Heart: Functional Physiology
The heart is a hollow muscular organ located in the mediastinum. It consists of four chambers: right atrium, right ventricle, left atrium, and left ventricle. The myocardium is the muscular wall of the heart; the pericardium is the protective fibrous sac surrounding it.
Normal Function
Functional Physiology perspective: Pumps oxygenated blood to the body via the left ventricle and receives deoxygenated blood from the right ventricle. Generates approximately 100,000 beats per day at rest.
Lunar Adaptations
On Arrival (First Weeks)
Initial cephalad fluid shift increases cardiac preload in the first days. Heart rate may temporarily decrease as cardiac output adjusts to reduced gravitational demands. Plasma volume redistribution is rapid.
6-Month Resident
Cardiac mass begins to decrease (cardiac atrophy) without mitigation. Stroke volume decreases. Maximum heart rate achievable during exercise declines measurably. Heart loses some Earth-specific adaptations to hydrostatic pressure gradients.
Long-Term Resident (2+ Years)
Significant cardiac remodeling: smaller left ventricular volume, reduced wall thickness, lower aerobic capacity. Some adaptations appear partially irreversible, complicating Earth return acclimation. Ongoing resistance exercise slows but does not fully prevent these changes.