Blood Basics: Hematocrit, Plasma, and Albumin (Study Notes)

Hematocrit and sex differences in red blood cells

  • Hematocrit definition: the percentage of red blood cells (RBCs) in the total blood volume, by volume. Measured by centrifuging a blood sample in a capillary tube and comparing the heights of layers.

    • Method: spin the tube; measure the height of the plasma layer (top) and the red blood cell layer (bottom); express the RBC layer height as a percentage of the total height of blood before spinning.

    • Formula (hematocrit by volume): \text{Hct} = \left( \frac{H{\text{RBC}}}{H{\text{total}}} \right) \times 100\%.

  • Reported values:

    • Males: ~47% RBCs in whole blood.

    • Females: ~42% RBCs in whole blood.

  • Why males tend to have higher hematocrit and RBC content:

    • On average, males are bigger than females, contributing to a higher total blood volume.

    • Males typically have more skeletal muscle, a highly metabolic tissue, which increases the need for oxygen transport and thus RBCs.

  • Practical implication: knowing these baseline values helps in diagnosing conditions related to anemia, polycythemia, and blood loss.

  • Study tip: start making flashcards for these values to commit them to memory (47% vs 42%, and the concept of percent by volume).

  • Summary takeaway: normal blood volume differences between sexes relate to body size and metabolic demands; higher hematocrit in males is part of this pattern.

Physical characteristics of blood

  • Blood is sticky (viscous), opaque, and salty due to dissolved salts (e.g., sodium chloride) and other salts.

  • Color variation:

    • Oxygen-rich blood is bright scarlet.

    • Oxygen-poor blood is a darker red.

  • Viscosity: blood is more viscous than water, meaning it flows more slowly and requires the heart to pump with greater effort.

  • pH of blood:

    • Normal healthy range: 7.35 \le \text{pH} \le 7.45.

    • The body regulates pH very tightly; deviations can make you feel unwell and, if extreme, can be life-threatening.

  • Carbon dioxide and pH relationship:

    • CO₂ produced by cells dissolves in blood and forms carbonic acid, which lowers pH.

    • Chemical relation (conceptual): \mathrm{CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons H^+ + HCO_3^-}.

  • Venous blood pH:

    • Venous blood pH is closer to 7.35 (more acidic) because of CO₂ accumulation from tissue metabolism.

    • This is contrasted with the tighter, slightly higher pH range and the exam-style question: venous blood is closer to 7.35 due to dissolved CO₂ lowering pH.

  • Quick recap question from lecture: Is venous blood pH closer to 7.35 or 7.45? Answer: closer to 7.35, because CO₂ lowers pH as it is carried away from tissue metabolism.

Blood composition: plasma vs formed elements

  • Blood components within vessels:

    • Formed elements: the cells (RBCs, white blood cells, platelets).

    • Plasma: the fluid in which the cells are suspended; this is the extracellular component outside the cells.

  • Centrifugation concept:

    • Whole blood can be separated into its components by centrifugation and filtration.

    • Plasma can be isolated and transfused as needed (plasma transfusion).

  • Visual cue in lecture: plasma bags shown as a separate pool (plasma bank).

  • Conceptual takeaway: by separating plasma from formed elements, clinicians can tailor transfusions to patient needs (RBCs vs plasma).

Plasma: composition and major proteins

  • Plasma composition:

    • Plasma is mostly water with dissolved substances (solutes).

    • The solutes in plasma form a solution with water as the solvent.

    • Major solutes include various dissolved substances; among the dissolved components, proteins are a major group.

  • Major plasma proteins:

    • Albumin is the most abundant plasma protein.

    • Other plasma proteins exist, but albumin is highlighted as the primary one to know for this course.

  • Liver’s role:

    • The liver synthesizes plasma proteins via transcription and translation.

    • Albumin is one of the key proteins produced by the liver.

  • Function of albumin:

    • Maintains osmotic (colloid osmotic) pressure.

    • Helps regulate water balance between the intravascular space and the interstitial spaces.

  • Practical implication: albumin helps keep fluid in the bloodstream; low albumin can contribute to edema due to reduced oncotic pressure.

Connections, implications, and study cues

  • Review and exam readiness:

    • Remember the hematocrit values: male ~47%, female ~42%.

    • Understand how hematocrit is measured and what it reflects about blood volume and oxygen-carrying capacity.

    • Be able to explain why sex differences exist in hematocrit and RBC content (size, muscle mass, metabolic demand).

    • Know the basic physical properties of blood (stickiness, opacity, salinity, color range, viscosity).

    • Remember the physiological pH range and why pH is tightly regulated; know the CO₂–carbonic acid mechanism lowering pH.

    • Be able to distinguish plasma from formed elements and understand the idea of plasma separation and transfusion.

    • Know that albumin is the most abundant plasma protein, produced by the liver, and its role in osmotic pressure and water balance.

  • Real-world relevance:

    • Hematocrit and RBC content relate to athletic performance, anemia evaluation, and blood donation considerations.

    • Plasma composition and albumin have clinical relevance for fluid management, edema, and liver function assessment.

  • Ethical/clinical note:

    • Transfusion practices rely on understanding blood components (RBCs, plasma) and compatibility (blood type AB0 and Rh factors); the example given (B positive) illustrates the ABO and Rh labeling in labeling bags.

  • Quick practical memory aid:

    • “Hct” = RBC proportion; “Albumin” = main plasma protein; “pH 7.35–7.45” is the narrow healthy window; venous pH ~7.35 due to CO₂.