# Lecture 1: Blood — Composition and Function
## Anatomy and Physiology II
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the general composition of blood and its major components
2. List the functions of blood in the body
3. Differentiate between plasma and formed elements
4. Describe the physical characteristics and properties of blood
5. Explain the composition and roles of plasma proteins
6. Identify the major types of blood cells and their general functions
7. Describe blood typing systems (ABO and Rh) and their clinical significance
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## Lecture Content
### I. Overview of Blood
Blood is a specialized connective tissue consisting of cells suspended in a liquid matrix called plasma. An average adult carries approximately 5 liters of blood, which accounts for roughly 8% of total body weight. This remarkable fluid is slightly alkaline, maintaining a pH between 7.35 and 7.45, and circulates at a temperature of approximately 38 degrees Celsius, slightly above core body temperature. Because blood contains formed elements and dissolved plasma proteins, it is 3 to 5 times more viscous than water. It holds the unique distinction of being the only fluid tissue in the human body.
#### Functions of Blood
Blood performs three broad categories of function: transport, regulation, and protection. As a transport medium, it delivers oxygen from the lungs to the tissues and carries carbon dioxide back to the lungs for exhalation. It shuttles nutrients absorbed from the gastrointestinal tract to every cell in the body, moves metabolic wastes to the kidneys and lungs for excretion, and carries hormones from endocrine glands to their target organs.
Blood also plays a central role in regulation. It helps maintain body temperature by absorbing heat from metabolically active tissues and distributing it throughout the body or to the skin for dissipation. Through its buffer systems, including the bicarbonate, phosphate, and protein buffers, blood maintains normal pH despite the continuous generation of acids and bases by cellular metabolism. Additionally, blood proteins, especially albumin, exert osmotic pressure that helps maintain adequate fluid volume within the circulatory system.
The protective functions of blood are equally vital. White blood cells patrol the circulation and migrate into tissues to defend against invading pathogens. Antibodies and complement proteins provide immune defense against specific threats. When a vessel is injured, clotting factors in the blood prevent excessive blood loss through the process of hemostasis.
A diagram showing a test tube of centrifuged blood. Panel A: The full test tube with three distinct layers labeled — the top straw-colored layer of plasma (~55%), a thin buffy coat in the middle (~1%), and the bottom layer of packed red blood cells (~45%). Panel B: An expanded view of each layer showing the specific components — plasma layer listing water, proteins, electrolytes, gases, nutrients, and wastes; buffy coat showing white blood cells and platelets; erythrocyte layer showing densely packed red blood cells. A hematocrit scale is shown alongside.
### II. Plasma
Plasma comprises approximately 55% of total blood volume and is about 90% water by weight. It serves as both the solvent for blood-borne substances and the suspending medium in which blood cells travel.
#### Plasma Composition
Water makes up roughly 90% of plasma and functions as the dissolving and suspending medium. It also absorbs, transports, and releases heat, making it central to thermoregulation.
Proteins account for 7 to 9% of plasma by weight and fall into three major classes. **Albumin** constitutes about 60% of plasma proteins and is produced by the liver. It is the chief determinant of colloid osmotic pressure (also called oncotic pressure), which draws water into the capillaries and prevents excessive fluid loss into the tissues. Albumin also acts as a carrier protein for fatty acids, hormones, and drugs, and contributes significantly to blood viscosity.
**Globulins** make up approximately 36% of plasma proteins. Alpha and beta globulins are produced by the liver and serve as transport molecules for lipids, metal ions, and fat-soluble vitamins. Gamma globulins, also known as immunoglobulins or antibodies, are produced by plasma cells (activated B lymphocytes) and are crucial for immune defense.
**Fibrinogen** represents about 4% of plasma proteins and is also liver-derived. It is a key clotting factor that is converted into insoluble fibrin threads during coagulation. When fibrinogen and other clotting factors are removed from plasma, the remaining fluid is called serum.
The remaining 1 to 2% of plasma consists of other solutes, including electrolytes such as sodium, potassium, calcium, chloride, and bicarbonate; nutrients such as glucose, amino acids, fatty acids, and vitamins; dissolved respiratory gases (oxygen and carbon dioxide); hormones; and waste products including urea, creatinine, uric acid, and bilirubin.
