The length of a vessel is directly proportional to its resistance: the longer the veessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood.
The kidneys secrete an enzyme called renin
The pair of characteristics that one must use to classify a fish is the presence of jaw and skeleton type. Agnatha fishes are also called jawless fishes and they lack bony skeletons. Cartilaginous fishes are composed of cartilage skeletons. Bony fishes have bony skeleton structures.
