These capillary beds form interconnecting networks linking terminal branches of pre-capillary arterioles and post-capillary venules. Adapted from Anand-Apte and Hollyfield with drawings by Dave Schumick. (B) Cross-sectional drawing of the retinal and choroidal vasculature at the level of the fovea. (A) Sagittal drawing of the human eye showing the retinal and choroidal circulation of the left eye. These further bifurcate into increasingly smaller arterioles, feeding eventually into a capillary bed as they extend towards the peripheral retina ( Figure 1). The central retinal artery branches into four principal intra-retinal arteries. At the optic nerve, there is a combination of choroidal and retinal arterial circulation, details of which are dealt with in other chapters. The central retinal artery traverses through the orbital portion of the optic nerve, entering the optic disc through the lamina cribrosa. Both of these supplies originate from the ophthalmic artery, which itself is a branch of the internal carotid artery. The photoreceptors and outer one third of the retina are supplied by the choroidal circulation. The inner two thirds of the retina is supplied by inherent intra-retinal vessels, fed by the central retinal artery, and drained via the central retinal vein. The metabolic demands and the oxygen requirement of the retina are met by two distinct vascular systems. Moreover, these feed-back loops may vary depending on the underlying disease processes in individuals with multiple co-morbidities. ![]() Second, in a complex system such as retinal circulation, many variables act as both dependent and as independent variables in many feed-back loops. Firstly, it is difficult to capture the data of multiple variables that may contribute to hemodynamics in a clinical setting. In this chapter, we will not delve into the detailed discussions of mathematical modeling, but will only use them to better understand the hemodynamics and its biologic consequences.Ĭlinical evidence for the relationship between intraocular pressure (IOP) and retinal hemodynamics remains inconsistent and difficult to interpret. In the retinal arterioles, the flow is often turbulent and blood itself is not truly a Newtonian fluid. However, it is important to know that Poisseuille’s equation is used mainly for Newtonian fluid in a system with laminar flow. Mathematically, this relationship is often simplified into Poisseuille’s equation, which we will discuss more in detail in the diabetic retinopathy section. ![]() Our understanding of their inter-relationship is derived from concepts used in fluid flow systems borrowed from engineering and from other physiologic studies of blood flow. Blood flow, arterial and venous pressure, vascular resistance, and blood viscosity all play important roles. Retinal hemodynamics are influenced by a number of factors. By understanding these processes and their disturbances, we can better characterize the pathological processes that occur in many ocular and systemic diseases such as glaucoma, age-related macular degeneration and diabetic retinopathy, to name a few. Retinal fundoscopy and digital imaging have allowed for retinal microvascular abnormalities to be directly and non-invasively identified and studied as a means of better understanding the manifestation of systemic microcirculatory disorders.Īlthough not yet completely understood, hemodynamic factors such as perfusion pressure, blood viscosity, vascular resistance and the variations on vessel caliber that ensue, determine the blood supply and flow to the retina. The retina shares similar anatomical features and physiological properties with other end organs such as the brain and the kidney, namely the presence of blood–brain, blood-kidney and blood-retina barrier as well as non-anastomotic end arteries.
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