In-depth Notes on Blood Composition and Respiratory Function

Unit 5: Chapter 15 - Overview of the Composition of Blood:

• Hematocrit - Definition: The fraction of blood volume occupied by red blood cells (erythrocytes), which are an essential component of the blood responsible for transporting oxygen and carbon dioxide throughout the body.

  • Erythrocytes: These are dense cells that settle at the bottom of a test tube during centrifugation, providing a clear measurement of hematocrit. Their primary function is gas exchange, utilizing hemoglobin to bind oxygen.

  • Buffy coat: This thin layer of leukocytes (white blood cells) and platelets is found between the plasma and erythrocytes. It plays a crucial role in the immune system and hemostasis.

  • Plasma: The least dense component, which sits atop the hematocrit, constitutes approximately 55% of blood volume. It is a yellowish fluid that facilitates the transport of cells, nutrients, hormones, and waste products throughout the body.

  • Normal hematocrit levels:

    • Men: 42% - 52%

    • Women: 37% - 47%

  • Polycythemia: This condition arises from an elevated concentration of erythrocytes, which can occur as a physiological adaptation to hypoxic environments (e.g., living at high altitudes) or due to pathological conditions.

Plasma Composition:

• Plasma resembles interstitial fluid (ISF) due to the permeable nature of capillary walls that allow small solutes to pass through, facilitating nutrient and waste exchange.

• Protein concentration: Plasma has a higher concentration of proteins compared to ISF, attributable to the limited permeability of larger proteins across capillaries, which helps maintain osmotic pressure.

• Major protein groups:

  • Albumins (most abundant proteins): Synthesized in the liver, albumins play a critical role in maintaining osmotic pressure, regulating fluid balance between blood and tissues, and serving as transport molecules for hormones and other substances.

  • Globulins: A diverse group of proteins that participate in the transport of substances (such as lipids and fat-soluble vitamins) and form antibodies essential for immune defense.

  • Fibrinogen: A soluble plasma glycoprotein that is converted into fibrin during the clotting process, a fundamental step for hemostasis following vascular injury.

• Serum: Plasma devoid of clotting factors following coagulation, it contains electrolytes, antibodies, antigens, and various hormones.

Composition Breakdown of Plasma:

• Water (90% of plasma volume): Serves as a solvent and medium for transporting solutes and cellular elements, supporting cellular functions and physiological processes.

• Solutes (8% by weight):

  • Nitrogenous waste products: Includes urea, uric acid, and creatinine, which are byproducts of protein metabolism that must be efficiently excreted by the kidneys.

  • Organic nutrients: Comprises glucose, amino acids, fatty acids, and vitamins, vital for cellular metabolism and energy production.

  • Electrolytes: Such as sodium, potassium, calcium, chloride, and bicarbonate, play key roles in maintaining membrane potentials, nerve impulse conduction, and acid-base balance.

  • Respiratory gases: Oxygen, predominantly transported by hemoglobin in red blood cells, and carbon dioxide that is mostly converted to bicarbonate in the plasma, facilitating efficient gas exchange and transport.

Erythrocytes (Red Blood Cells):

• These abundant cells lack nuclei and organelles, which maximizes space for hemoglobin and enhances their primary function of gas transport.

• Shape: The biconcave disc shape provides a large surface area-to-volume ratio, facilitating rapid diffusion of gases and allowing flexibility to navigate through narrow capillaries.

• Function: Erythrocytes transport oxygen bound to hemoglobin (each molecule can carry four O2 molecules) and assist in carbon dioxide transport from tissues back to the lungs for exhalation.

• Carbonic anhydrase: An enzyme present in erythrocytes that catalyzes the conversion of carbon dioxide and water into carbonic acid, enhancing CO2 transport in the blood.

• Life cycle: Erythrocytes are produced from hematopoietic stem cells in the bone marrow and have a lifespan of approximately 120 days before they are removed by the spleen.

• Dietary requirements: Essential nutrients for erythropoiesis include iron (for hemoglobin production), folic acid, and vitamin B12, which are crucial for DNA synthesis and cell division.

• Anemia: A condition characterized by a reduced capacity of blood to carry oxygen, which may result from deficiencies in iron, vitamin B12, or folate, excessive blood loss, or chronic disease.

Filtering and Removal of Erythrocytes:

• Spleen's role: Functions as a filtration system for blood, selectively removing aged or damaged erythrocytes through phagocytosis, a process integral to maintaining normal blood cell counts and overall blood health.

• Bilirubin formation: Bilirubin, a yellow compound that results from the breakdown of heme in erythrocytes, is processed by the liver and eventually excreted in bile.

• Jaundice: A condition in which elevated levels of bilirubin accumulate in the bloodstream, resulting from liver dysfunction or increased destruction of erythrocytes, manifesting as yellowing of the skin and eyes.

Chapter 16 - Overview of Respiratory Function:

• Respiration: The biological process involving the exchange of gases in the body, critical for cellular metabolism and energy production; involves both internal (cellular) and external respiration.