### III. Formed Elements
The formed elements comprise approximately 45% of total blood volume and include three main categories: erythrocytes, leukocytes, and platelets. Notably, only white blood cells are complete, functional cells possessing nuclei and a full complement of organelles. Erythrocytes lack nuclei and most organelles, and platelets are merely cell fragments.
#### A. Erythrocytes (Red Blood Cells)
Erythrocytes are by far the most abundant formed element, numbering approximately 4.5 to 5.5 million per microliter of blood. Each red blood cell adopts a distinctive biconcave disc shape, approximately 7.5 micrometers in diameter with a thin center and thicker periphery. This geometry maximizes the surface-area-to-volume ratio, which enhances gas exchange efficiency.
Erythrocytes are unusual cells because they eject their nucleus and most organelles during maturation. This means they can neither divide nor synthesize new proteins, which limits their lifespan to approximately 120 days. The interior of each erythrocyte is packed with roughly 280 million molecules of hemoglobin. Each hemoglobin molecule consists of four polypeptide chains (two alpha and two beta chains in adult hemoglobin A), and each chain contains a heme group with a central iron atom that binds one molecule of oxygen. Thus, a single hemoglobin molecule can carry up to four oxygen molecules. When hemoglobin is loaded with oxygen, it forms oxyhemoglobin and takes on a bright red color. In its deoxygenated state (deoxyhemoglobin), blood appears dark red with a bluish tinge. Hemoglobin can also bind carbon dioxide, forming carbaminohemoglobin.
Because erythrocytes lack mitochondria, they generate all their ATP through anaerobic glycolysis, ensuring they do not consume the very oxygen they are transporting. The hematocrit, which measures the percentage of blood volume occupied by erythrocytes, is typically 42 to 52% in males and 37 to 47% in females.
A detailed diagram of erythrocyte structure. Panel A: Side view and top view of the biconcave disc shape with dimensions labeled (7.5 micrometer diameter, 2 micrometer edge thickness, 1 micrometer center thickness). Panel B: Molecular structure of hemoglobin showing the four polypeptide chains (two alpha in one color, two beta in another), with each heme group highlighted and an iron atom at the center of each heme. Panel C: Oxygen binding to the iron atoms of hemoglobin, showing the transition from deoxyhemoglobin (T state) to oxyhemoglobin (R state).
#### B. Leukocytes (White Blood Cells)
Leukocytes are far less abundant than red blood cells, numbering only 4,500 to 11,000 per microliter of blood. Unlike erythrocytes, they are complete cells equipped with nuclei and organelles. Their primary function is immune defense, though most of their activity takes place in the tissues rather than in the blood itself. White blood cells are capable of amoeboid movement and can undergo diapedesis, squeezing through capillary walls to reach sites of infection or injury. They are classified into two groups based on the presence of visible cytoplasmic granules.
**Granulocytes** possess conspicuous cytoplasmic granules. **Neutrophils** are the most abundant white blood cells, comprising 60 to 70% of the total. They have a distinctive multilobed nucleus with 3 to 5 lobes and fine, pale-staining granules. As the first responders to bacterial infection, neutrophils are highly phagocytic, engulfing and destroying bacteria, though they are short-lived, surviving only hours to days. **Eosinophils** represent 2 to 4% of white blood cells, feature a bilobed nucleus and large red-orange granules that stain with eosin. They specialize in combating parasitic worm infections and help modulate allergic reactions. **Basophils**, the rarest of all white blood cells at less than 1%, possess a bilobed nucleus often obscured by their large, dark blue-purple granules. They release histamine, a vasodilator, and heparin, an anticoagulant, and participate in inflammatory and allergic responses.
**Agranulocytes** lack visible cytoplasmic granules. **Lymphocytes** account for 20 to 25% of white blood cells and are recognized by their large, round, dark-staining nucleus surrounded by a thin rim of cytoplasm. The three main types are T cells, B cells, and natural killer (NK) cells, all of which are central to adaptive immunity. **Monocytes**, the largest white blood cells, represent 3 to 8% of the total and have a distinctive kidney- or U-shaped nucleus. When they migrate into the tissues, monocytes differentiate into macrophages, which are powerful phagocytes that engulf pathogens, dead cells, and debris.
