Giraffes evolved a rete mirabile vascular structure to regulate cerebral blood pressure.
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REFUTED
the evidence says no
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Peer-reviewed literature indicates that the giraffe's carotid rete mirabile lacks the sympathetic innervation needed to play an active vasoconstrictor role during head postural changes, suggesting it does not actively regulate cerebral blood pressure as commonly theorized, though other unique vascular mechanisms are involved in regulating blood flow to the brain.
As discussed in this chapter, giraffes have, compared with any other mammal, a very high mean blood pressure of ~250 mmHg. Human blood pressure is ~90 mmHg. Its size is determined by the length of the neck, the height of the head above the heart, by hydrostatic pressure generated by gravity acting on the column of blood in the carotid artery, and contractions of the heart muscles: blood pressure must be high enough to ensure that blood reaches the brain. Uniquely in giraffes blood pressure is regulated by receptors that are located in both the carotid and occipital arteries. Once thought to be ~2.5% of body mass the heart is smaller (~0.5% of body mass) but its muscle walls, especially of the interventricular wall and left ventricle wall, are exceptionally thick (up to 8 cm). The relative cardiac output is the same as in other mammals (~5 L 100 kg–1 of body mass) through a combination of a higher than predicted heart rate (70 b min–1 vs 50 b min–1) and smaller than predicted stroke volume (~0.7 ml kg–1 body mass vs 1.2 ml kg–1). Stroke volume is small because the left ventricle muscle wall is thick. The origin of high blood pressure is the resistance to blood flow, which is about twice what it is in other mammals. The higher resistance results from a combination of the thick muscular walls and narrow lumens of a giraffe’s blood vessels and unique mechanisms that regulate blood flow to the brain.
This work was designed to characterize in anesthetized goats the hemodynamic response of the carotid rete during pharmacologically induced changes in systemic blood pressure or blood flow to the brain. Under control conditions, mean blood pressure in the middle cerebral artery (distal to rete) was 18% lower than that measured in the internal maxillary artery (proximal to rete). Pressure gradient and calculated resistance across the rete were unchanged when systemic arterial pressure was increased or decreased by intravenous administration of norepinephrine or isoproterenol, respectively. Hypercapnia or injections of isoproterenol and acetylcholine into the internal maxillary arteries increased blood flow and decreased middle cerebral arterial pressure, whereas injections of norepinephrine decreased blood flow and increased postrete pressure. Calculated resistance across the rete was unchanged. These observations indicate that the response of the carotid rete to the substances tested is negligible; they also suggest that the carotid rete may have a flow-damping effect by maintaining resistance to blood flow when a change in the caliber of brain vessels occurs.
Structure and sympathetic innervation of the intracranial arteries in the giraffe (Giraffa camelopardalis).
Fluorescence histochemistry discloses that the carotid rete mirabile in the giraffe has a poor sympathetic innervation. In contrast, the efferent artery of the rete (internal carotid artery) and the cerebral arteries show moderate sympathetic innervation. A certain degree of regional variability was noted in which the rostral arteries (anterior and middle cerebral) receive more sympathetic nerves than the caudal (posterior communicating and basilar) arteries. The sympathetic nerves on the giraffe cerebral vessels may constitute part of a host of mechanisms by which regional blood flow to the brain is regulated. Conversely, the paucity of sympathetic innervation of the carotid rete mirabile may indicate that this structure does not play an active role in vasoconstrictor responses during postural changes of the head.
Published in Journal of morphology (1991)
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