Comprehensive Study Notes: Blood – Structure, Function, and Clinical Aspects

Blood

Overview of Blood

Blood is the only fluid tissue in the human body and is classified as a connective tissue. It plays a vital role in transporting substances, regulating physiological parameters, and protecting the body against disease.

• Components of Blood:

  • Living cells: Formed elements (erythrocytes, leukocytes, platelets)

  • Non-living matrix: Plasma

• Blood Centrifugation:

  • Erythrocytes (red blood cells) sink to the bottom (about 45% of blood, known as the hematocrit)

  • Buffy coat (less than 1% of blood) contains leukocytes (white blood cells) and platelets

  • Plasma rises to the top (about 55% of blood)

Physical Characteristics of Blood

Blood has distinct physical properties that are important for its function and clinical assessment.

• Color Range:

  • Oxygen-rich blood is scarlet red

  • Oxygen-poor blood is dull red

• pH: Must remain between 7.35–7.45

• Temperature: Slightly higher than body temperature (about 100.4°F)

• Volume: In a healthy adult, about 5–6 liters (8% of body weight)

Blood Plasma

Composition and Function

Plasma is the non-living, straw-colored fluid matrix of blood, making up about 55% of its volume. It serves as a transport medium for nutrients, hormones, and waste products.

• Water: Approximately 90% of plasma

• Dissolved Substances:

  • Nutrients

  • Salts (electrolytes)

  • Respiratory gases

  • Hormones

  • Plasma proteins

  • Waste products

• Plasma Proteins:

  • Albumin: Regulates osmotic pressure

  • Clotting proteins: Help stem blood loss when a vessel is injured

  • Antibodies: Protect the body from pathogens

Blood pH Imbalances

• Acidosis: Blood becomes too acidic

• Alkalosis: Blood becomes too basic

• Both conditions are regulated by the respiratory system and kidneys to restore normal pH

Formed Elements of Blood

Erythrocytes (Red Blood Cells, RBCs)

Erythrocytes are specialized for oxygen transport and have unique structural features to optimize this function.

• Main Function: Carry oxygen

• Structure:

  • Biconcave disks

  • Essentially bags of hemoglobin

  • Anucleate (no nucleus)

  • Contain very few organelles

• Hemoglobin:

  • Iron-containing protein

  • Binds strongly, but reversibly, to oxygen

  • Each hemoglobin molecule has four oxygen binding sites

  • Each erythrocyte contains about 250 million hemoglobin molecules

Homeostatic Imbalance of RBCs

• Anemia: Decrease in the oxygen-carrying ability of blood

• Sickle Cell Anemia (SCA): Results from abnormally shaped hemoglobin

• Polycythemia: Excessive or abnormal increase in the number of erythrocytes

Leukocytes (White Blood Cells, WBCs)

Leukocytes are crucial for the body's defense against disease. They are complete cells with nuclei and organelles and can move in and out of blood vessels (diapedesis).

• Functions:

  • Respond to chemicals released by damaged tissues

  • Move by amoeboid motion

• Normal Count: 4,000 to 11,000 WBCs per cubic millimeter of blood

• Abnormal Numbers:

  • Leukocytosis: WBC count above 11,000/mm3 (usually indicates infection)

  • Leukopenia: Abnormally low WBC level (commonly caused by drugs such as corticosteroids and anticancer agents)

  • Leukemia: Bone marrow becomes cancerous, producing excess WBCs

Types of Leukocytes

• Granulocytes: Granules in cytoplasm can be stained; possess lobed nuclei

  • Neutrophils: Multilobed nucleus, fine granules, act as phagocytes at infection sites

  • Eosinophils: Large brick-red cytoplasmic granules, respond to allergies and parasitic worms

  • Basophils: Histamine-containing granules, initiate inflammation

• Agranulocytes: Lack visible cytoplasmic granules; nuclei are spherical, oval, or kidney-shaped

  • Lymphocytes: Nucleus fills most of the cell, important in immune response

  • Monocytes: Largest WBCs, function as macrophages, important in fighting chronic infection

Mnemonic for WBC abundance: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils)

Platelets

Platelets are cell fragments derived from ruptured multinucleate cells and are essential for blood clotting.

• Normal Count: 300,000/mm3

Hematopoiesis

Blood Cell Formation

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow.

