Key Takeaways
- Blood is composed of plasma (55%) and formed elements (45%): red blood cells, white blood cells, and platelets
- Red blood cells (erythrocytes) carry oxygen via hemoglobin; they lack a nucleus and live about 120 days
- White blood cells (leukocytes) fight infection; types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils
- Platelets (thrombocytes) are cell fragments that initiate blood clotting at injury sites
- The ABO blood typing system has four groups (A, B, AB, O) based on antigens on red blood cells; Rh factor adds + or -
- Type O negative is the universal donor; Type AB positive is the universal recipient
- The lymphatic system returns fluid to the blood, transports fats, and houses immune cells in lymph nodes
- Innate immunity provides nonspecific defense (skin, phagocytes, inflammation); adaptive immunity provides specific, targeted defense (antibodies, T cells)
Blood, Lymphatic & Immune Systems
Blood, the lymphatic system, and the immune system work together to transport nutrients, fight infection, and maintain homeostasis. These are among the most heavily tested topics on the HESI A2 Anatomy & Physiology section.
Blood Composition
Blood is a fluid connective tissue that makes up about 7-8% of body weight (approximately 5 liters in adults).
| Component | Percentage | Contents |
|---|---|---|
| Plasma | ~55% | Water (90%), proteins, electrolytes, nutrients, waste, hormones, gases |
| Formed Elements | ~45% | Red blood cells (99%), white blood cells, platelets (<1% combined) |
The percentage of blood volume occupied by red blood cells is called the hematocrit (normal: ~42% for women, ~47% for men).
Formed Elements of Blood
Red Blood Cells (Erythrocytes)
| Feature | Detail |
|---|---|
| Function | Carry oxygen from lungs to tissues via hemoglobin |
| Structure | Biconcave disc (no nucleus in mature RBCs) |
| Lifespan | ~120 days |
| Production | Red bone marrow (erythropoiesis) |
| Regulation | Erythropoietin (EPO) from kidneys stimulates production |
| Normal count | 4.5-5.5 million cells/mcL |
Hemoglobin (Hgb) is the iron-containing protein in RBCs that binds oxygen:
- Oxyhemoglobin — hemoglobin carrying oxygen (bright red)
- Deoxyhemoglobin — hemoglobin without oxygen (dark red/blue)
- Normal Hgb: 12-16 g/dL (women), 14-18 g/dL (men)
- Anemia — low RBC count or low hemoglobin → decreased oxygen-carrying capacity
White Blood Cells (Leukocytes)
| Type | Percentage | Function |
|---|---|---|
| Neutrophils | 60-70% | First responders to infection; phagocytize bacteria |
| Lymphocytes | 20-25% | B cells (antibodies), T cells (cell-mediated immunity), NK cells |
| Monocytes | 3-8% | Become macrophages; phagocytize debris and pathogens |
| Eosinophils | 2-4% | Fight parasites; involved in allergic reactions |
| Basophils | <1% | Release histamine; involved in inflammatory and allergic responses |
- Normal WBC count: 5,000-10,000 cells/mcL
- Leukocytosis — elevated WBC count (often indicates infection)
- Leukopenia — decreased WBC count (increased infection risk)
- "Never Let Monkeys Eat Bananas" — mnemonic for order by prevalence: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils
Platelets (Thrombocytes)
| Feature | Detail |
|---|---|
| Function | Initiate blood clotting (hemostasis) at injury sites |
| Structure | Cell fragments from megakaryocytes (no nucleus) |
| Normal count | 150,000-400,000 cells/mcL |
| Low count | Thrombocytopenia → increased bleeding risk |
| High count | Thrombocytosis → increased clotting risk |
Hemostasis (Blood Clotting)
Hemostasis is the process that stops bleeding after an injury:
- Vascular spasm — damaged blood vessel constricts to reduce blood flow
- Platelet plug formation — platelets adhere to the injury site and aggregate (stick together)
- Coagulation cascade — clotting factors activate in a chain reaction, converting fibrinogen → fibrin → mesh that reinforces the platelet plug
- Fibrinolysis — clot is gradually dissolved after healing
Blood Typing
ABO Blood Group System
| Blood Type | Antigens on RBCs | Antibodies in Plasma | Can Receive From | Can Donate To |
|---|---|---|---|---|
| A | A antigens | Anti-B antibodies | A, O | A, AB |
| B | B antigens | Anti-A antibodies | B, O | B, AB |
| AB | A and B antigens | No antibodies | A, B, AB, O (universal recipient) | AB |
| O | No antigens | Anti-A and Anti-B | O only | A, B, AB, O (universal donor) |
Rh Factor
- Rh positive (+): Has Rh antigen (D antigen) on RBCs
- Rh negative (-): Lacks Rh antigen
- Rh incompatibility: An Rh- mother carrying an Rh+ fetus can develop antibodies against fetal blood cells → hemolytic disease of the newborn (prevented with RhoGAM injection)
Type O negative is the universal donor for red blood cells (no A, B, or Rh antigens). Type AB positive is the universal recipient (has no antibodies against A, B, or Rh).
