Giraffe have unusually high heart rates for their size, at 150 beats per minute. When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint. The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered. Conversely, the blood vessels
Drug delivery to the brain drug that helps it pass through the barrier and then once inside the brain, is degraded in such a way that the drug cannot pass back through the brain. Once the drug cannot pass back through the barrier the drug can be concentrated and made effective for therapeutic use. However drawbacks to this exist as well. Once the drug is in the brain there is a point where it needs to be degraded to prevent overdose to the brain tissue. Also if the drug cannot pass back through the blood–brain barrier, it compounds the issues of dosage and intense monitoring