During the first 3 or 4 years after birth, almost all the bones of the body contain hematopoietic marrow, although regression of hematopoiesis begins in the distal phalanges of the digits before birth, and the red marrow of the phalanges of the toes is completely replaced by yellow, fatty marrow by 1 year of age. Shortly before puberty, yellow marrow appears in the distal ends of the long bones of the forearm, arm, leg, and thigh and gradually extends proximally until 20 years of age, by which age only the upper end of the humerus and femur still contain red marrow.
The other bones in which hematopoiesis occurs in the skeleton of the young adult are the vertebrae, ribs, sternum, clavicles, scapulae, coxal (hip) bones, and skull.
Blood reaches the marrow cavity of the diaphysis of a long bone via one or two relatively large diaphyseal nutrient arteries. The nutrient artery passes obliquely through the nutrient foramen of the bone, without branching and in a direction that usually points away from the end of the bone, where the greatest amount of growth is occurring at the epiphyseal plate. Once the nutrient artery enters the marrow cavity, it sends off branches that pass toward the two ends of the bone to anastomose with a number of branches of small metaphyseal arteries that pass directly through the bone into the marrow cavity at the two metaphyses. The arteries of the metaphysis supply the metaphyseal side of the epiphyseal growth plate of cartilage.
Numerous small epiphyseal arteries pass directly through the bone into the marrow cavity of the epiphyses at each end of the bone. The epiphyseal arteries supply the deep part of the articular cartilage and the epiphyseal side of the epiphyseal growth plate. In a growing bone with a relatively thick growth plate, there are few, if any, anastomoses between the epiphyseal and metaphyseal vessels. The growth plate also receives a blood supply from a collar of periosteal arteries adjacent to the periphery of the plate.
The branches of the diaphyseal nutrient arteries, which pass to each end of the bone to anastomose with the metaphyseal arteries, give off two sets of branches along the way, one peripheral and one central. The peripheral set passes directly to the bone as arterioles that give off the capillaries that enter Volkmann’s canals and branch to supply the central haversian canals, ultimately emerging at the outer surface of the bone and anastomosing with the periosteal vessels. The direction of the blood flow in these capillaries is from within the bone outward; thus, the blood flow through the canal system of the bony wall is relatively slow and at a low pressure.
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