## 4 Articular Synovial Parameters
### Synoial Fluid--Articular Cartilage Interactions  
Articular cartilage does not contain blood vessels, so nutrient delivery is ensured by transport from the SF by diffusion or by convective transport during mechanical loading [123; 138; 279]. Transport of macromolecules and nutrients in the form of oxygen, glucose and fatty acids takes place by penetrating deep into the tissue via the cartilage surface zone. These components are then used by chondrocytes in metabolic processes [280]. This process is dependent on the structure and the degree of hydration of the extracellular matrix, which provides a porous environment for migrating molecules [138]. The effectiveness of cartilage nutrition depends on the conditions of the joint environment, as changes in osmotic pressure and pH modify the flow of ions and metabolites [279]. Passive transport of molecules occurs in unloaded cartilage, although in the case of rhythmic loads or physical activity, mechanical compression occurs, which results in squeezing fluid towards the surface from deep zones and vice versa, which positively affects the convective flow of nutrients [281]. It has been shown that even a small movement of the joint affects the sliding of the cartilage surface, which leads to improved penetration of molecules into the matrix. This is a particularly important process, as it supports chondrocyte metabolism and also helps to limit degeneration [281]. Dynamic loading also has the effect of increasing the pressure gradients between the deep zones and the surface, resulting in increased fluid exchange and thus the delivery of nutrients [282]. In patients affected by OA, the matrix structure is damaged, thereby reducing the ability of the tissue to retain water. This translates into a reduction in the efficiency of nutrient transport, thereby exacerbating metabolic deficiencies in cells, accelerating degeneration [283]. The interaction between AC and SF is particularly important in the biomechanics of the joint, especially in the context of loading and unloading [123]. AC metabolism and homeostasis are significantly influenced by the mechanisms of mechanical transport of nutrients and removal of metabolites [123]. Articular cartilage obtains nutrients mainly from synovial fluid, which is transported to the cartilage by diffusion and convection, and these processes are assisted by cyclical loading and relaxation of the joint [121]. The ingredient that plays a special role in these processes is HA. As a result of mechanical loading of the joint, there is an increase in pressure within the AC and compression of the extracellular matrix, resulting in fluid from within the AC entering the joint cavity [284]. The mechanism of fluid expulsion is consistent with the principles of poroelasticity, which assumes that the tough collagen-proteoglycan matrix acts as a skeleton, while the displaced fluid acts as a flow phase [157]. When the joint is no longer under load, there is a diastolic phase in which the pressure in the tissue is reduced, resulting in the SF being sucked back into the AC [138]. [PERSON] et al. described this type of mechanism as a \"mechanical pump\", whereby waste products in the form of unnecessary metabolic products are removed in addition to nutrient transport [279]. Joints subjected to cyclical physiological loads increase the efficiency of this process [285]. Diffusion and fluid motion-assisted convection are influenced by the structural integrity of the AC. Proteoglycans contribute to water binding and water flow control [286]. In older people and those affected by OA, matrix components are degraded, with the result that AC reduces the capacity for effective fluid exchange [287]. In the case of prolonged non-loading, fluid transport is impaired, leading to hypoxia of the chondrocytes, resulting in their apoptosis and matrix degeneration [288].