Lymphatic System
The lymphatic system is a network of vessels, nodes, and organs that works alongside the cardiovascular system:
| Structure | Function |
|---|---|
| Lymph vessels | Collect excess interstitial fluid and return it to the bloodstream |
| Lymph nodes | Filter lymph; house immune cells (lymphocytes, macrophages) |
| Spleen | Filters blood; removes old RBCs; stores platelets; immune function |
| Thymus | T cell maturation (most active in childhood) |
| Tonsils | First line of defense against inhaled/ingested pathogens |
| Bone marrow | Produces all blood cells including immune cells |
Lymph is the fluid that flows through lymphatic vessels — it is similar to plasma but contains more white blood cells.
Edema occurs when excess fluid accumulates in tissues, often due to lymphatic system dysfunction or increased capillary permeability.
Immune System
The immune system protects the body against pathogens (bacteria, viruses, fungi, parasites).
Innate (Nonspecific) Immunity
Present from birth; provides immediate, general defense:
| Defense | Type | Mechanism |
|---|---|---|
| Skin | Physical barrier | Prevents pathogen entry |
| Mucous membranes | Physical/chemical barrier | Traps pathogens; contains lysozyme |
| Phagocytes | Cellular | Neutrophils and macrophages engulf and destroy pathogens |
| Inflammation | Vascular response | Redness, heat, swelling, pain → increased blood flow and immune cells |
| Fever | Systemic response | Elevated body temperature inhibits pathogen growth |
| Natural killer (NK) cells | Cellular | Destroy virus-infected and tumor cells |
| Complement system | Protein cascade | Enhances phagocytosis and causes cell lysis |
Adaptive (Specific) Immunity
Develops after exposure to specific pathogens; provides targeted, long-lasting defense:
| Type | Cells | Mechanism |
|---|---|---|
| Humoral immunity | B lymphocytes (B cells) | Produce antibodies that target specific antigens |
| Cell-mediated immunity | T lymphocytes (T cells) | Directly attack infected or abnormal cells |
Types of T cells:
- Helper T cells (CD4) — coordinate immune response; activate B cells and other T cells
- Cytotoxic T cells (CD8) — directly kill infected cells
- Memory T cells — remember past infections for faster future response
Antibodies (Immunoglobulins):
| Type | Location | Function |
|---|---|---|
| IgG | Blood, tissue | Most abundant; crosses placenta; provides passive immunity to fetus |
| IgA | Mucous membranes, saliva, breast milk | Protects mucosal surfaces |
| IgM | Blood | First antibody produced during primary immune response |
| IgE | Tissue | Involved in allergic reactions and parasitic infections |
| IgD | B cell surface | Involved in B cell activation |
Active vs. Passive Immunity
| Type | How Acquired | Duration | Example |
|---|---|---|---|
| Active natural | Having the disease | Long-lasting (memory cells) | Recovering from chickenpox |
| Active artificial | Vaccination | Long-lasting (memory cells) | Getting a flu vaccine |
| Passive natural | Mother to child | Temporary (weeks-months) | Antibodies in breast milk |
| Passive artificial | Injection of antibodies | Temporary (weeks) | RhoGAM, antivenom |
Which blood type is considered the "universal donor" for red blood cell transfusions?
Which type of white blood cell is the MOST abundant and acts as the first responder to bacterial infections?
A patient with Type B blood has which antibodies in their plasma?
The percentage of blood volume occupied by red blood cells is called the _____.
Type your answer below
Match each blood component to its primary function.
Match each item on the left with the correct item on the right
Vaccination produces which type of immunity?
Arrange the steps of hemostasis (blood clotting) in the correct order.
Arrange the items in the correct order
Which immunoglobulin is the most abundant in the blood and can cross the placenta?
Which of the following are components of innate (nonspecific) immunity? (Select all that apply)
Select all that apply