Chapter 17: Blood - Composition and Function

Blood and the Circulatory System

• Circulatory system implies a fluid (blood) circulating through hollow tubes (blood vessels: veins, arteries, capillaries) propelled by a central pump (the heart).
• Subdivisions:
  • Cardiovascular system: blood circulating in vessels powered by the heart.
  • Lymphatic system: lymph circulating in lymphatic vessels propelled by respiratory and skeletal muscle pumps.
• This lecture begins the examination of the cardiovascular system by exploring blood (Chapter 17 of the textbook).

Blood: Composition and Principal Functions

• Three principal functions of blood:
  • Transportation of materials throughout the body (e.g., nutrients such as glucose; respiratory gases such as oxygen).
  • Protection via immune and hemostatic components.
  • Homeostasis, including maintenance of stable internal conditions through buffering and temperature regulation.
• Blood components related to these functions:
  • Leukocytes (white blood cells) for immune defense.
  • Thrombocytes (platelets) for clotting and prevention of blood loss.
  • Buffers within the blood help maintain pH; several buffers resist pH changes to keep the blood in a narrow range.
• Key physiological targets:
  • pH stability: normal blood pH ranges from $7.35 \le \text{pH} \le 7.45$ with an average around $7.4$.
  • Temperature: blood helps regulate body temperature around $37^\circ\mathrm{C}$; maintaining this temperature is crucial because many blood and body proteins are pH- and temperature-sensitive.
• If pH or temperature deviate from the normal range, proteins may denature and lose function, disrupting homeostasis.
• Blood is a specialized form of connective tissue.
• Blood volume varies with body size: typically about $4-6\ \text{L}$, which is roughly $7-8\%$ of total body weight.
• Sex differences: males generally have greater blood volume than females due to larger skeletal musculature.

Blood: Plasma and Formed Elements

• Blood subdivides into two principal components:
  • Plasma (the liquid component, shown in yellow in diagrams).
  • Formed elements (solid components: erythrocytes, leukocytes, thrombocytes).
• Plan of study:
  • First examine plasma composition and function.
  • Then examine the structure and function of the formed elements (blood cells).

Plasma: Major Components

• Plasma consists of two major components:
  • Water
  • Plasma proteins
• Minor components (about 2% of plasma) include:
  • Electrolytes (charged ions such as Na^+ and bicarbonate) serving as buffers, nutrients, wastes, respiratory gases, regulatory substances (enzymes, hormones).
• Plasma proteins are subdivided into three classes:
  • Albumins
  • Globulins (alpha, beta, gamma)
  • Fibrinogen

Albumins

• Albumins are the most numerous plasma proteins.
• Functions of albumins:
  • Buffers to stabilize blood pH around 7.4 (resisting pH changes to maintain the narrow pH range).
  • Generate osmotic pressure to help retain water within the blood and prevent edema (a swelling of tissues).
  • Carrier proteins to bind and transport lipid-soluble substances in the blood (e.g., hormones and other lipophiles);
  • Hormones example: thyroid hormone and aldosterone (lipid-soluble).
  • Bilirubin: a toxic byproduct of hemoglobin breakdown is transported bound to albumin.

Globulins

• Globulins are subdivided into alpha, beta, and gamma globulins with distinct roles.

Alpha Globulins

• Alpha globulins primarily act as carrier proteins; they transport various substances in the blood.
• Examples:
  • Transcortin: binds to and transports cortisol.
  • High-density lipoprotein (HDL): transports cholesterol.
  • Transport of fat-soluble vitamins: vitamins A, D, E, and K.

Beta Globulins

• Beta globulins also function as carrier proteins and other roles:
  • Transferrin: binds and transports iron (Fe).
  • Low-density lipoprotein (LDL): transports cholesterol.
  • Transport of sex hormones derived from cholesterol (e.g., testosterone, estrogen, progesterone).
  • Some beta globulins act as complement proteins involved in immune responses.
  • Some beta globulins function as clotting factors necessary for coagulation.

Gamma Globulins

• Gamma globulins are antibodies produced by plasma cells and are involved in immune reactions.
• They contribute to humoral immunity and will be discussed in detail in the immunity section.

Fibrinogen

• Fibrinogen is intimately involved in blood clotting.
• It participates in the formation of the platelet plug and the coagulation cascade to stop blood loss after vessel injury.
• When fibrinogen is removed from plasma, the remaining liquid is the serum.

Minor Components of Plasma

• Minor components constitute about 2% of plasma and include:
  • Electrolytes (e.g., Na^+, bicarbonate) acting as buffers that work with albumin to stabilize pH.
  • Nutrients (e.g., glucose, amino acids, fatty acids).
  • Waste products (e.g., carbon dioxide, urea, bilirubin).
  • Respiratory gases (e.g., oxygen, carbon dioxide, nitrogen).
  • Regulatory substances (enzymes and hormones) transported within plasma.

Formed Elements: Erythrocytes, Leukocytes, and Thrombocytes

• Formed elements are the solid components of blood:
  • Erythrocytes (red blood cells)
  • Leukocytes (white blood cells)
  • Thrombocytes (platelets)
• These components will be studied in more detail after completing the plasma discussion.

Connections to Homeostasis and Clinical Relevance

• Homeostasis depends on maintaining:

  • Stable pH: buffers and proteins maintain a narrow pH range (around $7.35-7.45$; average $7.4$).
  • Stable temperature: body and blood temperature around $37^\circ\mathrm{C}$ to preserve protein function.

• Osmotic balance: albumin-driven osmotic pressure helps prevent edema by keeping water within the vascular compartment.

• Transport roles:

  • Plasma proteins transport lipid-soluble substances, iron, hormones, vitamins, and cholesterol via carrier proteins such as albumin, transferrin, HDL, and LDL.

• Immune defense:

  • Gamma globulins provide antibodies; beta globulins include some complement proteins involved in immune responses and certain clotting factors.

• Hemostasis and coagulation:

  • Fibrinogen is converted into fibrin during clot formation; removal yields serum.

• Metabolic waste disposal and gas exchange:

  • Blood carries wastes (e.g., bilirubin, urea), carbon dioxide, and oxygen to maintain tissue health.

• Real-world implications drawn from these topics include understanding edema formation, bilirubin toxicity, immune responses, and coagulation disorders, all central to clinical practice.

• Until next time, stay safe and be well.