From Lewis S Blevins, Jr MD – The pituitary gland is certainly quite small, and not metabolically active when compared to other organs, thus commands a very small fraction of the cardiac output. Pituitary blood flow is difficult to measure, really, and it is quite complex. I’ll try to simplify it and make it relevant to those who have pituitary disorders.
The arteries feeding the gland are tiny and come off of the carotid arteries. Interestingly, though atherosclerotic vascular disease is common and can be quite severe, nearly clogging the carotid arteries requiring a clean out, I’m not seen such patients referred for evaluation of pituitary function even though one would suppose there would be a risk for some degree of hypopituitarism. That’s an idea for a study! At any rate, these hypophysial arteries feed the pituitary systemic blood that has been derived from the heart. This is important because the blood delivered to the gland contains hormones from pituitary target glands.
When the blood vessels enter the gland they become smaller and smaller until they enter into a system of capillaries. The capillaries are dispersed throughout the gland and are intimately in contact with and amongst the groups of cells of the gland. Ultimately, the capillaries coalesce into small veins, that become larger veins, that empty into a complex system of sinuses and other veins that dump blood into the cavernous and related sinuses including the inferior petrosal sinuses. In fact, the inferior petrosal sinuses can be catheterized to obtain blood from the pituitary for diagnostic purposes. The most common reason to perform such a procedure is to determine if patients with documented Cushing’s syndrome due to ACTH secretion have a pituitary tumor or else a tumor elsewhere (syndrome of ectopic ACTH hypersecretion).
So, the blood bathing the pituitary that is derived from the systemic circulation (carotid arteries) contains those hormones I mentioned. The hormones are able to enter the interstitial space and then gain entry into the cells where they bind to receptors in the cells and can exert their effects on pituitary cells. In a way, the pituitary is measuring hormone levels in the blood stream. By the way, the same is going on in the hypothalamus. I’ll have more to say on that in a moment. Anyways, by way of example, T4 and T3 from the thyroid and peripheral conversion of T4 to T3 is carried to the pituitary. These hormones enter the cells of the pituitary, where T3 binds to its receptor in the nucleus of the pituitary cells, and exerts its effects. In the non TSH-producing cells T3 probably does a lot of things to keep cells healthy. In TSH-producing cells, T3 will regulate TSH production and secretion. In normal persons, if T4 and T3 levels are too high, then the TSH will be turned down if not completely off. If T4 and T3 levels are too low then the cells will sense this and TSH will be produced in greater quantities. This system of feedback also applies to cortisol and ACTH, and testosterone and LH. The female reproductive system and GH systems are a little more complex. It follows, then, that the hormones produced by the pituitary gland enter the venous drainage of the gland, ultimately find their way to he heart, and are then distributed to their target glands where there are receptors on the cell surfaces of the glands that are especially designed for each of the pituitary hormones. The hormones from the pituitary gland then exert their specific effects on the glands that it controls.
But, wait…that’s not all. The pituitary gland has a second blood supply that is also critical to its function. This system is known as the hypothalamic hypophysial portal system. Basically, there are small arteries that go to the hypothalamus. They break up into capillaries that intermingle with the hypothalamic nuclei composed of neurons that produce hypothalamic releasing hormones that are designed to exert their effects on the pituitary gland. These hormones are made by neurons and picked up by the capillaries in the hypothalamus. These capillaries form veins. In this case, they form long and short portal veins that travel down the pituitary stalk. The blood from these veins carries the hypothalamic hormones to the pituitary. Once in the pituitary gland, these veins form a network of capillaries that intermingle with the systemic capillaries. Thus, the hypothalamic hormones gain access to the cells of the pituitary gland. The capillaries then form the same veins mentioned before and drain pituitary blood that ultimately goes to the heart. By way of example, the hypothalamic neurons that make TRH are measuring T4 and T3 levels and decide how much TRH to make. TRH is carried to the anterior pituitary gland and, in conjunction with systemic blood T4 and T3 levels influences TSH production and secretion. A number of complex things do indeed happen in and on the pituitary TSH-producing cells and I’ve oversimplified to get the understanding across to the readers. Other hypothalamic hormones influence the pituitary in a number of different ways.
While the systemic supply carries oxygenated blood to the gland, the portal system contains less oxygen since it delivered oxygen to the hypothalamus.
You might ask….what can go wrong with this system? Let’s talk about some of those things.
Aneurysms, tumors, or surgical procedures in the vicinity of the circle of Willis or suprasellar cistern may disrupt hypothalamic or pituitary blood flow leading to hypopituitarism.
It is hypothesized that radiation to the pituitary region actually causes inflammation in the vessels of the hypothalamus, stalk, and pituitary that leads to hypopituitarism.
Surgery within the sella may disrupt normal blood vessels and lead to hypopituitarism.
Pituitary tumors grow their own vessels and may “steal” blood from the normal anterior pituitary. This whole system may be compromised by pituitary tumor apoplexy resulting in permanent hypopituitarism.
Some cases of vasculitis (Wegener’s, SLE) may be associated with hypopituitarism.
Sheehan’s syndrome is infarction of the enlarged pituitary of pregnancy and leads to hypopituitarism. This occurs as the pituitary may double in size during pregnancy, the blood supply is tenous, and if there is blood loss or low blood pressure during pregnancy or delivery the gland doesn’t get enough blood supply to nourish its cells and it dies.
Sickle cells in patients with Sickle Cell Anemia or Trait may clog up the vessels and cause hypopituitarism.
Traumatic or surgical transection of the pituitary stalk disrupts the portal vessels and the pituitary is no longer controlled by the hypothalamus and hypopituitarism develops.
I find the subject of pituitary blood flow and resultant function to be quite fascinating. I hope this has piqued your interest and also helped you understand a little more about this pea-sized gland that we call the “master gland.”
Photo Abstract by Sherrie Thai.
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