• Processes:

  1. Pulmonary ventilation: The mechanical process of air movement in and out of the lungs, driven by pressure changes created by the diaphragm and intercostal muscles.

  2. Diffusion of gases: The movement of oxygen and carbon dioxide between the alveoli and the blood happens according to concentration gradients.

  3. Transportation of gases: Oxygen and carbon dioxide are transported via the bloodstream to and from tissues, primarily bound to hemoglobin in red blood cells.

  4. Tissue gas exchange: The process in which oxygen diffuses from the blood into tissues and carbon dioxide from tissues into the blood, essential for cellular respiration.

• Additional respiratory functions include:

  1. Acid-base regulation: The respiratory system maintains blood pH within a narrow range by adjusting the exhalation of CO2.

  2. Vocalization: The production of sound through the vibration of vocal cords in the larynx.

  3. Pathogen defense: The respiratory tract has mechanisms, such as mucus and cilia, that trap and expel pathogens.

  4. Heat loss: Evaporation of water during respiration helps regulate body temperature.

  5. Venous return enhancement: The negative pressure generated during inhalation assists in drawing blood back to the heart through the venous system.

  6. Plasma protein activation: The lungs activate certain proteins involved in blood coagulation and other physiological processes.

Anatomy of the Respiratory System:

• Upper Airway: Comprised of the nasal and oral cavities, pharynx, and larynx; the larynx contains the vocal cords and acts as a gatekeeper to prevent food and liquids from entering the trachea during swallowing.

• Conducting Zone: Includes all airways leading to the lungs (trachea, bronchi, bronchioles), where no gas exchange occurs, but functions primarily to filter, warm, and humidify the air.

• Respiratory Zone: The site of gas exchange, characterized by alveoli that greatly increase surface area due to their extensive capillary network, promoting efficient diffusion of gases due to their thin epithelial walls.

• Alveoli: Microscopic structures composed of type I and type II epithelial cells; type I cells facilitate gas exchange, while type II cells secrete surfactant to lower surface tension and prevent alveolar collapse.

Mechanics of Breathing:

• Pressure Gradient and Airflow: Airflow into and out of the lungs is driven by pressure differences; inspiration occurs when alveolar pressure becomes negative relative to atmospheric pressure.

  • Boyle's Law: Describes the inverse relationship between gas pressure and volume in a sealed environment; as lung volume increases during inhalation, pressure decreases, allowing air to flow in.

  • Pulmonary pressures:

    • Patm: Atmospheric pressure (approximately 760 mmHg at sea level).

    • Palv: Alveolar pressure fluctuates with the respiratory cycle; negative during inhalation, positive during exhalation.

    • Pip: Intrapleural pressure, which is typically negative and necessary to maintain lung inflation.

• Lung Compliance: Refers to the ease with which the lungs can expand; it is influenced by factors such as lung tissue structure, surface tension due to surfactant, and the elasticity of the lung parenchyma.

• Airway Resistance: Resistance encountered as air flows through the airways; it is determined by the diameter of the airways—bronchoconstriction leads to increased resistance, while bronchodilation decreases resistance, improving airflow.

Transport and Exchange of Gases:

• Oxygen Transport: The vast majority of oxygen is carried by hemoglobin in red blood cells; oxygen loading and unloading depend on the partial pressure of oxygen in the lungs and tissues.

• Carbon Dioxide Transport: Mainly transported as bicarbonate ions in the plasma, formed via carbonic anhydrase activity, which facilitates the conversion of CO2 into a more transportable form, aiding in the maintenance of acid-base balance in the blood.

• Regulation of Ventilation: Ventilation is finely regulated by central and peripheral chemoreceptors that detect changes in CO2 and O2 levels, adjusting respiratory rate and depth accordingly to maintain homeostasis.

• Gas Exchange Efficiency: The high efficiency of gas exchange is due to the extensive surface area of the alveoli and the thinness of the alveolar-capillary membrane, which minimizes diffusion distance; it can be impaired in conditions such as pulmonary edema, where fluid accumulation interferes with gas exchange.

Chemoreceptor Control and Ventilation:

• Types of Chemoreceptors:

  • Peripheral Chemoreceptors: Located in carotid and aortic bodies, these receptors monitor levels of arterial PO2, PCO2, and pH, prompting adjustments to breathing patterns.

  • Central Chemoreceptors: Located in the medulla oblongata, these receptors respond primarily to changes in hydrogen ion concentration, which reflects shifts in CO2 levels in arterial blood.

• Responses to CO2: Elevated levels of CO2 stimulate hyperventilation, enhancing removal of CO2; conversely, decreased levels can result in hypoventilation, leading to CO2 retention and respiratory acidosis.

• Haldane Effect: This physiological phenomenon describes how the presence of oxygen affects the loading and unloading of carbon dioxide; essentially, the binding of oxygen to hemoglobin facilitates the release of carbon dioxide in the lungs and improves carbon dioxide uptake in peripheral tissues, enhancing overall gas exchange efficiency.