CASE 7 PREDATOR–PREY: A GAME OF LIFE AND DEATH

An animal that is dehydrated or has lost blood following an injury may experience a drop in blood pressure. In such cases, the posterior pituitary gland releases antidiuretic hormone (ADH), also known as vasopressin (Chapter 38), into the circulation. ADH causes the arteries to constrict, increasing their resistance to blood flow. The higher resistance elevates blood pressure throughout the body.

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Even under less stressful conditions, blood pressure is continually being adjusted to remain within normal physiological limits, another example of homeostasis. The autonomic nervous system plays an important role in keeping blood pressure at healthy levels (Chapter 35). If blood pressure in the general circulation drops, animals reduce the supply of blood to the limbs by constricting arterioles that supply the limbs. This response helps maintain blood pressure to the heart, brain, and kidneys. In this case, sympathetic neurons synapsing on the smooth muscles of arterioles in the limbs stimulate these muscles to contract. In contrast, when blood pressure is too high, sympathetic neurons that synapse on the smooth muscles are inhibited. The smooth muscles relax, reducing resistance in the arterioles and increasing blood flow. These two responses are called vasoconstriction and vasodilation, respectively.

In some circumstances, it may be advantageous to an animal to have blood pressure rise higher than normal levels. One example is exhibited during the fight-or-flight response of animals faced with a stressful situation, as when prey are alerted to the presence of predators, and when predators locate a potential prey. The sympathetic nervous system stimulates the adrenal gland to release the hormone adrenaline (also called epinephrine). The surge of adrenaline into the bloodstream—the “adrenaline rush”—causes an animal to become alert and aroused. At the same time, the sympathetic nervous system acts to increase heart rate and respiratory rate and inhibit digestive processes. Arterioles supplying limb muscles dilate to enhance blood flow to exercising muscles, while arterioles supplying organs within the digestive system constrict. The increase in heart and breathing rates increases the rate of O₂ delivery to the exercising muscles to meet their increased demand for ATP. These physiological changes, along with a heightening of sensory acuity, prepare an animal for the more intensive activity needed to chase or flee.

In contrast, when an animal has recently eaten a meal, its sympathetic nervous system is inhibited and its parasympathetic nervous system is stimulated. In response, digestive secretions increase and smooth muscle in the gut becomes more active (Chapter 40). Blood flow to the digestive system increases as well. In this way, the regulation of blood flow to different organ systems is coordinated with more general feedback from the nervous and endocrine systems, providing regulation of blood pressure linked to the physiological state of the animal. These shifts in blood flow are readily controlled by vasoconstriction or dilation of the arterioles supplying the different organs.