Vascular Supply and Circulation in Bone

All tissues in the body include bone requires vascular support to survive. The blood vessels in bones are necessary for all skeletal functions like homeostasis and repair. The generation of blood vessels, if lost due to trauma gets regenerated with the help of the vascular system. 

The blood supply of long bone is done by three separate systems. They are nutrient artery, periosteal vessels, and epiphyseal vessels. The nutrient artery passes through the cortex into the medullary cavity and sends it out through Haversian and Volkmann canals to supply the cortex. This nutrient artery nourishes the diaphysis and metaphysis. 

The blood vessels inside the bones are highly active, it does not only deliver the nutritions. The rate of blood vessels delivers oxygen, nutrients, growth factors and circulation cells to bones determine its vascularization. Usually, the size of the blood vessels determines the blood pressure and rate of blood flow. Though the skeletal disorders defect the general vascular system, it alters and focuses on the blood supply of bone through the vessels to support bone repair and regenerations. 

Role of Vascular Supply and Circulation in Bone 

Bone is a high vascularized connective tissue. The process of bone development is extremely supported by the skeletal vasculature. Approximately, the skeletal system receives 10 - 15% of total cardiac output. It provides specific hormones, growth factors, and neurotransmitters secreted by other tissues also with nutrients to support skeleton development. 

Blood Supply of Bone

The vascular system network delivers oxygen and all essential nutrients to all 209 bones in the human body. It requires 10 - 15 percent of total cardiac output. The pattern of blood supply of bones is mainly centrifugal. From the marrow cavity, the blood supplies to the cortical bone through nutrient arteries and it gets returned by the periosteal vessels. Usually, the blood supply of long bone has specific arterial inlets and veinous outlets. The structure of the vascular network gets varies depending on the skeletal site. 

Researchers have shown that depending on the hemodynamic conditions of bone they can change the blood flow direction, which shifts from centrifugal to centripetal. This hypothesis was tested experimentally by intramedullary reaming in the ovine tibia. After the nutrient supply through vascular tissues, the blood flow at the periosteum increases rapidly through local centripetal blood flow to compensate for the loss. The stress fracture healing was related to the disruption of normal centrifugal flow in the ulna. The blood supply of bones are site-specific and dynamically change in response to trauma, metabolic demands, and ageing.

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Mechanisms of Vascularization

The production of bone, development or repair, entails the generation of new blood vessels to support the tissue. There are at least four distinct mechanisms for the generation of new vessels. Vasculogenesis or neovascularization is the generation of de nova blood vessels during the development. Likewise, angiogenesis is a process used to describe the generation of new vessels from the existing vasculature. Angiogenesis is of two types. Sprouting angiogenesis, here the new vessel branches develop from an existing vessel. During Intussusceptive angiogenesis, the vessels split into two or more vessels. Arteriogenesis is a different process, in which the functional collateral arteries are generated between existing arteriolar anastomoses. 

Angiogenesis is triggered by hypoxia through HIF - 1α mediated gene transcription. Whereas, arteriogenesis appears to occur through the response of physical forces, independent of hypoxia. The structure of blood vessels can alter through the vascular remodelling process. Usually, vascular remodelling either increases or decreases the vessel diameter. It is termed as outward and inward respectively. The remodelling can be characterized as hypertrophic, eutrophic, and hypotrophic. 

The short-term rapid expansion or contraction of blood vessels are known as vascular tone or vasomotor, this makes changes in the blood flow and can cause injury. The regulation and treatment process is complex. 

Techniques For Measuring Bone Blood Flow

The blood flow rate can be determined as volume per time. It is mainly calculated through a product of the cross-sectional area of the vessel and the fluid velocity. As it determines the pressure. This relationship was expressed in Poiseuille’s law, a simplification of the Navier-strokes equation.  There are various techniques available in the market to calculate the bone blood flow like Laser Doppler Flowmetry, Positron emission tomography (PET), non-newtonian fluids..etc

The Newtonian fluids equation. 

Q=πr4ΔP / 8μL

Q → flow rate

r →  radius of the vessel

ΔP → difference in pressure

μ → viscosity of the fluid 

L→ length of the vessel

Blood Flow During Fracture

Bone Fracture of skeletal fracture occurs due to the disruption of soft tissues around the marrow components of the blood supply of long bones. As a result, the vascular network around the bone is affected. The decreased perfusion associated with an increase in metabolic demands leads to hypoxia near the fracture site. This helps to restore blood flow through angiogenic mechanisms and acts as a key for the fracture healing process. To fix the canine tibial fracture, intramedullary rods can be used to increase blood flow at the fracture site. It is comparatively better to fic compression-plate. The nutrient artery and periosteal vessels are responsible for quick recovery of fracture. In some cases, it may require new blood vessels for vascularizing the fracture callus and repairing the vascular damage. But it can repair through the pre-existing vascular network during a short span of time. 

Blood Flow for Maintenance of Bone

Osteoporosis is a metabolic disorder, it occurs due to the loses of bone or the thinning of the bone. The bone blood flow plays a predominant role to cause Osteoporosis. The researchers studied blood flow for maintenance of bone and found that the increase in bone loss, particularly at the hip may result to decrease vascular support.