Oxygen tension (mmHg)

Optical density

60

50

40

30

1 mm. temporal of optic nerve, M and SEM, 16 sections. V = vitreous, C = choroid.

Fig. 11.35 Flickering light stimulation increases retinal arteriovenous oxygen difference (top left) and inner retinal glucose uptake (bottom left) and elicits a retinal

functional hyperemia (right top and bottom) [69Ð71] (upper right graph data from Bill and Sperber [69])

after brief (Fig. 11.29) and long periods of ischemia (Fig. 11.37) [56]. Surprisingly, the retinal pressure-ßow relationships under lightand dark-adapted conditions have not been determined, but it seems likely that the dark-adapted curve would be shifted upward due to the heightened oxygen consumption. Similarly, reactive hyperemic responses to brief ischemic periods would be expected to be larger in the dark (Fig. 11.35).

11.4.4 Ciliary Body

While the retina has essentially one function (processing light information for the central nervous system) with a dual blood supply, the ciliary body has two functions (accommodation and aqueous secretion) with a single blood supply. Distinguishing between ciliary muscle and secretory

tissue is fairly straightforward by histology, but difÞcult in vivo, and measuring the blood ßow to the two tissue types is even harder. Microsphere measurements at normal and high IOP suggest that both ciliary muscle and ciliary processes autoregulate, but the measurements vary widely, perhaps due to the small number of spheres that can be trapped in such a small amount of tissue (Fig. 11.38).

Laser Doppler ßowmetry measurements in the rabbit also show evidence of ciliary autoregulation. However, it is also clear that the ciliary circulation is under neural control since ganglionic blockade caused an upward and leftward shift in the pressure-ßow relationship indicative of tonic neuroconstrictor tone (Fig. 11.39). This is an interesting example of the interplay between neural and local control. Ciliary blood ßow provides the oxygen and nutrient delivery to support aqueous production, and ciliary blood ßow can

Fig. 11.36 Feline retinal blood ßow responses in cats consistent with metabolic local control. Retinal perfusion decreases during hyperoxia (top left and right) and

increases during hypoxia (bottom left) and hypercapnia (bottom right) [72Ð75]