Final answer:
Renin is the hormone that regulates blood pressure by causing the retention of sodium and water, which increases blood volume.
Step-by-step explanation:
Renin is part of the renin-angiotensin-aldosterone system, which increases blood pressure by causing the retention of sodium (Na+) and water, subsequently raising blood volume. The kidneys release renin in response to reduced blood volume or blood pressure. While other hormones like erythropoietin (EPO) and atrial natriuretic hormone (ANH) influence blood volume and red blood cell production, it is renin specifically that has a direct role in regulating blood pressure.