Final answer:
The number of active marrow cavities declines with age as red marrow converts to fatty yellow marrow, reducing hematopoietic activity. The liver and spleen may compensate hematopoiesis under certain conditions. Bone mass peaks around age 30 and then declines, especially in individuals with XX chromosomes post-menopause.
Step-by-step explanation:
The number of active marrow cavities decreases as age increases. In youth, the medullary cavity is filled with red bone marrow which is actively involved in hematopoiesis, the process of blood cell formation. As a person ages, the red bone marrow gradually gets replaced by yellow marrow, which is fatty and does not produce blood cells. This conversion process, referred to as the yellowing of bone marrow, represents a decrease in active marrow cavities.
During the aging process, calcifies can occur, making it difficult for nutrients to reach chondrocytes, leading to their death and the eventual formation of the medullary cavity. Throughout adulthood, while the number of active marrow cavities declines, organs like the liver and spleen take over some of the hematopoietic function through extramedullary hematopoiesis to compensate, particularly under disease conditions like bone cancer.
Bone mass peaks around the age of 30, as shown in graphs of bone mass and age relation. After this age, the bone mass starts to decline, a trend that is more marked in individuals with XX chromosomes due to lower estrogen levels post-menopause. Despite this reduction in bone mass and the decrease in active marrow cavities, bones can still increase in thickness through processes such as bone remodeling in response to physical activity.