Week 8: Exercise Physiology
Observe this point on the graph in Figure 3. At a PO2 of 100 mmHg, the hemoglobin saturation is approximately 100%.
At the tissues at rest, a typical PO2 is about 40 mmHg. Hemoglobin saturation is approximately 75%. This means that 25% of the hemoglobin subunits released an oxygen molecule to be used by the tissues. At skeletal muscle during a time of exercise, the oxygen concentration in the blood is lower - 20 mmHg. At this point, hemoglobin saturation dramatically drops to 25%. Think of it this way: at this range, oxygen is more readily released from hemoglobin to surrounding tissue. The fact that hemoglobin can "store" extra oxygen between 100mmHg PO2 and 40 mmHG PO2 and readily release it during times of need (like exercise, down to 20mmHg) helps to buffer oxygen changes. Tissues will always receive the amount of oxygen they need with this system. To summarize, hemoglobin stores oxygen during transport around the body. In times of rest, it parts with some of the oxygen for the tissues. In times of exercise, hemoglobin gives up much more oxygen to tissues because of hemoglobin's ability to fine-tune how much oxygen it binds. This mechanism allows for the same amount of O2 to be carried in blood from the lungs to the tissues, but a greater amount of O2 to be released from hemoglobin and delivered to the muscles to meet demands.
Temperature and Gas Exhalation Responses due to Metabolism
Cell harness energy from oxygen and the building blocks of our food in a process called cellular respiration. The process generates carbon dioxide (CO2) and heat. During exercise, there is increased demand for energy and metabolic reactions ramp up to fuel muscle cells to continue functioning.
Background
Changes in heart rate, breathing rate, blood pressure, and the capillary beds will all occur to help the body adapt and compensate for the stress created by strenuous activity.
Q - Observe Figure 4 below. When the curve shifts to the right, the affinity of hemoglobin to oxygen generally...
Decreases
Changes during Exercise
During exercise, CO2 increases, blood pH decreases, and body temperature increases. In all of these cases, the hemoglobin saturation curve shifts to the right.
Metabolic Changes
During exercise, metabolism increases to meet demand. As a result, more CO2 is being produced and exhaled. The concentration of CO2 in exhaled air is expressed as a percentage (you will see it be referred to as "ETCO2" in the lab). Typically, about 0.5-2% of the air we exhale is CO2. During exercise, exhaled air can consist of up of 3-5% CO2. Due to the increases in metabolism, more of the oxygen that is inhaled will be used. Therefore, less oxygen will be exhaled. The percent or concentration of O2 in exhaled air (or ETO2) decreases from about 20% to no lower than 15%. To put new words to these concepts, CO2 CLEARANCE from the body increases and O2 CONSUMPTION by the body increases during exercise. CO2 clearance and O2 consumption are expressed in liters per minute and describes the amount of these gases that are being dumped or kept by the body, respectively. Neither of these can be measured directly - they are calculated based on the percentage of the gases in the exhaled air, described above. You will learn how to calculate them in lab. CO2 clearance hovers around 0.03 L (or 30 mL) to 0.05 L per minute at rest. Considering the resting tidal volume is 0.5 L, carbon dioxide does not make up a huge percentage of exhaled air. During exercise, CO2 clearance can peak at 0.3 L per minute - ten times more than at resting state. O2 consumption ranges from 0.02 to 0.05 L per minute at rest. But it increases to around 0.3 L per minute during exercise.
Q - The affinity (willingness to bind) of hemoglobin to oxygen is the same at the capillaries of the lungs and of the tissues of the body.
False
Cardiac Output
In response to proprioceptors and mechanoreceptors in the limbs, the cardiovascular control center in the medulla stimulates the sympathetic nerves extending to the SA node of the heart. The sympathetic input increases cardiac output. Cardiac output can increase from 5 L/min to 20 L/min in a healthy participant and up to 40 L/min in a trained athlete. It is the decreased parasympathetic activity that allows the heart to start to increase from 70 to 100 beats per minute in a healthy person. Increases in heart rate over 100 beats per minute can be attributed to the sympathetic nerves communicating with the heart.
Q - This makes intuitive sense, because then hemoglobin is _______ prone to unload oxygen to the muscles and tissues that need it.
More
Temperature Changes
Much of the energy created is converted to the waste product heat. Body temperature can increase to 40-42˚C compared to rest which is normally 37˚C. The body will sweat to increase EVAPORATIVE COOLING. Vasodilation of the cutaneous (skin) blood vessels will increase CONVECTIVE COOLING
Exercise hyperpnea
Occurs when both breathing rate and breathing depth increase when compared to rest. Movement of joints and limbs stimulates specialized neurons called proprioceptors.
