Sometimes red juices appear on the surface of meats as they cook, or they ooze out into the skillet or pan. For novice cooks or squeamish eaters, this can be disconcerting, because the appearance of blood isn't always appetizing. In truth the red liquid is seldom blood, and its appearance is perfectly normal when meats are cooked.
Heat and Protein
The reason liquids escape from your meat as it cooks has to do with the behavior of proteins when heated. When they're warmed gently, the proteins in your meat begin to become firm, just as they do in an egg as it's boiled. As the heat continues to rise it strengthens the bonds within the protein molecules and that, in turn, causes them to contract. This is why hamburger patties usually shrink on the grill, and why a well-done steak is smaller than an identical steak cooked to medium-rare. The muscle tissues in the meat contain a relatively high percentage of liquids, and when the cells contract and shrink, a portion of those liquids will be forced out.
It's Usually Not Blood
It can be disconcerting to see what appears to be blood coming out of your food, but that usually isn't what you see on your plate or in the pan. Usually it's water containing myoglobin and cytochromes, two substances that transfer the oxygen needed to fuel the muscles as they do their work. Both contain iron, which means they turn red in the presence of oxygen. As the meat progresses from rare to well-done, they undergo a chemical change, losing their rosy color. This is why, in fully cooked meats, the juices become clear. Until that point, juices escaping from the meat will look red or pink in your cooking pan.
But Sometimes It Is
Occasionally, you will see genuine blood ooze from your meats as they're cooking. You'll sometimes see a spot of dark red blossoming in a piece of beef or pork as it grills, or watch a few drops seep from the bone of a chicken leg as it fries. The blood is contained inside small capillaries, veins and other blood vessels that sometimes aren't trimmed from the cut meats. In chicken legs, for example, it's usually from the femoral artery that runs through the thigh. As long as you cook the meat to its recommended temperature, this is nothing to be worried about.
Knowing the Difference
Once you understand there is a difference between the two types of fluids, it's easy to recognize. The red juices pooling at the top of your hamburger are always myoglobin, from within the muscle tissues of the ground beef. Real blood is always a deeper, more vivid red, and it will come from a specific spot rather than from the cut as a whole. Pink cooking juices are fine in beef or even pork, but must be avoided in chicken. If the juices run clear from a bird's leg, but contain a small pool of bright red blood, the chicken is still probably safe. If you're uncertain, use an instant-read thermometer to check the meat has reached the recommended safe temperature.