7.2 Blood composition and hemodynamics

Blood composition and hemodynamics are crucial aspects of the cardiovascular system. Understanding the components of blood and how they interact is key to grasping the system's function in oxygen transport, immune defense, and maintaining homeostasis.

Blood flow dynamics, including pressure, viscosity, and cardiac output, play a vital role in circulation. These factors influence how efficiently blood moves through vessels, delivering nutrients and removing waste products throughout the body.

Blood Cells and Plasma

Erythrocytes and Leukocytes

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• Erythrocytes, also known as red blood cells, are the most abundant cells in the blood
  • Responsible for transporting oxygen to tissues throughout the body
  • Contain hemoglobin, a protein that binds to oxygen (lungs) and releases it in areas of low oxygen concentration (tissues)
• Leukocytes, or white blood cells, play a crucial role in the body's immune response
  • Can be categorized into granulocytes (neutrophils, eosinophils, and basophils) and agranulocytes (lymphocytes and monocytes)
  • Each type of leukocyte has specific functions in defending the body against pathogens and foreign substances (bacteria, viruses, and fungi)

Platelets and Plasma

• Platelets, also called thrombocytes, are small, disc-shaped cell fragments that contribute to blood clotting
  • When a blood vessel is damaged, platelets adhere to the injury site and release chemicals that attract more platelets
  • Platelets aggregate and form a plug, which helps stop bleeding (hemostasis)
• Plasma is the liquid component of blood, consisting primarily of water, proteins, and dissolved substances
  • Transports nutrients, hormones, and waste products throughout the body
  • Contains clotting factors essential for blood coagulation (fibrinogen)

Hematocrit

• Hematocrit is the volume percentage of red blood cells in the blood
  • Normal hematocrit ranges are approximately 40-50% for men and 35-45% for women
  • Low hematocrit may indicate anemia, while high hematocrit can be a sign of dehydration or polycythemia vera
  • Hematocrit can be measured using a centrifuge to separate blood components (packed cell volume)

Hemoglobin and Blood Flow

Hemoglobin

• Hemoglobin is an iron-containing protein found in red blood cells that binds to oxygen
  • Each hemoglobin molecule can carry up to four oxygen molecules
  • Oxygen binding to hemoglobin is influenced by factors such as pH, temperature, and the partial pressure of oxygen (oxygen-hemoglobin dissociation curve)
  • Abnormalities in hemoglobin structure can lead to disorders like sickle cell anemia and thalassemia

Blood Viscosity and Flow

• Blood viscosity refers to the thickness and stickiness of blood, which affects its ability to flow through blood vessels
  • Viscosity is influenced by factors such as hematocrit, plasma proteins, and temperature
  • High blood viscosity can increase resistance to blood flow and contribute to cardiovascular problems (atherosclerosis)
• Laminar flow describes the smooth, orderly movement of blood through vessels, with layers of fluid sliding past each other
  • Laminar flow is characterized by a parabolic velocity profile, with the highest velocity in the center of the vessel
• Turbulent flow occurs when blood flow becomes chaotic and disorganized, often due to obstructions, sharp turns, or high velocities
  • Turbulent flow can cause damage to blood vessel walls and contribute to the development of atherosclerotic plaques

Cardiovascular Dynamics

Blood Pressure

• Blood pressure is the force exerted by blood against the walls of blood vessels
  • Systolic blood pressure is the pressure during heart contraction, while diastolic pressure is the pressure during relaxation
  • Normal blood pressure is considered to be 120/80 mmHg (systolic/diastolic)
  • High blood pressure, or hypertension, can lead to increased risk of cardiovascular events (heart attack, stroke)

Cardiac Output and Stroke Volume

• Cardiac output is the volume of blood pumped by the heart per minute, calculated as the product of heart rate and stroke volume
  • Cardiac output = Heart rate × Stroke volume
  • Typical resting cardiac output is around 5 liters per minute
• Stroke volume is the volume of blood ejected from the left ventricle with each heartbeat
  • Stroke volume is influenced by factors such as ventricular contractility, preload (end-diastolic volume), and afterload (resistance to ejection)
  • Increased stroke volume can occur due to enhanced contractility (exercise) or increased preload (increased venous return)