Blood: Types,Composition & Functions

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.

Composition of Blood

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

White Blood Cell (WBC)

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

  1. American Society of Hematology. (2022). Blood Basics. https://www.hematology.org/education/patients/blood-basics
  2. National Heart, Lung, and Blood Institute. (2023). Blood Diseases and Disorders. https://www.nhlbi.nih.gov/health-topics/blood-diseases-and-disorders
  3. World Health Organization. (2024). Blood Safety and Availability. https://www.who.int/news-room/fact-sheets/detail/blood-safety-and-availability
  4. American Red Cross. (2024). Blood Types. https://www.redcrossblood.org/donate-blood/blood-types.html
  5. Mayo Clinic. (2023). Complete Blood Count. https://www.mayoclinic.org/tests-procedures/complete-blood-count/about/pac-20384919
  6. National Library of Medicine. (2024). Hematopoietic Stem Cells. https://medlineplus.gov/ency/article/002244.htm