1. Introduction to Human Blood
Blood is a vital, specialized bodily fluid that circulates through the cardiovascular system, sustaining life by delivering oxygen, nutrients, and hormones while removing waste products. It makes up about 7-8% of an adult's body weight—roughly 4-6 liters (about 1-1.5 gallons) in an average adult. Blood is considered a connective tissue because it's composed of cells suspended in a liquid matrix called plasma. It's constantly in motion, pumped by the heart, and plays a crucial role in homeostasis (maintaining the body's internal balance).
Blood's color comes from hemoglobin, a protein in red blood cells that binds oxygen—arterial blood (oxygen-rich) is bright red, while venous blood (oxygen-poor) is darker red or purplish.
2. Composition of Blood
Blood is a mixture of liquid and cellular components. When centrifuged (spun in a lab to separate parts), it divides into layers: plasma at the top (55%), a thin "buffy coat" of white blood cells and platelets in the middle (1%), and red blood cells at the bottom (44%).
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| Human blood |
a. Plasma
What it is plasma: The liquid portion of blood, making up about 55% of total blood volume. It's a pale yellow fluid that's 90-92% water.
Components:
Proteins(7-8%): Include albumin (maintains osmotic pressure and transports substances), globulins (involved in immunity and transport), and fibrinogen (essential for clotting).
-Electrolytes: Salts like sodium, potassium, calcium, and chloride, which help regulate fluid balance, nerve function, and muscle contraction.
Nutrients and Waste: Glucose, amino acids, lipids, vitamins, hormones, and waste products like urea and carbon dioxide.
Other: Gases (oxygen and carbon dioxide), hormones, and clotting factors.
Functions: Acts as a solvent for transporting substances and helps regulate body temperature and pH (blood pH is tightly maintained at 7.35-7.45).
b. Red Blood Cells (Erythrocytes)
What they are: The most abundant cells in blood (about 4-6 million per microliter). They're biconcave discs (like a donut without a hole) for flexibility and maximum surface area. They lack a nucleus, so they live only about 120 days.
Key Component: Hemoglobin, an iron-containing protein that binds oxygen (each red blood cell carries about 250 million hemoglobin molecules).
Functions: Transport oxygen from the lungs to tissues and carry carbon dioxide back to the lungs for exhalation. They also help buffer blood pH.
Production and Recycling: Made in bone marrow; old cells are broken down in the spleen and liver, with iron recycled.
c. White Blood Cells (Leukocytes)
What they are: Fewer in number (4,000-11,000 per microliter) but crucial for defense. They have nuclei and can move independently. There are five main types, divided into granulocytes (with granules) and agranulocytes (without).
Granulocytes: Neutrophils (fight bacteria via phagocytosis), eosinophils (combat parasites and allergies), basophils (release histamine for inflammation).
Agranulocytes: Lymphocytes (T-cells for cell-mediated immunity, B-cells for antibody production), monocytes (become macrophages to engulf pathogens).
Functions: Protect against infection, inflammation, and foreign invaders. They can leave blood vessels to patrol tissues.
Lifespan: Varies; some live days, others years (e.g., memory lymphocytes).
d. Platelets (Thrombocytes)
What they are: Tiny cell fragments (150,000-450,000 per microliter) derived from megakaryocytes in bone marrow. Not true cells, but essential for clotting.
Functions: Form plugs at injury sites and release chemicals to promote blood clotting (hemostasis). They help prevent blood loss and initiate vessel repair.
Lifespan: About 7-10 days.
3. Functions of Blood
Blood performs three primary roles: transportation, regulation, and protection.
Transportation:
- Delivers oxygen and nutrients (e.g., glucose, fats) from lungs and digestive system to cells.
Removes waste (e.g., CO2 to lungs, urea to kidneys).
- Transports hormones (e.g., insulin) and heat throughout the body.
Regulation:
Maintains body temperature by distributing heat (e.g., vasodilation in skin to cool down).
Regulates pH and fluid balance via buffers and osmotic pressure.
Helps control blood pressure through volume and vessel constriction.
Protection:
Clotting prevents excessive bleeding (via platelets and fibrinogen forming fibrin clots).
