Final answer:
The statement about vertebrates regulating at about 300 mOsm is true. Vertebrates have developed osmoregulatory mechanisms, including the action of kidneys and osmoreceptors, to maintain internal water and salt balance.
Step-by-step explanation:
The statement 'All vertebrates regulate at about 300 mOsm' is true, as this is the average osmolarity level that vertebrates, including mammals, maintain. Vertebrates have developed various osmoregulatory mechanisms for maintaining water and salt balance within their bodies. The kidneys are crucial organs for this process as they filter the blood and adjust the water and electrolyte contents to maintain a constant internal environment, or homeostasis. Osmoreceptors in the brain play a vital role in monitoring blood solute concentration and signaling hormonal responses to modify kidney function, thereby controlling water retention or loss. Moreover, albumin in the blood, due to its size, significantly affects osmotic pressure, which is another important aspect of osmoregulation. Some animals adapt their osmoregulation to different environments; for example, freshwater fish excrete dilute urine and actively transport salts through their gills to achieve electrolyte balance, while marine fish ingest seawater and excrete excess salts to accomplish this balance.