Capillary Exchange and Fluid Movement

Osmosis and Capillary Filtration

Capillary exchange is the process by which materials move between blood and tissues. This exchange is governed by physical forces, primarily osmotic pressure and hydrostatic pressure.

• Osmosis: The movement of water from an area of high water concentration (low solute) to low water concentration (high solute) across a semipermeable membrane.

• Osmotic Pressure: In capillaries, plasma proteins and solutes create a higher concentration inside the vessel, drawing water in from the interstitial space. This is typically a constant negative pressure (e.g., -25 mmHg), representing net absorption into the vessel.

• Hydrostatic Pressure: The pressure exerted by blood against the capillary wall, higher on the arterial end (e.g., 32 mmHg) and lower on the venous end (e.g., 15 mmHg). This pressure pushes fluid out of the capillary (filtration).

Combining these forces determines the net movement of fluid:

• Net Filtration: Occurs on the arterial side where hydrostatic pressure exceeds osmotic pressure (e.g., 32 - 25 = +7 mmHg).

• Net Absorption: Occurs on the venous side where osmotic pressure exceeds hydrostatic pressure (e.g., 15 - 25 = -10 mmHg).

Physiological Significance:

• Net filtration on the arterial side delivers oxygen and nutrients to tissues.

• Net absorption on the venous side removes waste products like carbon dioxide.

Summary Table: Capillary Pressures

Additional info: These values are typical for systemic capillaries; actual values may vary by tissue and physiological state.

Blood Composition and Hematopoiesis

Components of Blood

Blood is a specialized connective tissue composed of plasma and formed elements.

• Plasma: The liquid matrix, mostly water, containing proteins (albumin, globulins, fibrinogen), nutrients, electrolytes, hormones, and waste products.

• Red Blood Cells (Erythrocytes): Biconcave, anucleate cells specialized for oxygen transport via hemoglobin.

• White Blood Cells (Leukocytes): Immune cells present in lower numbers than RBCs.

• Platelets (Thrombocytes): Cell fragments involved in blood clotting.

Hematopoiesis: Formation of Blood Cells

Blood cells are produced in the bone marrow (pelvis, spine, ribs, cranium, proximal long bones) through a process called hematopoiesis.

• Stem Cells: Hematopoietic stem cells differentiate into various blood cell lineages.

• Erythropoiesis: Formation of erythrocytes from erythroblasts (which lose their nucleus before maturing).

• Thrombopoiesis: Formation of platelets from megakaryocytes (large cells that fragment to release platelets).

• Hormonal Regulation:

  • Erythropoietin (EPO): Produced by the kidneys; stimulates RBC production.

  • Thrombopoietin: Produced by the liver; stimulates platelet production.

Red Blood Cell Structure and Function

• Biconcave Shape: Maximizes surface area for gas exchange and facilitates smooth flow through capillaries.

• Hemoglobin: The oxygen-carrying protein; each molecule has four subunits, each with a heme group that binds one O2 molecule.

• Iron Metabolism: Dietary iron is absorbed in the intestine, transported by transferrin in the blood, and delivered to erythroblasts in the bone marrow for hemoglobin synthesis.

Iron Absorption and Transport

• Absorption: Iron enters intestinal cells via a divalent metal transporter (DMT1) on the apical side.

• Export: Iron exits via ferroportin on the basal side into the bloodstream.

• Transport: In blood, iron binds to transferrin, which delivers it to developing erythroblasts in the bone marrow.

Red Blood Cell Lifespan and Destruction

• Lifespan: ~120 days.

• Destruction: Old RBCs are broken down in the spleen; amino acids are recycled, and heme is converted to bilirubin (excreted via bile or urine).

• Clinical Note: Accumulation of bilirubin can cause jaundice, especially in newborns.

Respiratory System Structure and Gas Exchange

Anatomy of the Respiratory System

The respiratory system is designed to maximize oxygen intake and carbon dioxide removal.

• Major Structures: Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.

• Branching: Airways branch repeatedly, decreasing in diameter and increasing in number, culminating in alveoli (site of gas exchange).

• Surface Area: The total alveolar surface area is approximately the size of a football field, maximizing gas exchange efficiency.

• Alveoli: Thin-walled sacs surrounded by capillaries; site of O2 and CO2 exchange.

Airway Defense Mechanisms

• Mucus: Produced by goblet cells; traps pathogens and particles.

• Ciliated Epithelial Cells: Move mucus toward the pharynx for removal.

• Saline Solution: Mucus rides on a saline layer, facilitating its movement; the mechanism of saline production involves ion transport (to be discussed in detail in subsequent lectures).

Gas Laws and Respiratory Physiology

Boyle's Law

Describes the inverse relationship between pressure and volume of a gas at constant temperature.

• Formula:

• As volume decreases, pressure increases, and vice versa.

• Application: Explains how changes in thoracic volume during breathing affect lung pressure and airflow.

Dalton's Law of Partial Pressures

The total pressure of a gas mixture equals the sum of the partial pressures of each component gas.

• Formula:

• Atmospheric pressure at sea level is 760 mmHg, composed mainly of nitrogen (78%), oxygen (21%), and carbon dioxide (<1%).

• Partial Pressure Calculation:

• Example: mmHg

Gas Solubility and Transport

• Oxygen: Poorly soluble in water/plasma; most O2 is transported bound to hemoglobin.

• Carbon Dioxide: More soluble in plasma than O2; transported dissolved, as bicarbonate, or bound to proteins.

• Equilibration: At equilibrium, the partial pressure of a gas is equal in air and water, but the concentration depends on solubility.

Carbon Dioxide and Acid-Base Balance

• CO2 reacts with water to form carbonic acid, catalyzed by carbonic anhydrase:

• Increased CO2 leads to increased H+ (acidity), affecting blood pH.

Additional info: The body tightly regulates CO2 and H+ to maintain acid-base homeostasis.

Summary Table: Gas Properties in Blood

Key Terms and Concepts

• Filtration: Movement of fluid out of capillaries into tissues.

• Absorption: Movement of fluid into capillaries from tissues.

• Hemoglobin: Oxygen-carrying protein in RBCs; each molecule binds four O2 molecules.

• Bilirubin: Breakdown product of heme; excess causes jaundice.

• Boyle's Law:

• Dalton's Law:

• Carbonic Anhydrase Reaction:

Study Tips

• Practice drawing and labeling diagrams of capillary exchange, blood cell development, and respiratory structures.

• Memorize key equations and understand their physiological applications.

• Use flashcards for terminology and processes.

• Relate physical principles (e.g., gas laws) to their biological significance in oxygen and carbon dioxide transport.