Your heart beats and your blood circulates with little or no conscious awareness on your part. Even though circulation is an involuntary function, it is a dynamic one. Your cardiovascular system is constantly adjusting to changes in the external environment or to demands you place on it. It adapts quickly, or directs other systems to adapt to chang- ing conditions in order to maintain a constant ?ow of blood to body tis- sues. Even the simple act of standing up requires increased blood ?ow to the legs, because the heart must work harder to counteract the effects of gravity. This means that either blood ?ow to other parts of the body must be decreased or the heart must pump blood faster or in greater volume to accommodate the activity.
The two main systems that help regulate cardiac function are ?rst, the brain and the nervous system, and second, the kidneys.
The Brain and the Nervous System
Nervous system receptors throughout your body constantly gather information about factors such as stretching of the arterial walls or the amount of oxygen in the blood. This information is relayed to the brain by chemicals called neurotransmitters. In the brain stem, at the base of the brain, regulatory centers involved with automatic body functions including heart rate, blood pressure, and respiration receive the mes- sages and formulate a response. Neurotransmitters such as adrenaline carry messages back that direct a response in the target tissue, such as commands to constrict the blood vessels or increase the rate of respira- tion to deliver more oxygen to your lungs.
The Kidneys
The kidneys in?uence the volume of ?uids in the body, so they can change the volume of circulating blood. In this way, they signi?cantly affect blood pressure. They release enzymes that can raise blood pres- sure by constricting blood vessels, raising sodium levels, and increasing water retention. The kidneys can adapt to changing environmental conditions by, for instance, concentrating your urine if your body is dehydrated. If, on the other hand, you eat a lot of salty foods and start to retain water, your kidneys will produce less urine.