Cardiorespiratory Fitness & Energy Systems

Cardiorespiratory fitness is composed of two parts 1) How efficient your heart and lungs are at delivering oxygen to your body and 2) how efficient your body is at creating the ATP, or energy, your muscles need in order to contract. As you participate in workouts that elevate your heart rate, aka cardio workouts, your heart will get stronger and be able to pump more of the life sustaining blood throughout your body. As you walk, row, bike, or run your body will also become more efficient at creating the ATP needed for your muscles to contract.

First, let's check out the cardiorespiratory system which is composed of two parts: the respiratory system (lungs) and the cardiovascular system (heart, blood vessels & capillaries.) The main function of the respiratory system is to allow us to breathe in oxygen and exhale carbon dioxide. The cardiovascular system delivers oxygen and nutrients to the cells of our bodies and helps with the removal of carbon dioxide from the cells. There has to be a "transfer point" where the oxygen and carbon dioxide can move from the cardiovascular system to the lungs. This transfer point is where the alveoli of your lungs and the capillaries of your cardiovascular system meet. Cardiorespiratory endurance represents how efficient these two systems are at supplying oxygen and nutrients to the muscles so muscular activity can be continued. To improve cardiorespiratory endurance, engage in walking, running, biking, swimming or rowing workouts.

The Respiratory System

Many people think that their two lungs are just big empty sacks like balloons on the far side of their wind pipe. Actually your windpipe, aka trachea (which connects your mouth to the lungs) branches to the left and right lobes of your lungs. All in all, it branches 23 times. Sixteen of these branchings are air passageways, think hallways. The last 7 branchings are respiratory branches which end in about 300 million alveoli - miniature sacks with gas exchange surfaces. You can think of your lungs as a giant upside down oak tree. The trunk of the tree is the trachea, the major branches are the air passageways and all the leaves are the alveoli. It's at the alveoli that the oxygen and carbon dioxide transfer from the respiratory system to the cardiovascular system. The walls of the alveoli and the walls of the capillaries are very thin, so the oxygen and carbon dioxide can "jump" from one system to the other. Oxygen comes into your trachea as you breathe air. It travels through the branches of your lungs to the alveoli sacks where the membranes are so thin that the oxygen molecules can cross on over to the capillaries. Once inside the capillaries the oxygen attaches to the hemoglobin of the red blood cells. The cardiovascular system delivers this oxygen along with the other nutrients throughout your body by circulating the blood throughout your body.

Your blood isn't going to circulate on it's own. That's where the big pump known as your heart comes into the picture. The job of your heart is to send the oxygen rich blood out to your body: your brain, muscles, organs, heart, etc. (It's interesting that with all that blood pumping through your heart, it still needs it's own supply of blood.) The oxygen rich blood travels through arteries which branch into arterioles which branch into capillaries with thin walls. At this point the oxygen & nutrients can be delivered to the cells of your body. The cells get rid of carbon dioxide (which ends up at the lungs) and other waste products (which end up at the kidneys.) On the return trip these capillaries turn into venules which turn into veins, which go back to the heart and lungs. Then the process starts all over again.

The cardiovascular system is under pressure. Your blood pressure is the force the blood exerts on the walls of your arteries. As you can imagine, high blood pressure is not a good thing. Your blood pressure is made up of two numbers: systolic and diastolic. Systolic, the first number in your blood pressure, is the pressure when the heart beats. Diastolic, the second number in your blood pressure, is the pressure when the heart rests. A systolic pressure less than 120 is normal. A diastolic pressure less than 80 is normal. Blood pressure does fluctuate. If your numbers stay above these normal readings most of the time, you are at risk and should consult a doctor.

Every time your heart contracts, it sends blood through the vascular system. Your heart rate, or pulse, is the result of your heart contracting. You can monitor your heart rate with a heart rate strap, by holding the contacts on some exercise equipment, or by taking your pulse at the carotid artery in your neck. Make sure and use your fingers and not your thumb when checking your heart rate. Stroke volume is the amount of blood pumped each time your heart contracts. Cardiac Output is the amount of blood pumped each minute. A person who is fit has a higher stroke volume than an unfit person.

Energy Systems

So far, we've covered how oxygen, carbon dioxide and nutrients travel around your body via the highway known as the cardiovascular system. To really understand cardiorespiratory fitness, two more issues need to be addressed: A) How do the muscles contract and B) why do we start breathing harder when we exercise.

In order for a muscle to contract, it must have access to ATP, adenosine triphosphate. When ATP is broken down it creates energy so the muscles can contract. There is a small amount of ATP stored in each muscle, but only enough for a couple seconds of movement. After those few seconds are up, your muscle needs more ATP. Now, the last time I read a food label, ATP wasn't listed. So the question becomes "How does the body get ATP?" The answer is your body has to create ATP from another source. Your body can use glycogen, fat, or protein to create ATP. There is glycogen and fat stored in your body. Glycogen is manufactured by the liver from amino acids and carbohydrates you eat. Glycogen is stored in your muscles and liver. Fat is created when you eat more than is needed to fill the glycogen stores.

There are three different energy systems at work in your body that can create ATP: A) the phosphagen system, B) the anaerobic system and C) the aerobic system. In a healthy individual glycogen and fat stores are the primary energy source for the creation of ATP.

