You wouldn't run a marathon or swim laps on a full stomach. Aside from the discomfort you would feel, you would put undue strain on your cardiovascular system. Although you know this intuitively, it helps to know the physiology of the human body that explains the role of circulation in digestion.
Your stomach houses millions of tiny blood vessels, called arterioles. Like all the organs in your body, it needs oxygen to function. The circulatory system, controlled by your heart, receives oxygen from your lungs that is carried throughout your body via your blood. Throughout the day, your various organs demand oxygenated blood to ensure their peak performance. Your heart receives these messages and adapts to serve the needs of the organs.
As you chew and swallow your food and drink, you fill up your stomach, which breaks down the food into digestible matter. In order for this to occur, oxygenated blood must flood the stomach's vessels. The influx of oxygen enables the stomach to release digestive acids, which degrade the food into smaller molecules. This prepares the food for the small intestines, which absorb the nutrients and send the waste on to the large intestine for evacuation.
The human body's circulatory system functions by three basic principles. First, blood flow increases to any tissue or organ according to its needs. If you are between meals, your gastrointestinal, or GI, system requires less blood than after a meal, so the heart sends less. The second principle says that the total of all of the body's blood/oxygen needs at any given time controls the amount of cardiac output. Lastly, the pressure in the arteries, which carry the oxygen-rich blood to the tissues, operates independently from those that supply blood locally.
Effects of Eating and Drinking
Because your heart puts out what your organs need at any given time, it sends more blood to your GI system when you eat and drink. Your body has an intricate communication system that not only signals when your stomach needs more blood, but also supplies feedback to your heart when it meets the tissue's need. Blood pressure is a major player in this system and increases as needed. Your heart can supply blood, but it takes the work of the arterial walls to get the blood to your stomach. Your arteries accomplish this by raising the pressure to increase the rate of blood flow. Your elevated blood pressure returns to normal when the digestive process is complete.
Just as your stomach needs more blood after you eat and drink, the muscles in your legs require extra blood when you walk, run, bike or swim. Eating puts demands on your heart and raises your blood pressure in the same way exercise does. Because the heart tries to supply all the blood you need where and when you need it, it pumps harder and raises your blood pressure for each action. For this reason, it is advisable to do one thing at a time. Avoid strenuous exercise directly after eating. Two hours after a meal is the best time to exercise. By then your stomach will have emptied and your heart can redirect the blood to your muscles without excessive demand.