Blood Composition and Function Study Pack
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Last updated May 27, 2026
Blood Composition and Function Study Guide
Break down the composition and functions of blood, from plasma proteins like albumin and fibrinogen to the five leukocyte types, hemoglobin-based oxygen transport, platelet-driven hemostasis, and hematopoiesis in red bone marrow.
Key Takeaways
- •Blood is a specialized connective tissue composed of plasma (about 55% of volume) and formed elements — erythrocytes, leukocytes, and platelets — suspended within it.
- •Erythrocytes transport oxygen bound to hemoglobin and carry carbon dioxide back to the lungs, making them the most numerous cells in blood at roughly 4.5–5.5 million per microliter.
- •Leukocytes defend the body through innate and adaptive immune responses; the five major types — neutrophils, eosinophils, basophils, monocytes, and lymphocytes — each perform distinct functions.
- •Platelets are anucleate cell fragments derived from megakaryocytes that initiate hemostasis by aggregating at sites of vascular damage and releasing clotting factors.
- •Plasma proteins — albumin, globulins, and fibrinogen — collectively regulate osmotic pressure, immune defense, and coagulation.
- •Blood maintains homeostasis by regulating body temperature, buffering pH near 7.4, and distributing hormones, nutrients, and metabolic waste products throughout the body.
- •All formed elements originate from a common pluripotent hematopoietic stem cell in red bone marrow through the process of hematopoiesis.
Physical Properties and Overall Composition of Blood
Blood is a fluid connective tissue whose unique properties allow it to simultaneously serve as a transport medium, a buffer system, and a cellular delivery vehicle across every tissue in the body.
Basic Physical Characteristics
- •Blood is slightly alkaline, with a normal pH range of 7.35–7.45; values outside this range impair enzyme function and cellular metabolism.
- •Whole blood has a viscosity roughly 4–5 times greater than water, a property that influences vascular resistance and blood pressure.
- •The temperature of blood (~38 °C) is slightly higher than core body temperature and helps distribute heat generated by metabolic activity.
- •Total blood volume in an average adult is approximately 4–6 liters, representing about 7–8% of body weight.
Two Major Compartments: Plasma and Formed Elements
- •Centrifuging a blood sample separates it into plasma (the straw-colored liquid upper layer, ~55% of volume) and formed elements (the cellular lower layer, ~45%).
- •The percentage of blood volume occupied by erythrocytes is called the hematocrit; normal values are roughly 42–52% in males and 37–47% in females.
- •A thin buffy coat of leukocytes and platelets sits between the plasma and erythrocyte layers but accounts for less than 1% of total volume.
Plasma: Composition and Protein Functions
Plasma is the liquid matrix of blood, consisting of approximately 92% water and a complex mixture of dissolved proteins, electrolytes, nutrients, gases, and waste products that together regulate fluid balance and support clotting and immunity.
Plasma Proteins and Their Roles
- •Albumin is the most abundant plasma protein (~60% of total), produced by the liver; it generates colloid osmotic pressure that retains fluid within the vasculature and serves as a carrier molecule for hormones, fatty acids, and drugs.
- •Globulins include alpha and beta globulins, which transport lipids, hormones, and metal ions, and gamma globulins (immunoglobulins/antibodies) produced by plasma cells as part of adaptive immunity.
- •Fibrinogen is a soluble plasma protein that the liver synthesizes; during coagulation it is converted by thrombin into insoluble fibrin threads that form the structural mesh of a blood clot.
Other Dissolved Solutes in Plasma
- •Electrolytes — principally sodium, potassium, calcium, chloride, and bicarbonate — regulate membrane potentials, osmolarity, and acid-base balance.
- •Nutrients such as glucose, amino acids, and lipoproteins circulate in plasma for delivery to tissues, while metabolic waste products such as urea and creatinine travel to the kidneys for excretion.
- •Dissolved gases including oxygen, carbon dioxide, and nitrogen are present at low concentrations; most oxygen and CO₂ are instead transported by erythrocytes rather than dissolved in plasma.
Erythrocytes: Structure and Oxygen Transport
Erythrocytes, or red blood cells, are the most abundant formed elements and are exquisitely structured to maximize gas exchange between the lungs and peripheral tissues.
Distinctive Structural Features of Erythrocytes
- •Mature erythrocytes lack a nucleus and most organelles, freeing the entire interior for hemoglobin and maximizing oxygen-carrying capacity.
- •Their biconcave disc shape increases surface-area-to-volume ratio and allows flexible deformation as they squeeze through narrow capillaries (~6–8 µm in diameter).
- •Each erythrocyte contains approximately 250 million hemoglobin molecules, giving blood its characteristic red color.
Hemoglobin and Gas Transport
- •Hemoglobin is a tetramer of four globin chains, each bearing a heme group with a central iron atom (Fe²⁺) that reversibly binds one oxygen molecule, allowing each hemoglobin to carry up to four O₂ molecules.
- •In peripheral tissues, oxyhemoglobin releases oxygen in response to lower partial pressure of O₂, higher CO₂ concentration, lower pH (Bohr effect), and higher temperature.
- •Carbon dioxide is transported primarily as bicarbonate ions (HCO₃⁻) in plasma after CO₂ diffuses into erythrocytes and is converted by carbonic anhydrase; a smaller fraction binds hemoglobin as carbaminohemoglobin.
Erythrocyte Life Cycle
- •Erythrocytes circulate for approximately 120 days before aging changes — loss of membrane flexibility, oxidative damage — signal macrophages in the spleen and liver to phagocytose them.
- •During destruction, heme is converted to bilirubin (excreted in bile) and iron is recycled via transferrin back to red bone marrow for new hemoglobin synthesis.
Leukocytes: Classification and Immune Defense
Leukocytes are nucleated cells that circulate in blood but perform most of their functions after migrating into tissues; they are classified by the presence or absence of visible cytoplasmic granules and by nuclear morphology.
Granulocytes
- •Neutrophils are the most numerous leukocyte (~50–70% of the total), characterized by a multilobed nucleus; they rapidly migrate to infection sites and destroy bacteria by phagocytosis and by releasing antimicrobial enzymes from their granules.
- •Eosinophils (2–4%) combat parasitic infections and modulate allergic responses by releasing histaminase and other mediators from their large, orange-staining granules.
- •Basophils (<1%) release histamine and heparin from their dark-staining granules, promoting inflammation and preventing local clotting during immune responses; they are the least common circulating leukocyte.
Agranulocytes
- •Monocytes (2–8%) are the largest leukocytes; after leaving the bloodstream they differentiate into macrophages and dendritic cells that perform phagocytosis and antigen presentation in tissues.
- •Lymphocytes (20–35%) are the central cells of adaptive immunity: B lymphocytes produce antibodies, T lymphocytes kill infected cells or regulate immune responses, and natural killer (NK) cells destroy virus-infected and tumor cells without prior sensitization.
Leukocyte Movement into Tissues
- •Diapedesis (extravasation) is the process by which leukocytes squeeze through capillary walls; it is directed by chemical signals called chemokines released at sites of infection or injury.
- •A significant increase in circulating leukocyte count above ~11,000 per microliter is called leukocytosis and typically signals active infection, inflammation, or certain malignancies.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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What is the normal pH range of blood, and what happens when values fall outside this range?
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Blood Composition and Plasma
Explain what blood is made of in your own words. What are the two major compartments, what does each contain, and how do plasma proteins like albumin, globulins, and fibrinogen each contribute to the body's functioning?
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