Bone requires a constant, rich supply of blood to maintain its structure and function. This vascular network is important because the internal medullary cavity houses the bone marrow, the primary site for hematopoiesis (blood cell production). Spongy bone (cancellous bone), found primarily at the ends of long bones, also contains marrow and relies heavily on blood flow. The system of arteries and veins ensures oxygen and nutrients are delivered while waste is removed, supporting the health of the bone structure.
The Central Supply Route to the Medullary Cavity
The dominant pathway for blood entering the central region of a long bone is the high-pressure nutrient artery system. This artery penetrates the dense outer layer of bone, the cortex, through a small opening called the nutrient foramen, typically located near the middle of the shaft (diaphysis). Once inside the medullary cavity, the nutrient artery immediately splits into ascending and descending branches that run parallel to the bone’s long axis.
These longitudinal branches are the main suppliers of the bone marrow within the central cavity. They divide into smaller arterioles that extend radially, feeding the central sinusoids of the marrow. This high-pressure system is also responsible for supplying the inner two-thirds of the compact bone forming the shaft wall.
The arterioles serving the inner cortex connect with the vessels of the periosteal system, which supply the outer third of the bone. This connection creates an anastomotic network, ensuring redundancy in the blood supply to the cortical bone. The nutrient artery system is the primary source for the bone marrow and contributes significantly to the structural integrity of the main shaft.
Vascular Supply at the Bone Ends
While the nutrient artery focuses on the central shaft, the spongy bone at the bone ends (metaphyses and epiphyses) is supplied by separate vascular networks. These regions are nourished by the metaphyseal and epiphyseal arteries, derived from vessels supplying adjacent joints and muscles. These arteries penetrate the bone directly, forming extensive capillary beds that weave through the porous structure of the spongy bone.
The metaphyseal arteries enter the region where the shaft widens, creating a dense network that makes the metaphysis highly vascular. These vessels provide oxygen and nutrients directly to the trabeculae (delicate struts of spongy bone) and the red marrow housed within. The epiphyseal arteries supply the ends of the bone, ensuring the health of the subchondral bone and the joint surface.
In children, the blood supply in these regions is separated from the nutrient artery system by the growth plate (physis). This separation means the epiphyseal and metaphyseal vessels supply the growing bone and cartilage, which is important for skeletal development. In adults, after the growth plate closes and fuses, the metaphyseal and epiphyseal vessels anastomose (connect) with the nutrient artery system, creating a single, interconnected supply network.
Microcirculation and Venous Drainage
After the arteries deliver blood into the bone and marrow, the flow transitions into a specialized microcirculation network. Within the bone marrow, terminal arterioles empty into unique, wide, and highly permeable capillaries called sinusoids. This sinusoidal network is the site of exchange, delivering oxygen and nutrients to the bone cells and marrow stem cells, and collecting waste products.
The structure of the sinusoids is adapted for marrow function, featuring thin walls that allow newly formed blood cells to pass from the hematopoietic tissue into the central circulation.
Blood flow within the diaphysis generally follows a centrifugal pattern. This means the flow moves outward from the high-pressure central nutrient artery system toward the lower-pressure vessels in the cortex and periosteum.
The blood is collected into veins that mirror the arterial pathways. The majority of deoxygenated blood drains out through the large nutrient veins, which accompany the nutrient artery out of the diaphysis. Blood from the metaphyses and epiphyses drains through separate metaphyseal and epiphyseal veins. These exit the bone ends through numerous small foramina, completing the circulatory loop.