As you exercise, these systems work together to provide ATP so your muscles can contract. In review: the anaerobic system is used for all out, high-intensity work. In general, the anaerobic system can provide up to 3 minutes of ATP. The aerobic system provides the ATP for long-term, low-intensity exercise. After you exercise for 3 minutes, the majority of the fuel is supplied by breaking down glycogen while a small portion of the fuel is supplied by breaking down fat. After an hour, the glycogen stores in your system become depleted and the majority of ATP is produced from the break down of fat. The amount of glycogen and fat used will be different from person to person. Most activities require ATP to be created by both the aerobic and anaerobic systems. For example, soccer is a long game (aerobic system) of many sprints (anaerobic system.) A volleyball match is three games (aerobic system) with jumps and fast movements (phosphagen system.)

The cardiorespiratory and energy systems are at work even when you are at rest. When you aren't exercising, your body still needs oxygen and ATP. That's because sitting up requires some of your muscles to contract. At rest 15 - 20% of your cardiac output is needed by your skeletal muscles. A healthy person's ambient heart rate is generally in the 70's, blood pressure is 128/80 and breathing is 12 - 20 breaths per minute. Your aerobic metabolic system supplies the ATP for your body by burning fat when you are at rest. Hmm... does that mean you can lose weight by just sitting around? The answer is no. That's because you don't need much energy to just sit around. So the quantity of fat converted to ATP is a very small amount.

When you exercise, you require considerably higher amounts of oxygen and ATP in order for your muscles to contract. When you exercise, your skeletal muscles require 85 - 90% of your cardiac output. Your body responds to this need by restricting blood flow to your stomach, etc and increasing blood flow to your muscles. This blood carries oxygen and nutrients. To increase the blood flow, your heart beats faster and pumps out more blood with each heart beat, resulting in increased cardiac output. This increase in output results in higher blood pressure when you work out. To provide more oxygen to your system, you start breathing faster. This increased respiration rate also allows your body to get rid of the extra carbon dioxide produced by aerobic metabolism.

As you continue your exercise program you become more fit and your cardiorespiratory system becomes more efficient. As you become more fit, your resting heart rate decreases. Because your heart is a muscle, it becomes stronger and the stroke volume increases. The muscles responsible for breathing become stronger, so you can breathe harder without tiring out. As you workout at both aerobic and anaerobic levels, these energy systems become more efficient at producing ATP. You can choose either weight bearing or non-weight bearing cardio workouts. An added benefit of weight bearing exercises is that this kind of exercise can also help prevent osteoporosis, a disease which results in the reduction of bone density, resulting in increased risk of fracture.

To improve your cardiorespiratory endurance you can workout by

Health Benefits of Improved Cardiorespiratory Endurance

Warm Ups

Warm up by doing a lower intensity of the same type of workout you are planning on doing. So if you are planning on a 30 minute walk, walk slowly for at least 3 minutes before you begin your workout. You are not warmed up until you perspire lightly and breathe more deeply. Make sure and warm up for at least 3 minutes. The older you are, the longer your warm up period should be.

Work Out

Before you workout, decide how long you will work out and at what intensity you will exercise. When working out, make sure you are "in the zone" by either using the Talk Test, the Perceived Rate of Exertion, or monitoring your heart rate. See the Handout "Understanding Heart Rates" for more information on training zones. When you are finished with your workout, record your cardio workout in your journal.

If you are new to exercise, begin with a 3-5 minute cardio workout session and work up to 12 minutes of continuous cardio exercise. The American Heart Association suggests that people just beginning an exercise program start out at 60-75% of Max Heart Rate, Max HR. Aim for 60% for the first few months and gradually increase it to 75%. According to the AHA exercising at more than 75% of Max HR may be too strenuous unless you are in excellent physical condition.

Recent studies have shown that interval training, short bursts of high-intensity workouts interspersed with more moderate-intensity workouts burns fat and improves cardio fitness quicker than constant moderately-intense workouts.

Terminology Associated with Cardiorespiratory Workouts

Cool Down, Recovery & Stretching

After a cardiorespiratory workout, it is important to cool down. A cool down will gradually lower your heart rate and relax muscles. Slowly reduce your workload until your heart rate is below 60% of your maximum HR. Cool down for at least 5 minutes. If your heart rate is more than 120 bpm 5 minutes after you have quit exercising, your exertion may been too strenuous for your current level of fitness. As with all exercises, stretch after you have worked out. Incorporate stretching into your cool down routine.

Several things must happen for your body to recover:

Most of the ATP stored in the muscles is replaced in the first 3 - 5 minutes of recovery. After exhausting exercise, 60% of the liver glycogen is restored in the first 10 hours and it is completely restored in 24 - 48 hours. You must have some carbohydrates in your diet to restore the glycogen in your liver.

In addition to restoring glycogen and ATP levels, lactic acid must be removed from your blood stream and muscles. Remember, it's the high levels of lactic acid that cause fatigue in the first place. If you rest and just sit around (ie a passive recovery) it'll take more than an hour to remove this lactic acid. If you complete an active warm-down period, ie exercise-recovery, you will remove the lactic acid. This lactic acid will be turned into glycogen which can be stored in your muscles and liver. So the active recovery gives you a 2-for-1 bonus. For untrained people, this active recovery should be at about 50-60 % HR max. For trained individuals, this active recovery period should be at a higher level.



alveoli of the lungs
Alveoli of the Lungs Heart and Lungs
This image has been released via Wikipedia into the public domain by its author, LadyofHats, Mariana Ruiz Villarreal. Gray's Anatomy
Picture in Public Domain


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