Blood supply pituitary

The pathway by which MSH-RIF and MSH-RF reach the pituitary gland is unclear. Unlike the pars distalis, the pars intermedia is well supplied with neurones from the hypothalamus but its blood supply is poor. Accordingly it has been suggested that the hypothalamic hormones are released from nerve endings within the pars intermedia, but further experimental evidence is needed to substantiate such a hypothesis. 

Transcortin acts as a reservoir from which a constant supply of unbound cortisol may be provided to target cells. In addition, when serum albumin levels are low, less circulating cortisol becomes bound, which yields a greater physiological effect. Not only does protein binding control the amount of biologically active cortisol available, but it also reduces the rate at which steroids are cleared from the blood and thus limits steroid suppression of corticotrophin release from the pituitary gland. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

As the free T3 level in the blood bathing the thyrotrophs of the anterior pituitary gland rises, the feedback loop is closed. Secretion of TSH is inhibited until the free T3 levels in the systemic circulation fall just below a critical level, which once again signals the release of TSH. This feedback mechanism ensures an uninterrupted supply of biologically active free T3 in the blood (Fig. 43.11). High levels of T3 also inhibit the release of TRH from the hypothalamus. 

Neiurosarcoidosis may present as an acute explosive illness or in a slow, chronic fashion (143). Any part of the nervous system may be affected including the cranial nerves, meninges (Fig. 11), pituitary gland, hypothalamus, parenchyma of the brain, brainstem (Fig. 12), spinal cord (Fig. 13), subependymal layer of the ventricular system, peripheral nerves, and blood vessels supplying the nervous structures (143). 

Lyophilized, anatomically Intact, whole pituitary glands from hogs were kindly supplied by Canada Packers Anterior and posterior were carefully dissected from these when necessary. Isolation of Fractions H and L was exactly as described by Rudman et al. as was the preparation of the rabbits used in the bioassay. The extracts were dissolved in 0.9% NaCI and injected into the marginal ear vein. Blood samples were drawn from the marginal ear vein (except in 10 rabbits by cardiac puncture) and analyzed for FFA (Dole), triglycerides (Van Handel), cholesterol (Abell etal Levine and Zak), lipoproteins (Shandon), glucose, urea and uric acid (E.Seifter et al.). 

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