Blood: Structure, Function, and Blood Groups

Blood: Structure and Function

Overview of Blood as a Connective Tissue

Blood is a specialized fluid connective tissue that plays a vital role in transporting substances throughout the body and in immune defense. It is unique among connective tissues because it is fluid and circulates within the cardiovascular system.

• Tissue Definition: A tissue is a group of similar cells that perform a common function. Blood qualifies as a tissue because it contains multiple types of cells working together.

• Connective Tissue: Blood is classified as a connective tissue because it consists of cells suspended in an extracellular matrix (plasma).

• Extracellular Matrix: The matrix in blood is called plasma, which is mostly water but also contains dissolved proteins, nutrients, hormones, and waste products.

Blood Cells

Main Types of Blood Cells

Blood contains several types of cells, each with specialized functions essential for survival.

• Erythrocytes (Red Blood Cells, RBCs): Responsible for transporting oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs. They contain the protein hemoglobin for oxygen binding.

• Leukocytes (White Blood Cells, WBCs): Involved in immune defense. There are several subtypes, including lymphocytes, neutrophils, monocytes, eosinophils, and basophils, each with specific roles in fighting infection and disease.

• Platelets (Thrombocytes): Small cell fragments important for blood clotting and wound repair.

Blood Cell Type	Main Function
Erythrocytes (RBCs)	Transport oxygen and carbon dioxide
Leukocytes (WBCs)	Defend against pathogens and foreign substances
Platelets	Initiate blood clotting

Plasma

Composition and Function of Plasma

Plasma is the liquid component of blood, making up about 55% of its total volume. It serves as the medium for transporting cells and dissolved substances.

• Water: Composes about 90% of plasma, providing a solvent for other components.

• Solutes: Includes electrolytes, nutrients, hormones, waste products, and proteins.

• Plasma Proteins: The most abundant is albumin, which helps maintain osmotic pressure and transport substances.

Recognizing Pathogens and Immune Response

Antigens and Antibodies

The immune system protects the body by recognizing and responding to foreign substances called antigens.

• Antigen: Any substance (usually a protein or polysaccharide) that the immune system recognizes as foreign and can provoke an immune response.

• Antibody: A protein produced by lymphocytes (a type of WBC) that specifically binds to an antigen to neutralize or mark it for destruction.

• Immune Response: When the body detects an antigen, lymphocytes produce antibodies to target and eliminate the threat.

Blood Groups

ABO Blood Group System

Blood groups are determined by the presence or absence of specific antigens on the surface of red blood cells. The ABO system is the most important for transfusions.

• Type A: Has A antigens on RBCs and anti-B antibodies in plasma.

• Type B: Has B antigens on RBCs and anti-A antibodies in plasma.

• Type AB: Has both A and B antigens; no anti-A or anti-B antibodies.

• Type O: Has no A or B antigens; has both anti-A and anti-B antibodies.

Blood Type	Antigens on RBCs	Antibodies in Plasma
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B

• Transfusion Reactions: If incompatible blood is transfused (e.g., type A blood into a type B person), the recipient's antibodies attack the donor RBCs, causing agglutination and potentially dangerous reactions.

Rh Blood Group System

In addition to ABO antigens, the Rh factor (D antigen) is another important antigen on RBCs.

• Rh Positive (Rh+): RBCs have the D antigen.

• Rh Negative (Rh-): RBCs lack the D antigen.

• Immune Response: An Rh- person exposed to Rh+ blood will develop anti-Rh antibodies. The first exposure may not cause a reaction, but subsequent exposures can lead to serious immune responses.

• Clinical Importance: This is especially important in pregnancy (hemolytic disease of the newborn) and in blood transfusions.

Special Blood Groups: Universal Donor and Recipient

Universal Donor and Recipient

Some blood types are considered universal donors or recipients based on their antigen and antibody profiles.

• Type O-: Considered the universal donor because it lacks A, B, and Rh antigens, minimizing the risk of transfusion reactions.

• Type AB+: Considered the universal recipient because it has all major antigens (A, B, and Rh), so it does not form antibodies against any common blood type.

Blood Type	Universal Status
O-	Universal Donor
AB+	Universal Recipient

Additional info:

• Hormones are chemical messengers transported in the blood to regulate physiology and behavior. The two major classes are peptide (protein-based) and steroid (lipid-based) hormones, which differ in their mechanisms of action and solubility in plasma.

• Blood volume in an average adult is about 5 liters.

• Plasma proteins also include globulins (immune function) and fibrinogen (clotting).