis)
Blood cells are continuously produced in the bone marrow through a process called hematopoiesis:
| Cell Type | Lifespan | Production Rate |
|---|---|---|
| Red Blood Cells | 120 days | 2-3 million cells/second |
| Platelets | 8-10 days | 100 billion/day |
| Neutrophils | 6-8 hours (circulating) | 100 billion/day |
| Lymphocytes | Days to years | Variable |
Hematopoietic stem cells in the bone marrow differentiate into various blood cell types through a complex process regulated by hormones like erythropoietin and cytokines.
Blood Disorders
There are numerous disorders that can affect blood and its components:
Anemia
Characterized by a reduction in red blood cells or hemoglobin:
| Type | Cause | Symptoms |
|---|---|---|
| Iron Deficiency Anemia | Insufficient iron | Fatigue, weakness, pale skin |
| Sickle Cell Anemia | Genetic mutation in hemoglobin | Pain crises, increased infections |
| Pernicious Anemia | Vitamin B12 deficiency | Fatigue, neurological problems |
| Aplastic Anemia | Bone marrow damage | Fatigue, increased bleeding, infections |
Leukemia
Cancer of blood-forming tissues, characterized by abnormal proliferation of white blood cells:
| Type | Characteristics | Progression |
|---|---|---|
| Acute Lymphoblastic Leukemia (ALL) | Affects lymphoid lineage | Rapid progression |
| Acute Myeloid Leukemia (AML) | Affects myeloid lineage | Rapid progression |
| Chronic Lymphocytic Leukemia (CLL) | Affects lymphoid lineage | Slow progression |
| Chronic Myeloid Leukemia (CML) | Affects myeloid lineage | Initially slow, can accelerate |
Bleeding Disorders
Conditions that affect the body’s ability to form blood clots:
| Disorder | Cause | Symptoms |
|---|---|---|
| Hemophilia | Genetic deficiency of clotting factors | Prolonged bleeding, joint damage |
| Von Willebrand Disease | Deficiency of von Willebrand factor | Mucosal bleeding, easy bruising |
| Thrombocytopenia | Low platelet count | Petechiae, easy bruising, bleeding |
Blood Clotting Disorders
Conditions that lead to excessive or inappropriate clotting:
| Disorder | Characteristics | Risks |
|---|---|---|
| Deep Vein Thrombosis | Blood clot in deep veins | Pulmonary embolism, post-thrombotic syndrome |
| Pulmonary Embolism | Blood clot in lungs | Respiratory failure, death |
| Disseminated Intravascular Coagulation | Widespread clotting followed by bleeding | Multiple organ failure, severe bleeding |
Blood Tests and Diagnostics
Various tests are performed on blood to diagnose diseases and assess overall health:
Complete Blood Count (CBC)
| Measurement | Normal Range (Adults) | Clinical Significance |
|---|---|---|
| Red Blood Cell Count | 4.5-5.5 million/μL (men); 4.0-5.0 million/μL (women) | Anemia, polycythemia |
| Hemoglobin | 13.5-17.5 g/dL (men); 12.0-15.5 g/dL (women) | Oxygen-carrying capacity |
| Hematocrit | 38.8-50.0% (men); 34.9-44.5% (women) | Blood volume status |
| White Blood Cell Count | 4,500-11,000/μL | Infection, inflammation, leukemia |
| Platelet Count | 150,000-450,000/μL | Bleeding or clotting disorders |
Blood Chemistry Tests
| Test | What It Measures | Clinical Significance |
|---|---|---|
| Electrolytes | Sodium, potassium, chloride, bicarbonate levels | Fluid and electrolyte balance |
| Blood Glucose | Blood sugar level | Diabetes, hypoglycemia |
| Lipid Panel | Cholesterol, triglycerides | Cardiovascular risk |
| Liver Function Tests | Enzymes and proteins produced by the liver | Liver disease |
| Kidney Function Tests | Blood urea nitrogen, creatinine | Kidney disease |
Blood in Medical Treatments
Blood Transfusion
The transfer of blood components from a donor to a recipient:
| Component | Used For | Storage Life |
|---|---|---|
| Whole Blood | Rarely used; major blood loss | 21-35 days |
| Packed Red Blood Cells | Anemia, blood loss | 42 days |
| Platelets | Bleeding disorders, thrombocytopenia | 5-7 days |
| Fresh Frozen Plasma | Clotting factor deficiencies | 1 year |
| Cryoprecipitate | Specific clotting factor replacement | 1 year |
Stem Cell Transplantation
Used to treat various blood disorders:
- Autologous: Patient’s own stem cells
- Allogeneic: Stem cells from a donor
- Umbilical cord blood transplantation: Stem cells from umbilical cord blood
Blood Donation
Blood donation is a vital process that helps maintain adequate blood supplies for medical use:
| Donation Type | What’s Collected | Recovery Time |
|---|---|---|
| Whole Blood Donation | All blood components | 56 days before next donation |
| Plateletpheresis | Primarily platelets | 7 days before next donation |
| Plasmapheresis | Primarily plasma | 28 days before next donation |
| Double Red Cell Donation | Red blood cells | 112 days before next donation |
FAQs About Blood
1. How much blood is in the human body? An average adult has about 5-6 liters of blood, constituting approximately 7-8% of total body weight.
2. How often does blood completely renew itself? Different components renew at different rates. Red blood cells have a lifespan of about 120 days, while some white blood cells may last only a few hours or days.
3. Why do some people faint at the sight of blood? This is called vasovagal syncope, a response triggered by the vagus nerve that causes a sudden drop in heart rate and blood pressure, reducing blood flow to the brain.
4. Can lifestyle factors affect blood health? Yes. Diet, exercise, hydration, smoking, and alcohol consumption all impact blood health. A balanced diet rich in iron, vitamins B12 and B9 (folate) supports healthy blood cell production.
5. What makes blood appear red? Hemoglobin, the iron-containing protein in red blood cells, gives blood its red color. Oxygenated blood is bright red, while deoxygenated blood appears darker.
6. Can blood type change over time? Generally, blood type remains constant throughout life. However, certain situations like bone marrow transplants or some rare cancers might seemingly change a person’s blood type.
7. Why can’t all blood types be given to all people? Incompatible blood types can cause severe immune reactions when antibodies in the recipient’s plasma attack antigens on the donor’s red blood cells.
8. Is artificial blood available? While there are blood substitutes and oxygen carriers under development, none can fully replace all functions of human blood. Research continues on creating effective blood substitutes.
References
- American Society of Hematology. (2022). Blood Basics. https://www.hematology.org/education/patients/blood-basics
- National Heart, Lung, and Blood Institute. (2023). Blood Diseases and Disorders. https://www.nhlbi.nih.gov/health-topics/blood-diseases-and-disorders
- World Health Organization. (2024). Blood Safety and Availability. https://www.who.int/news-room/fact-sheets/detail/blood-safety-and-availability
- American Red Cross. (2024). Blood Types. https://www.redcrossblood.org/donate-blood/blood-types.html
- Mayo Clinic. (2023). Complete Blood Count. https://www.mayoclinic.org/tests-procedures/complete-blood-count/about/pac-20384919
- National Library of Medicine. (2024). Hematopoietic Stem Cells. https://medlineplus.gov/ency/article/002244.htm