Immunity: White blood cells fight infections; antibodies in plasma neutralize toxins.
Inflammation response: Coordinates healing after injury.
4. Blood Types and Groups
Blood is classified by antigens (proteins) on red blood cell surfaces, which determine compatibility for transfusions.
- ABO System:
Type A: A antigens (can receive A or O).
Type B: B antigens (can receive B or O).
Type AB: A and B antigens (universal recipient; can receive any).
Type O: No A or B antigens (universal donor; can receive only O).
About 45% of people are O, 40% A, 11% B, and 4% AB (varies by ethnicity).
Rh Factor: A separate antigen; Rh-positive (85% of people) have it, Rh-negative don't. Important in pregnancy (e.g., Rh incompatibility can cause hemolytic disease in newborns).
Other Systems: Over 30 blood group systems exist (e.g., Kell, Duffy), but ABO and Rh are most critical for transfusions. Incompatible transfusions can cause severe reactions like hemolysis (red blood cell destruction).
5. Blood Circulation and the Cardiovascular System
Blood flows in a closed loop powered by the heart, which pumps about 5 liters per minute at rest (up to 25 liters during exercise).
Systemic Circulation: Oxygen-rich blood from the left ventricle travels via arteries (e.g., aorta) to body tissues, exchanges gases/nutrients in capillaries, and returns oxygen-poor via veins to the right atrium.
Pulmonary Circulation: Oxygen-poor blood from the right ventricle goes to lungs via pulmonary arteries, picks up oxygen in alveoli, and returns to the left atrium via pulmonary veins.
Key Vessels: Arteries (thick, high-pressure), veins (thinner, with valves to prevent backflow), capillaries (tiny for exchange).
Blood Pressure: Measured as systolic (contraction, ~120 mmHg) over diastolic (relaxation, ~80 mmHg). Regulated by heart rate, vessel elasticity, and blood volume.
6. Blood Production (Hematopoiesis)
Where it Happens: Primarily in red bone marrow (in flat bones like the pelvis, sternum, and vertebrae). In fetuses, it's in the liver and spleen; in adults, these can reactivate if needed.
Process: Stem cells in marrow differentiate into blood cells under hormonal control (e.g., erythropoietin from kidneys stimulates red blood cell production in response to low oxygen).
Regulation: Kidneys, liver, and hormones like thrombopoietin (for platelets) and colony-stimulating factors (for white cells) maintain balance.
Daily Output: Bone marrow produces about 200 billion red blood cells, 10 billion white blood cells, and 400 billion platelets per day.
7. Blood Disorders and Diseases
Blood issues can arise from genetics, infections, lifestyle, or environment. Common ones include:
Anemia: Low red blood cell count or hemoglobin (e.g., iron-deficiency from poor diet, sickle cell from genetic mutation causing misshapen cells).
Leukemia: Cancer of white blood cells, leading to overproduction of abnormal cells.
Hemophilia: Genetic clotting disorder (missing factors like VIII or IX), causing excessive bleeding.
Thrombosis: Abnormal clotting (e.g., deep vein thrombosis), risking blockages like pulmonary embolism.
Infectious Diseases: Affecting blood, like HIV (attacks white cells), malaria (parasites in red cells), or sepsis (blood infection).
Polycythemia: Too many red blood cells, thickening blood.
Diagnosis: Via blood tests (e.g., complete blood count, CBC) measuring cell counts, hemoglobin, etc.
8. Blood Donation, Transfusion, and Other Facts
Donation: Healthy adults can donate whole blood (450 ml) every 8 weeks or components like plasma/platelets more frequently. It's screened for diseases and typed.
Transfusion: Used in surgeries, trauma, or conditions like anemia. Universal donor is O-negative; universal recipient is AB-positive.
Interesting Facts:
Blood regenerates quickly: Full volume replaces in 4-6 weeks after donation.
In space, low gravity affects blood flow, causing issues like reduced red cell production.
Blood doping (illegal in sports) involves boosting red cells for better oxygen delivery.
Historical note: The first successful transfusion was in 1818, but blood typing (discovered by Karl Landsteiner in 1901) made it safe.
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