Blood Overview and Components

Blood Overview

• Blood is a fluid tissue, a type of connective tissue found in the human body.
• It transports essential components throughout the body, including nutrients, wastes, hormones, and body heat.

Blood Components

• Blood consists of formed elements (living cells) and plasma (non-living fluid matrix).
• Plasma makes up 55% of blood volume and is primarily water (90%). It transports other substances, including salts, nutrients, respiratory gases (oxygen and carbon dioxide), hormones, plasma proteins, and waste products.
• Plasma proteins perform vital roles, including maintaining osmotic pressure, aiding in blood clotting, and producing antibodies for immune function. Albumin and fibrinogen are vital plasma proteins.

Formed Elements

• Formed elements constitute 45% of blood and include:
  • Erythrocytes (red blood cells): These carry oxygen and carbon dioxide. They lack a nucleus and are shaped like biconcave discs. Normal count: 5 million per cubic millimeter of blood.
  • Leukocytes (white blood cells): These are crucial for body defense against disease. They are complete cells with nuclei and organelles. They can move in and out of blood vessels and respond to chemicals released by damaged tissues. Their number is 4,800-10,800 per cubic millimeter of blood.
• Platelets: These are cell fragments needed for blood clotting.
• Types of leukocytes (white blood cells): Granulocytes (with granules in cytoplasm) - Neutrophils, eosinophils, basophils. Agranulocytes (without granules in cytoplasm) - Lymphocytes, monocytes.

Erythrocytes (Red Blood Cells)

• Their function is oxygen transport via hemoglobin, an iron-bearing protein. Each hemoglobin molecule binds four oxygen molecules.
• Normal blood contains 12-18 grams of hemoglobin per 100 milliliters of blood.
• RBCs live for approximately 120 days. They are eliminated in the spleen or liver when worn out.
• Imbalances can lead to conditions like anemia, a decrease in the oxygen-carrying capacity of the blood, often due to low RBC count or abnormal hemoglobin. Polycythemia, an abnormally high RBC count, can result from bone marrow cancer or high altitudes.

Leukocytes (White Blood Cells)

• Leukocytosis is an abnormally high WBC count, generally indicating an infection.
• Leukopenia is an abnormally low WBC count, often caused by certain drugs.
• Leukemia is a cancer of the bone marrow, producing numerous immature WBCs.

Platelets

• Platelets are cell fragments crucial for blood clotting.

Blood Volume

• The average healthy adult has about 5-6 liters (6 quarts) of blood.
• About 8% of a person's body weight consists of blood.

Blood Characteristics

• Appearance: Sticky, opaque fluid, denser and thicker than water.
• Color: Oxygen-rich blood is scarlet red; oxygen-poor blood is dull red or purple.
• Taste: Metallic, salty.
• Temperature: Slightly higher than body temperature, roughly 38°C (100.4°F).

Hemostasis

• Hemostasis is the process of stopping bleeding from a blood vessel injury. It involves three phases:
  1. Vascular spasms: The immediate response constricts the blood vessel.
  2. Platelet plug formation: Platelets adhere to collagen fibers, then release chemicals to stimulate more platelet aggregation.
  3. Coagulation: Injured tissues release tissue factor activating a cascade of reactions to form a fibrin mesh to trap red blood cells and platelets. This creates a blood clot.

Disorders of Hemostasis

• Undesirable clotting: Thrombus (clot in an unbroken vessel) which can be life-threatening if it occurs in vital areas or if it breaks away (embolus) and travels to critical areas.
• Bleeding disorders: Thrombocytopenia (insufficient platelets) and Hemophilia (hereditary deficiency in clotting factors).

Blood Groups and Transfusions

• Significant blood loss requires transfusions.
• Blood has genetically determined proteins known as antigens. The immune system recognizes these as foreign and may attack them.
• Antibodies are the "recognizers" that bind foreign antigens.
• ABO (A, B, AB, O) blood types categorize blood based on the presence or absence of two antigens. Type O blood is considered a universal donor and AB blood is considered a universal recipient.
• Rh factor (Rh+, Rh-) refers to the presence or absence of another antigen. Rh-negative individuals can receive Rh-positive blood for the first time, but receiving Rh-positive blood a second time will cause the body's immune system to produce anti-bodies attacking the Rh+ blood cells with serious consequences.

Hematopoiesis

• Hematopoiesis is the process of blood cell formation, occurring in the red bone marrow. All blood cells derive from a common stem cell. RBCs have a lifespan of approximately 120 days.