#### C. Platelets (Thrombocytes)
Platelets are not true cells but rather cell fragments derived from enormous precursor cells called megakaryocytes in the bone marrow. They circulate at concentrations of 150,000 to 400,000 per microliter and are small, disc-shaped structures measuring 2 to 4 micrometers across. Platelets lack a nucleus and survive for approximately 5 to 10 days. Their cytoplasm contains granules loaded with clotting factors, ADP, serotonin, and calcium. Platelets are essential for hemostasis: they form platelet plugs at sites of vascular injury and facilitate the coagulation cascade that produces a stable fibrin clot.
### IV. Blood Typing
#### ABO Blood Group System
The ABO blood group system is based on the presence or absence of specific surface antigens, called agglutinogens, on the red blood cell membrane. Individuals with **Type A** blood carry A antigens on their red blood cells and anti-B antibodies in their plasma. Those with **Type B** blood carry B antigens and anti-A antibodies. **Type AB** individuals express both A and B antigens and produce neither anti-A nor anti-B antibodies, making them universal recipients. **Type O** individuals express neither antigen but carry both anti-A and anti-B antibodies, making them universal donors.
A critical feature of the ABO system is that the antibodies are preformed, meaning they exist in the plasma even without prior exposure to foreign blood. If donor red blood cells carrying the wrong antigens enter a recipient's circulation, the recipient's antibodies will attack them, causing agglutination (clumping) followed by hemolysis (rupture of red blood cells). Such transfusion reactions can be severe, potentially leading to renal failure and death.
#### Rh Blood Group System
The Rh blood group system depends on the presence or absence of the Rh antigen (also called the D antigen) on the red blood cell surface. Approximately 85% of the population is Rh-positive, meaning the D antigen is present, while the remainder is Rh-negative. Unlike the ABO system, anti-Rh antibodies are not preformed; they develop only after an Rh-negative individual is exposed to Rh-positive blood.
The most important clinical consequence of Rh incompatibility is hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis. This condition arises when an Rh-negative mother carries an Rh-positive fetus. During the first pregnancy, the mother is typically sensitized when fetal red blood cells cross the placenta during delivery, but the fetus is usually unaffected. In subsequent pregnancies with Rh-positive fetuses, however, the mother's anti-Rh antibodies can cross the placenta and attack fetal red blood cells, causing hemolysis. This dangerous scenario is now largely preventable by administering RhoGAM (anti-D immunoglobulin) to Rh-negative mothers during and after pregnancy.
A chart showing the ABO blood typing system. Panel A: A table with four rows (one for each blood type: A, B, AB, O) and columns showing: antigens present on RBC surface (illustrated), antibodies in plasma, can donate to, and can receive from. Panel B: Illustration of an agglutination reaction showing anti-B antibodies cross-linking Type B red blood cells, leading to clumping and subsequent hemolysis. Panel C: Diagram of Rh incompatibility in pregnancy showing first pregnancy sensitization and second pregnancy immune response against fetal Rh+ cells.
### V. Clinical Correlations
Several clinically significant disorders affect blood composition and function. **Anemia** refers to a decrease in the oxygen-carrying capacity of blood and comes in many forms. Iron-deficiency anemia results from insufficient iron for hemoglobin synthesis. Pernicious anemia arises from a lack of intrinsic factor, which impairs vitamin B12 absorption. Sickle cell anemia is caused by a defective hemoglobin variant (HbS) that causes red blood cells to assume a rigid, sickle shape under low-oxygen conditions. Aplastic anemia involves destruction or inhibition of the red bone marrow itself.
**Polycythemia** is the opposite problem: an abnormal excess of erythrocytes that increases blood viscosity and raises the risk of clotting. Polycythemia vera is a bone marrow cancer, while secondary polycythemia occurs as a compensatory response to chronic hypoxia, as seen at high altitude or in the setting of lung disease. **Leukemia** involves the cancerous overproduction of abnormal white blood cells. **Thrombocytopenia**, a low platelet count, increases the risk of uncontrolled bleeding. Finally, the **complete blood count (CBC)** is a standard clinical test that measures all formed elements, including the red blood cell count, white blood cell count, platelet count, hematocrit, hemoglobin concentration, and differential white blood cell count.
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