Capillary Exchange and Fluid Movement

Osmosis and Hydrostatic Pressure in Capillaries

Capillary exchange is the process by which materials move between blood and tissues. This exchange is governed by two main forces: osmotic pressure and hydrostatic pressure. Understanding these forces is essential for explaining how nutrients, gases, and waste products are transported to and from cells.

• Osmosis: The movement of water from an area of high water concentration (low solute) to low water concentration (high solute) across a semi-permeable membrane. In capillaries, plasma (mostly water) moves toward areas with higher solute concentration inside the vessel.

• Hydrostatic Pressure: The force exerted by fluid against the capillary wall. It is higher on the arterial end (about 32 mmHg) and lower on the venous end (about 15 mmHg).

• Osmotic Pressure: Primarily due to plasma proteins and solutes inside the capillary, typically around -25 mmHg (negative sign indicates absorption into the vessel).

At the arterial end, hydrostatic pressure exceeds osmotic pressure, resulting in net filtration (fluid moves out of the capillary). At the venous end, osmotic pressure exceeds hydrostatic pressure, resulting in net absorption (fluid moves into the capillary).

• Net Filtration (Arterial End):

• Net Absorption (Venous End):

This mechanism ensures delivery of oxygen and nutrients to tissues (filtration) and removal of waste products like carbon dioxide (absorption).

Physiological Importance

• Oxygen Delivery: Net filtration on the arterial side delivers oxygen to cells for cellular respiration and ATP production.

• Waste Removal: Net absorption on the venous side collects carbon dioxide and other waste products for removal from tissues.

Example: If the hydrostatic pressure at the arterial end is 32 mmHg and the osmotic pressure is -25 mmHg, the net filtration pressure is +7 mmHg, favoring movement of fluid out of the capillary into the tissue.

Blood Composition and Hematopoiesis

Components of Blood

Blood is a specialized connective tissue composed of plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets. Each component has distinct functions essential for homeostasis.

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

• Red Blood Cells (Erythrocytes): Specialized for oxygen transport; lack a nucleus in their mature form and contain hemoglobin.

• White Blood Cells (Leukocytes): Involved in immune defense; present in much lower numbers than RBCs.

• Platelets (Thrombocytes): Small cell fragments involved in blood clotting and vessel repair.

Hematopoiesis: Blood Cell Formation

Blood cells are produced in the bone marrow (hematopoietic tissue), found in the pelvis, spine, ribs, cranium, and proximal ends of long bones. The process is regulated by hormones and involves several stages of cell differentiation.

• Stem Cells: Pluripotent cells that give rise to all blood cell types.

• Erythroblasts: Precursors to erythrocytes; lose their nucleus before maturing into RBCs.

• Megakaryocytes: Large cells that fragment to form platelets.

• Hormonal Regulation:

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

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

Red Blood Cell Morphology and Function

• Biconcave Shape: Increases surface area for gas exchange and allows flexibility through capillaries.

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

Red Blood Cell Life Cycle and Spleen Function

• Lifespan: Approximately 120 days.

• Destruction: Old RBCs are destroyed in the spleen; amino acids are recycled, and heme is converted to bilirubin for excretion via bile or urine.

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

Respiratory System Structure and Function

Anatomy of the Respiratory Tract

The respiratory system is designed to maximize oxygen intake and carbon dioxide removal through a highly branched network ending in alveoli.

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

• Branching: The airway branches from the trachea (15 mm diameter) to bronchi, bronchioles, and finally to alveoli (0.3 mm diameter), greatly increasing surface area for gas exchange.

• Alveoli: Grape-like sacs with a surface area equivalent to a football field; lined with thin epithelial cells (type I pneumocytes) for efficient gas diffusion.

• Elastic Fibers: Allow expansion and recoil of the lungs during breathing.

Mucociliary Clearance

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

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

• Saline Solution: Underlies mucus, allowing it to move smoothly; its production will be discussed in detail in subsequent lectures.

Gas Laws and Respiratory Physiology

Boyle's Law

Boyle's Law describes the inverse relationship between pressure and volume of a gas at constant temperature:

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

• Application: During inhalation, lung volume increases and pressure decreases, drawing air in.

Dalton's Law of Partial Pressures

Dalton's Law states that the total pressure of a mixture of gases equals the sum of the partial pressures of each gas:

• Atmospheric pressure at sea level: 760 mmHg.

• Composition: 78% nitrogen, 21% oxygen, <1% carbon dioxide.

• Partial pressure of oxygen: mmHg.

Gas Solubility and Transport

• Oxygen: Poorly soluble in water (plasma); requires hemoglobin for efficient transport.

• Carbon Dioxide: More soluble in water than oxygen; can be transported dissolved, as bicarbonate, or bound to proteins.

• Equilibration: Gases diffuse until partial pressures are equal across the air-water interface, but the actual amount dissolved depends on solubility.

Key Equations

• Boyle's Law:

• Dalton's Law:

• CO2 Hydration (Carbonic Acid Formation):

Clinical Application: Acid-Base Balance

• Excess CO2 leads to increased H+ (acidity) in blood.

• Maintaining proper CO2 levels is crucial for acid-base homeostasis.

Summary Table: Capillary Exchange Forces

Key Terms

• Osmosis: Movement of water across a semi-permeable membrane from low to high solute concentration.

• Hydrostatic Pressure: Pressure exerted by a fluid in a closed system.

• Osmotic Pressure: Pressure required to prevent osmosis; in blood, mainly due to plasma proteins.

• Hemoglobin: Oxygen-carrying protein in RBCs.

• Transferrin: Plasma protein that transports iron.

• Bilirubin: Breakdown product of heme, excreted in bile and urine.

• Boyle's Law: Relationship between pressure and volume of a gas.

• Dalton's Law: Law of partial pressures in a gas mixture.

Additional info: Some explanations and terminology were expanded for clarity and completeness, including the clinical relevance of bilirubin and the role of hemoglobin in overcoming oxygen's low solubility in plasma.