Blood Physiology and Hemodynamics

Overview of Blood and Circulation

The cardiovascular system is responsible for transporting blood, nutrients, gases, and waste products throughout the body. Blood flow is regulated by pressure gradients and vascular resistance, forming a closed-loop system essential for maintaining homeostasis.

• Closed-loop system: Blood circulates continuously through the heart and vessels, as established by William Harvey in the 1600s.

• Historical models: Early models (Galen, Colombo) proposed open or partially closed systems, but Harvey demonstrated the true closed nature of human circulation.

• Cardiac output (CO): The volume of blood pumped by the heart per minute.

• Stroke volume (SV): The amount of blood ejected per heartbeat.

• Heart rate (HR): Number of heartbeats per minute.

Key Equations:

• EDV: End-diastolic volume; ESV: End-systolic volume

Example: If the heart pumps 70 mL/beat and beats 70 times per minute, cardiac output is mL/min.

Blood Flow, Pressure, and Resistance

Blood flow through vessels is determined by the pressure gradient and resistance. These relationships are described by physical laws and are fundamental to understanding hemodynamics.

• Pressure gradient (): Drives blood from areas of higher to lower pressure.

• Resistance (R): Opposition to flow, primarily due to vessel diameter and blood viscosity.

• Flow (Q):

Units and Conversions:

• Pressure: Measured in mmHg (millimeters of mercury), Pa (Pascals), or Torr.

• Conversions:

  • 1 mmHg = 133.3 Pa

  • 1 mmHg = 1.36 cm water

  • 1 mmHg = 1.316 × 10-3 atm

  • 1 mmHg = 1.93 × 10-2 psi

• Density of blood: ~1060 kg/m3

Resistance Formula:

• Where: = viscosity, = length, = radius

Viscosity: Expressed in Poise (P) or centipoise (cP); SI unit is Pa·s.

Example Calculation:

• Given Pa·s, m, m:

• Convert to mmHg·s/m3:

Blood Composition and Cellular Elements

Components of Blood

Blood is a specialized connective tissue composed of plasma and formed elements. Each component plays a vital role in physiological processes.

• Plasma: Liquid matrix (~55% of blood), contains water (92%), proteins (7%), and other solutes (1%).

• Proteins: Albumins, globulins, fibrinogen, transferrin.

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

Plasma vs. Interstitial Fluid: Plasma contains more proteins than interstitial fluid.

Complete Blood Count (CBC)

A CBC is a common diagnostic test that provides information about the cellular components of blood.

• Hematocrit: Percentage of blood volume occupied by RBCs (normal: 40-50%).

• Hemoglobin: Concentration of hemoglobin in whole blood (g/dL).

• Red cell count: Number of erythrocytes per mL.

• White cell count: Number of leukocytes per mL.

Plasma Proteins

Formed Elements

• Red blood cells (erythrocytes): Transport oxygen and carbon dioxide.

• White blood cells (leukocytes): Immune defense.

• Platelets (thrombocytes): Involved in clotting.

Hematopoiesis

Hematopoiesis is the process of blood cell formation, occurring primarily in bone marrow.

• Erythropoiesis: Production of RBCs.

• Leukopoiesis: Production of WBCs.

• Thrombopoiesis: Production of platelets from megakaryocytes.

• Regulation: Controlled by cytokines and growth factors.

Bone marrow: Red marrow is active in hematopoiesis; yellow marrow contains adipose tissue and is inactive.

Red Blood Cells and Hemoglobin

Structure and Function of Erythrocytes

Red blood cells are specialized for oxygen transport. Their biconcave shape increases surface area for gas exchange.

• No nucleus: Mature RBCs lack nuclei to maximize space for hemoglobin.

• Lifespan: ~120 days; removed by spleen.

• Hemoglobin: Protein that binds oxygen; consists of four subunits, each with a heme group containing iron.

Hemoglobin Equation:

Iron metabolism: Iron is absorbed from diet, transported by transferrin, stored in liver, and incorporated into hemoglobin in bone marrow.

Disorders: Anemia (low RBC or hemoglobin), sickle-cell disease (abnormal hemoglobin).

Platelets, Hemostasis, and Coagulation

Platelets and Megakaryocytes

Platelets are cell fragments derived from megakaryocytes and are essential for blood clotting.

• Function: Initiate clot formation at sites of vessel injury.

• Regulation: Platelet production is controlled by thrombopoietin.

Hemostasis

Hemostasis is the process that prevents blood loss following vessel injury. It involves three main steps:

1. Vasoconstriction: Blood vessels constrict to reduce blood flow.

2. Platelet plug formation: Platelets adhere to exposed collagen and aggregate.

3. Coagulation: Cascade of enzymatic reactions leading to fibrin clot formation.

Coagulation Pathways:

• Intrinsic pathway: Triggered by contact with collagen.

• Extrinsic pathway: Triggered by tissue factor from damaged cells.

• Common pathway: Both pathways converge to activate thrombin, which converts fibrinogen to fibrin.

Fibrinolysis: Breakdown of clots by plasmin, activated from plasminogen by tissue plasminogen activator (tPA).

Summary Table: Blood Parameters and Significance

Additional info:

• Blood flow, pressure, and resistance calculations are foundational for understanding cardiovascular physiology and pathology.

• Disorders of blood composition or hemostasis can lead to clinical conditions such as anemia, thrombosis, or bleeding disorders.