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
Bronchial veins Bronchial veins The bronchial veins are small vessels that return blood from the larger bronchi and structures at the roots of the lungs. The right side drains into the azygos vein, while the left side drains into the left superior intercostal vein or the accessory hemiazygos vein. Bronchial veins are thereby part of the bronchial circulation, carrying waste products away from the cells that constitute the lungs. The bronchial veins are counterparts to the bronchial arteries. However, they only carry ~13% of the blood flow of the bronchial arteries. The remaining blood is returned to the heart via the pulmonary