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Blood Bank

Circulation & Volume

The circulatory system acts as the body’s vital transport network, propelled by the heart, delivering oxygen and nutrients while removing waste. Tied closely to this is blood volume, the total amount of fluid within this network, which must be carefully regulated. Understanding both circulation dynamics and normal blood volume is foundational for appreciating patient physiology, the reasons behind blood transfusions, and the potential consequences of having too little or too much volume

The Circulatory System: The Body’s Highway Network

  • What it is: A closed loop system designed to transport blood throughout the body, delivering vital substances (like oxygen, nutrients, hormones) to tissues and removing waste products (like carbon dioxide, metabolic wastes)
  • Key Components
    • The Heart: The central pump! This muscular organ generates the pressure needed to propel blood through the vessels. It has four chambers (two atria, two ventricles) that ensure blood flows in one direction
    • Blood Vessels: The “pipes” or “roadways.”
      • Arteries: Carry oxygenated blood away from the heart (except for the pulmonary artery). They are thick-walled and elastic to withstand high pressure. They branch into smaller
      • Arterioles: Smaller arteries that regulate blood flow into capillary beds by constricting or dilating
      • Capillaries: Tiny, thin-walled vessels forming vast networks within tissues. This is where the magic happens! Their thin walls allow for the exchange of gases (O2/CO2), nutrients, and waste products between the blood and the body’s cells
      • Venules: Small veins that collect deoxygenated blood from the capillaries. They merge to form larger
      • Veins: Carry deoxygenated blood back towards the heart (except for the pulmonary veins). They have thinner walls than arteries and operate under lower pressure. Many veins (especially in the limbs) have valves to prevent the backflow of blood against gravity. Veins also act as a volume reservoir, holding a significant portion of the blood at rest
  • Two Main Circuits
    • Pulmonary Circulation: Blood flows from the right ventricle -> pulmonary artery -> lungs (picks up O2, drops off CO2) -> pulmonary veins -> left atrium
    • Systemic Circulation: Blood flows from the left ventricle -> aorta -> rest of the body (delivers O2/nutrients, picks up CO2/waste) -> vena cavae -> right atrium

Blood Volume: How Much Fluid is in the Pipes?

  • Definition: The total amount of blood circulating within the arteries, arterioles, capillaries, venules, and veins
  • Components: Blood volume consists of two main parts:
    • Plasma: The liquid component (~55% of total volume). It’s mostly water (~92%) but also contains vital proteins (albumin, globulins including antibodies, fibrinogen), electrolytes, nutrients, hormones, and waste products. Plasma volume is crucial for maintaining blood pressure and transporting substances
    • Cellular Elements: The formed elements suspended in the plasma (~45% of total volume)
      • Red Blood Cells (Erythrocytes): By far the most numerous cells; responsible for oxygen transport via hemoglobin. Their percentage of the total blood volume is called the hematocrit
      • White Blood Cells (Leukocytes): Immune system cells (neutrophils, lymphocytes, monocytes, eosinophils, basophils)
      • Platelets (Thrombocytes): Cell fragments essential for blood clotting
  • Normal Values
    • In average adults, total blood volume is roughly 5 liters, but this varies significantly based on:
      • Body Size: Larger individuals generally have more blood volume
      • Sex: Adult males typically have slightly higher blood volume per kilogram than adult females, partly due to hormonal influences and differences in muscle mass
      • Age: Infants and children have a higher blood volume relative to their body weight but a lower absolute volume than adults. Blood volume calculations in pediatrics often use mL/kg
    • A common estimate is about 7-8% of ideal body weight in kilograms
  • Distribution: At rest, a large portion of the blood volume (~60-70%) resides in the systemic veins and venules, acting as a reservoir that can be mobilized if needed (e.g., during exercise or blood loss)

Regulation of Blood Volume: Maintaining Balance

The body works constantly to keep blood volume within a narrow range, primarily by regulating water and salt balance. This involves complex interplay between:

  • Kidneys: The main regulators! They adjust water and sodium excretion based on the body’s needs
  • Hormones
    • Antidiuretic Hormone (ADH) / Vasopressin: Released from the pituitary gland when the body detects increased blood concentration (osmolality) or low blood pressure. It tells the kidneys to reabsorb more water, reducing urine output and conserving volume
    • Aldosterone: Released from the adrenal cortex, often triggered by low blood pressure/volume (via the renin-angiotensin system). It tells the kidneys to reabsorb more sodium, and water follows passively, increasing volume
    • Atrial Natriuretic Peptide (ANP): Released from the heart’s atria when they are stretched by high blood volume/pressure. It promotes sodium and water excretion by the kidneys, lowering volume
  • Thirst Mechanism: Stimulated by increased blood osmolality or low volume, prompting fluid intake