How does a protein (hemoglobin) "know" when to pick up oxygen in the lungs and when to drop it off in the tissues? Why does the hemoglobin sometimes bind to oxygen and why is it sometimes willing to give oxygen up to the surrounding tissues?
Proteins can change their 3D shapes under different conditions. This allows them to change their behavior and binding abilities in different environments. Hemoglobin does this! Once an oxygen molecule binds to one of the four subunits, the hemoglobin molecule slightly changes its 3D shape. Now, hemoglobin binds much better to the second oxygen molecule. Once that oxygen binds, hemoglobin changes again, and so on. In other words, once hemoglobin binds to oxygen, it becomes greedy! It wants more oxygen!
Blood Pressure
Recall that blood pressure is influenced by cardiac output as well as vasodilation and constriction. Above, we noted that cardiac output increases during exercise. This would serve to increase blood pressure. However, the drop in arterial blood pressure due to vasodilation in skeletal muscle arterioles cancels out some of the expected increase in blood pressure. The baroreceptors do not function as they do in rest and allow blood pressure changes to compensate for the skeletal muscle demands during exercise. Therefore, blood pressure only rises slightly during exercise. Mean arterial pressure (MAP) will be approximately 90 mmHg (a millimeter of mercury is a unit of pressure). In times of exercise it will generally only slightly increase to about 100-110 mmHg.
This increase in breathing due to limb movement often precedes the actual increased demand of exercising muscles.
That way the extra oxygen will be available for tissues when they do need it. Peripheral chemoreceptors monitoring blood O2, CO2, and pH concentrations also communicate to the respiratory control center to change rate and depth of breathing to maintain homeostasis.
Why is this important for hemoglobin's function? When there is a lot of oxygen available in the blood, hemoglobin will suck it all up for transport.
The partial pressure of a gas indicates how much of that gas in dissolved in blood. You can think of it like the concentration of gas in the blood. As previously mentioned, a unit of pressure is millimeters of mercury (mmHg). In this hemoglobin discussion, we are interested in the partial pressure of oxygen, or PO2. In the capillaries of the lungs, the PO2 is very high. This is because the oxygen is rapidly diffusing across the respiratory membranes in the alveoli. In this case, there are plenty of oxygen molecules floating around in the blood and all of the heme groups on the red blood cells in the area will bind oxygen. Hemoglobin saturation, or the percentage of hemoglobin subunits bound to oxygen, is nearly 100%.
Hemoglobin Dissociation Curves
Therefore, oxygen binding to a population of hemoglobin molecules is not a linear relationship. A straight line would indicate that hemoglobin does not change its affinity for oxygen once the one oxygen binds. In reality, if we plot how oxygen binds to hemoglobin, it is an S-shaped curve. To the left, there is a steep incline. Once one oxygen ion binds, the next binds even better! But to the right of the curve, almost all hemoglobin subunits are bound to oxygen and the line plateaus.
Vasoconstriction and Vasodilation
Vasodilation and vasoconstriction in different blood vessels of the body help direct more blood flow, and therefore oxygen, to the skeletal muscle during exercise. The sympathetic input also promotes vasoconstriction in peripheral arterioles. The constriction will raise blood pressure and also help direct blood flow to exercising muscle and divert blood flow from the digestive and urinary systems. In skeletal muscles, vasodilation occurs. Due to vasodilation, skeletal muscles will experience increased blood flow. At rest muscles receive 21% of the cardiac output, but during exercise 88% of cardiac output is delivered.
Blood Returning to the Heart
Veins have lower pressure than arteries and do not contain smooth muscle. There are two different mechanisms that help venous blood make its way back to the heart. Venous return is increased by the increased muscle contractions in the skeletal muscle pumps and respiratory muscle pumps. For SKELETAL MUSCLE PUMPS, your leg muscles contract, causing venous blood to be pushed towards your heart. For RESPIRATORY MUSCLE PUMPS, muscles of the chest and the diaphragm case changes in pressure in the chest. This in turn promotes blood flow to the heart. During exercise, like running, the pumps work much faster. The heart is already compensating for this increased venous return of blood by increasing heart rate. Thus, the body is able to move more blood faster during exercise.
Q - The greatest decrease in hemoglobin saturation occurs...
between tissues at rest and skeletal muscle during exercise
Hemoglobin (Hb)
is a protein on red blood cells that functions to bind and carry oxygen and carbon dioxide in the blood. There is a limit to how much gas can dissolve in a liquid, and hemoglobin is the solution for carrying concentrations needed for life.
Proprioceptors
sensing movement, send information from the limbs to the CNS and communicate to the respiratory control center in the medulla to increase breathing rate and depth. At rest, a typical person breathes 12 breaths per minute. You also learned last week that tidal volume (TV) is typically 500 mL (0.5 L) per breath. During exercise, breathing rate and depth can increase to as much as 45 breaths per minute and one liter per breath.
