 Hey everybody, Dr. O. I want to talk about cardiac output. We'll talk about it here just in normal circumstances, then I want to talk about what happens during exercise as well. So first the equation, your cardiac output or how much blood you can move per minute is your heart rate, how many times your heart beats per minute times your stroke volume, which is how much blood you move per beat. So it's the amount of blood you move per beat times how many beats you have in a minute determines your cardiac output. So an average person, their stroke volume is going to be about 70 milliliters, their heart rate is going to be about 75. So that means for a healthy person, you're looking at a normal cardiac output somewhere between five and 5.25 liters of blood per minute. And as you see, lots of things can affect them both. So let's walk through them quickly, not all of them, but factors affecting heart rate, so autonomic innervation, so stress, sympathetic innervation is going to increase your heart rate, fitness level clearly is going to have an impact, hormones have an impact like if you have hyperthyroidism, excess thyroid hormones going to cause your heart rate to go up, hypothyroidism would cause your heart rate to go down. On the other side, the size of your heart and your fitness levels are going to impact stroke volume, gender will as well, usually man's heart, male's heart is a little bit larger, a little larger stroke volume, all these things are going to have an impact. So the two are separate, but in the end, your body's primary concern is maintaining cardiac output. So if the stroke volume were to go up, the heart rate would go down and vice versa. So look at like athletes, well trained athletes, runners, long distance athletes, often their heart doesn't beat 75 times a minute, it might beat 50 times a minute. I know someone whose heart rate normal, perfectly healthy is in the high 30s and that's because their heart is such a powerful efficient pump, it moves so much blood per beat that doesn't have to beat that many times per minute. So if you're like an endurance athlete, sitting at rest is pretty easy peasy for you, right? So if the stroke volume goes up, the heart rate can go down. Then on the flip side, you're going to see other issues, like if someone is losing blood, for example, they're bleeding or their heart is an inefficient pump, the response is going to be to increase the heart rate. So if you find someone and they have a really high heart rate and what's called the thready pulse, the concern is they don't have a lot of blood, which means maybe they're bleeding or really dehydrated. So what's happening is every time their heart beats, the stroke volume is pretty low. Their response is to beat faster and faster and faster. So the heart rate and stroke volume will often, one will go up as the other goes down and vice versa. But with exercise, let me show you some numbers here with exercise and blood flow. When exercising, both will go up. So you can see a huge increase in cardiac output. So a normal healthy person is going to move about five liters of blood per minute, same thing with an athlete. But then when a non-athlete is involved in physical activity, their stroke volume and heart rate can go up in a way where their maximum cardiac output can maybe go to 20 liters per minute. But a well-trained athlete, they can get their cardiac output as high as 30 liters per minute. So imagine why if you, you know, it's New Year's resolution time and you start jogging or something, imagine, you know, a well-trained athlete with a higher fitness level, they can move 10 more liters of blood per minute, which is why when you first start training, it's hard and it gets easier and it gets easier. But I wanted to show you here just that the amount of blood flow. So you look at maximal exercise, look at those totals. This is a healthy young individual, not a well-trained athlete. You're looking at moving 17,500 milliliters of blood per minute compared to 5,800 per minute at rest. But I like to look at where the changes occur. Hopefully you understand why you're exercising. Your muscles are going to need a lot more blood. Your heart's going to need a lot more blood because it's working a lot harder. Your skin's going to need a lot more blood because remember, your skin is going to function as that radiator to help get rid of excess heat. Those are going to be the three areas where you see a huge increase in blood flow. Notice that the blood flow of the brain here doesn't change at all. The brain just takes what it needs. It might change where the blood is flowing to in the brain, but the overall blood flow doesn't change. The brain is pretty particular about what it gets. It won't share its blood flow or its energy sources. But then notice, we only have the same amount of blood. So if we're moving more blood to the muscles in the heart and the skin, then look, we're moving less blood to the kidneys. I think my dog's having a bad dream. Less blood to the kidneys, less blood to the GI tract, et cetera. That's why when you're exercising, all your blood or all the available blood is being shunted to your musculoskeletal system. All right. So that is cardiac output, stroke volume times heart rate, and then some of the things that can influence it. I hope this helps. Have a wonderful day. Be blessed.