Pathophysiology: When Volume Goes Wrong

Imbalances in blood volume are critical in clinical medicine and transfusion practice:

  • Hypovolemia (Low Blood Volume)
    • Causes: Hemorrhage (blood loss from trauma, surgery, GI bleeding), severe dehydration (vomiting, diarrhea, burns, inadequate intake), excessive fluid loss (e.g., certain kidney diseases)
    • Consequences: Reduced venous return to the heart -> decreased cardiac output -> low blood pressure (hypotension) -> reduced tissue perfusion (delivery of oxygen and nutrients). If severe, leads to hypovolemic shock, a life-threatening condition where organs don’t receive enough oxygen
    • Blood Bank Relevance: This is a primary reason for transfusion! We replace lost volume and/or oxygen-carrying capacity. Acute hemorrhage often requires rapid volume replacement (crystalloids, colloids, plasma) and red blood cells if oxygen delivery is compromised
  • Hypervolemia (High Blood Volume)
    • Causes: Excessive fluid intake (especially IV fluids), kidney failure (inability to excrete fluid), heart failure (impaired pumping ability leading to fluid backup), excessive sodium intake, overly rapid or excessive blood transfusion
    • Consequences: Increased blood pressure (hypertension), edema (fluid leaking into tissues), shortness of breath (pulmonary edema if fluid backs up into the lungs), increased workload on the heart, potentially leading to heart failure
    • Blood Bank Relevance: A major concern is Transfusion-Associated Circulatory Overload (TACO). Giving blood products (especially whole blood or large volumes of plasma) too quickly or to a patient who already has fluid overload can overwhelm the circulatory system, leading to respiratory distress and other complications. Careful monitoring and slower infusion rates are crucial in at-risk patients (e.g., elderly, those with heart or kidney disease)

Connecting Circulation & Volume to Blood Banking

Understanding this physiology helps us:

  • Recognize signs and symptoms related to volume depletion or overload
  • Appreciate the goals of transfusion – restoring oxygen-carrying capacity (RBCs), correcting coagulopathy (plasma, platelets), and/or restoring volume (plasma, crystalloids)
  • Understand the risks associated with transfusion, particularly TACO, and identify patients needing careful monitoring
  • Interpret laboratory values (like hematocrit) in the context of the patient’s overall volume status (e.g., hematocrit might appear falsely normal or high in dehydration)

Key Terms

  • Circulation: The movement of blood through the vessels of the body, driven by the heart’s pumping action
  • Blood Volume: The total amount of blood (plasma + cellular elements) present in the circulatory system
  • Plasma: The liquid matrix of blood, composed mainly of water, proteins, electrolytes, etc
  • Hematocrit: The proportion of the blood volume occupied by red blood cells, expressed as a percentage
  • Hypovolemia: Abnormally low circulating blood volume
  • Hypervolemia: Abnormally high circulating blood volume
  • Hypovolemic Shock: A life-threatening condition caused by insufficient circulating blood volume, leading to inadequate tissue perfusion
  • Perfusion: The passage of fluid (like blood) through the circulatory or lymphatic system to an organ or tissue, usually referring to the delivery of blood to a capillary bed
  • Transfusion-Associated Circulatory Overload (TACO): Acute respiratory distress occurring during or after transfusion due to the circulatory system’s inability to handle the transfused volume, often resulting in pulmonary edema
  • Opsonization: The coating of a particle (e.g., cell, bacterium) with molecules (opsonins like C3b or IgG) that enhance its uptake by phagocytic cells. (Included here as it relates to cell clearance which affects circulating cells, though primarily an immune term)
  • Extravascular Hemolysis: Destruction of red blood cells outside the blood vessels, primarily by macrophages in the spleen and liver recognizing opsonized (C3b/IgG coated) cells. (Included as it affects circulating RBC volume)
  • Intravascular Hemolysis: Destruction of red blood cells within the blood vessels, typically mediated by MAC formation. (Included as it affects circulating RBC volume)