and left ventricular output. The lungs have a unique gravity dependence in several ways, including blood flow distribution, alveolar gas exchange, and inhaled particulate deposition and clearance. The perfusion pressure of deoxygenated blood entering the pulmonary artery from the right ventricle is relatively low, typically 10 to 20 mm Hg. In an individual standing upright, this pressure may be insufficient to overcome hydrostatic gradients, and so very little flow reaches the upper regions of the lungs, but a relatively large portion of pulmonary blood flow perfuses the dependent portions of the lungs. Air in the alveoli flows somewhat preferentially into the middle and upper regions of the lungs. These regional differences create a mismatch of air ventilation (V) and blood perfusion (Q) and are the basis for the system of classification of lung zones.¹ This classification system does not address differences between central and peripheral ventilation. These ventilation/perfusion (V/Q) mismatches account for the observation that clinical conditions associated with excessive fluid in the lungs (e.g., congestive heart failure) tend to affect the bases or dependent portions of the lungs, whereas airborne infections, such as tuberculosis, occur mainly in the upper regions of the lungs where the oxygen content is relatively high. The lungs have both intrinsic and extrinsic mechanisms to match ventilation and perfusion, but the main mechanism for accommodating changes in gravity and demand is redundancy, which provides a substantial excess capacity for gas exchange.

The lungs also have an elaborate system for clearing inhaled particulate materials existing as aerosols. (An aerosol is any system of solid or liquid particles sufficiently small to maintain stability as a suspension in air.^{2 3}) The particles have to be too large to diffuse but must be sufficiently small to remain suspended (0.01 to 10 μm). Dust, smoke, chemicals, and inhaled bacteria all represent common threats to the lungs, both on Earth and, perhaps even more so, in microgravity. Three factors determine the location and extent of deposition of inhaled particles in the lungs: (1) the anatomy of the upper and lower respiratory tract, (2) patterns of inhalation and air flow, and (3) characteristics of the inhaled particles, including size, density, electric charge, and the tendency to absorb water. The effective size of a particle reflects its diameter d and density r, and is described by its aerodynamic diameter d_a,

Particles being deposited by gravitational sedimentation reach a terminal velocity V_t where gravitational acceleration g is balanced by air resistance and is described by the equation

where α and γ are the density and viscosity of air, and ρ and d are the density and diameter of the particle. Particles larger than 5 to 10 μm are filtered by small hairs in the nares. Sharp bends in the nasal passages, trachea, and bronchi also cause these larger particles to impact the airway walls, where they adhere and are cleared by the mucocilary sweeping action of the bronchial lining cells. Inertial impaction of particulates leads to highly localized deposition at airway bifurcations and accounts for the observation that most smoking-related lung cancers occur at such locations. Flow velocity in the lower airways falls progressively as the airway cross-sectional area rises. Particles of 1 to 5 μm are deposited in the terminal bronchioles and alveolar regions mainly by sedimentation, with Brownian diffusion transporting particles smaller than 1 μm.

Given the gravity dependence of the cardiovascular and pulmonary systems, it is not surprising that humans exposed to altered gravity show significant cardiovascular and pulmonary changes. These were

Front Matter (R1-R20)

Executive Summary (1-20)

1 Introduction (21-28)

2 Cell Biology (29-36)

3 Developmental Biology (37-48)

4 Plants, Gravity, and Space (49-62)

5 Sensorimotor Integration (63-79)

6 Bone Physiology (80-96)

7 Skeletal Muscle (97-117)

8 Cardiovascular and Pulmonary Systems (118-131)

9 Endocrinology (132-155)

10 Immunology (156-170)

11 Radiation Hazards (171-193)

12 Behavioral Issues (194-230)

13 Setting Priorities in Research (231-236)

14 Programmatic and Policy Issues (237-250)

Appendix A: Acronyms and Abbreviations (251-252)

Appendix B: Glossary (253-265)

Appendix C: Workshops (266-271)

Appendix D: Committee Biographies (272-276)