• All blood cells are derived from a common stem cell (hemocytoblast)

• Lymphoid stem cell produces lymphocytes

• Myeloid stem cell produces all other formed elements

Formation of Erythrocytes

• RBCs are unable to divide, grow, or synthesize proteins

• Wear out in 100 to 120 days

• Eliminated by phagocytes in the spleen or liver

• Replaced by division of hemocytoblasts in red bone marrow

• Control of Production:

  • Rate controlled by hormone erythropoietin

  • Kidneys produce most erythropoietin in response to reduced oxygen levels

  • Homeostasis maintained by negative feedback from blood oxygen levels

Formation of White Blood Cells and Platelets

• Controlled by hormones

• Colony stimulating factors (CSFs) and interleukins prompt bone marrow to generate leukocytes

• Thrombopoietin stimulates production of platelets

Hemostasis

Mechanism of Blood Clotting

Hemostasis is the process of stopping bleeding resulting from a break in a blood vessel. It involves three phases:

1. Vascular Spasms: Vasoconstriction causes blood vessel to spasm, narrowing the vessel and decreasing blood loss

2. Platelet Plug Formation:

   • Platelets become "sticky" and cling to fibers

   • Anchored platelets release chemicals to attract more platelets

   • Platelets pile up to form a platelet plug

3. Coagulation (Blood Clotting):

   • Injured tissues release tissue factor (TF)

   • PF3 (a phospholipid) interacts with TF, blood protein clotting factors, and calcium ions to trigger a clotting cascade

   • Prothrombin activator converts prothrombin to thrombin (an enzyme)

   • Thrombin joins fibrinogen proteins into hair-like molecules of insoluble fibrin

   • Fibrin forms a meshwork (basis for a clot)

Blood usually clots within 3 to 6 minutes.

Undesirable Clotting

• Thrombus: A clot in an unbroken blood vessel; can be deadly in areas like the heart

• Embolus: A thrombus that breaks away and floats freely in the bloodstream; can clog vessels in critical areas such as the brain

Bleeding Disorders

• Thrombocytopenia: Platelet deficiency; even normal movements can cause bleeding from small blood vessels

• Hemophilia: Hereditary bleeding disorder; normal clotting factors are missing

Blood Groups and Transfusions

Transfusions

Transfusions are the only way to replace blood quickly. Transfused blood must be of the same blood group.

• Loss of over 30% causes shock, which can be fatal

Human Blood Groups

Blood contains genetically determined proteins called antigens, which may be attacked by the immune system if recognized as foreign.

• Antibodies: "Recognizers" that cause blood with certain proteins to clump (agglutination)

• Over 30 common red blood cell antigens; most vigorous transfusion reactions are caused by ABO and Rh blood group antigens

ABO Blood Groups

Based on the presence or absence of two antigens: A and B.

Blood Type	Antigen(s) Present	Antibody Produced	Can Receive From
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	A, B, AB, O (Universal recipient)
O	None	Anti-A and Anti-B	O (Universal donor)

Blood Typing

• Blood samples are mixed with anti-A and anti-B serum

• Coagulation or no coagulation leads to determining blood type

• Typing for ABO and Rh factors is done in the same manner

Rh Blood Groups

Named for the presence or absence of one of eight Rh antigens (agglutinogen D) originally defined in Rhesus monkeys.

• Most Americans are Rh+ (Rh positive)

• Problems can occur in mixing Rh+ blood into a body with Rh– (Rh negative) blood

Rh Dangers During Pregnancy

• Danger occurs only when the mother is Rh– and the father is Rh+, and the child inherits the Rh factor

• RhoGAM shot can prevent buildup of anti-Rh antibodies in mother’s blood

• Mismatch of an Rh– mother carrying an Rh+ baby can cause problems for the unborn child

  • First pregnancy usually proceeds without problems

  • Immune system is sensitized after the first pregnancy

  • In second pregnancy, mother’s immune system produces antibodies to attack the Rh+ blood (hemolytic disease of the newborn)

Developmental Aspects of Blood

Sites of Blood Cell Formation

• Fetal liver and spleen are early sites of blood cell formation

• Bone marrow takes over hematopoiesis by the seventh month

Fetal Hemoglobin

• Fetal hemoglobin differs from hemoglobin produced after birth

• Physiologic jaundice results in infants whose liver cannot rid the body of hemoglobin breakdown products fast enough