*Blood flow is regulated in all three types of blood vessels through constriction and dilation.
*Contraction of blood vessels slows blood flow, while relaxation of blood vessels, or dilation, allows free flow of blood.
*Muscle fibers control the constriction and dilation of vessels. In the capillaries, rings of muscle called capillary sphincters control blood flow.
*The directional flow of blood in the veins is controlled with valves , which prevent backward blood flow when closed, and allow blood to flow only toward the heart when open.
Vasoconstriction and vasodilatation allow blood vessels (arteries and veins) to control blood flow and thus control the blood pressure. This is possible because of the smooth muscle cells that surround the vessels. The contraction and dilatation of those muscles is controlled by chemical signals (e.g. norepinephrine constricts the vessels, while NO dilates them).
Option A, The pressures of a hatchery environment and of the wild environment select for different traits.
The various pressures in wild environment include the competition for food and survival, threat of prey etc. The effect of domestication on salmon can also be a reason for its low productivity in wild environment. Some other factors that might have affected the regular productivity of salmon in wild environment include- captive rearing, inbreeding among close relatives and ability of some fish to adapt to the unique hatchery environment.
populations were prevented from interbreeding by geographic isolation
