EXAM 3 SHORT ANSWER

Compare and contrast central and peripheral fatigue. Pay special attention to the a) sites of fatigue, b) molecules, ions, or substrates involved in each, and c) the prevalence of each in everyday life. Also, provide an example each.

Central fatigue occurs when the central nervous system decreases the signals being sent to the muscles, limiting their ability to produce force. This can be caused by changes in neurotransmitters like serotonin and dopamine, low blood glucose, increased cytokines, or sensory feedback from pain and temperature. Central fatigue plays a major role during long-duration exercise and may be responsible for much of the fatigue we experience in everyday life. An example would be feeling exhausted during a long run and slowing down even though your muscles still have some ability to produce force.

Peripheral fatigue occurs within the motor neuron or muscle itself. It can result from disrupted action potential propagation, decreased acetylcholine release at the neuromuscular junction, or problems with excitation-contraction coupling. Hydrogen ions, calcium handling problems, phosphocreatine depletion, and glycogen depletion can all contribute. An example would be losing muscle force during repeated all-out sprints because phosphocreatine becomes depleted.

During exercise stroke volume will increase as exercise intensity increases up to approximately 40-60% of maximal work rate. Explain how/why stroke volume increases at low to moderate exercise intensities, and how/why it will eventually plateau as intensity increases above 40-60%

During low to moderate exercise, stroke volume increases because more blood returns to the heart through the skeletal muscle pump and thoracic pump. This increases preload, stretching the ventricles and causing stronger contractions through the Frank-Starling mechanism.

Sympathetic stimulation also increases contractility, and vasodilation in working muscles reduces resistance to blood flow. However, once exercise intensity reaches around 40–60% of maximum, stroke volume begins to plateau. This happens because HR becomes so high that there is less time for the ventricles to fill, and contractility approaches its maximum. After this point, increases in cardiac output mainly come from increases in heart rate

Describe 2 things that happen during exercise that help to increase venous return to the heart

The skeletal muscle pump works when contracting muscles squeeze nearby veins and push blood back toward the heart. One-way valves in the veins prevent blood from flowing backward.

The thoracic pump uses pressure changes in the chest during breathing to help move blood toward the heart.

Together, these mechanisms increase venous return, preload, and stroke volume

Blood is composed primarily of what 2 things? What is the main function of blood and what molecule(s) allows it to perform this function?

Blood is primarily composed of plasma and formed elements, which include red blood cells, white blood cells, and platelets. Plasma makes up about 55–60% of blood volume, while formed elements account for most of the remaining volume.

One of blood's main functions is transporting oxygen, nutrients, and waste products throughout the body.

RBCs contain hemoglobin, which allows oxygen to be transported from the lungs to the tissues. Each hemoglobin molecule can bind up to four oxygen molecules. Blood also helps regulate body temperature and maintain proper pH.

List the 4 variables discussed in class that work together to determine/regulate stroke volume and explain the factors that affect each of those variables

1. Preload is the amount of stretch in the ventricle before contraction. It is affected by venous return, blood volume, the muscle and thoracic pumps, posture, heart size, and venous tone. Increased preload generally increases stroke volume through the Frank-Starling mechanism.

2. Afterload is the resistance the heart must overcome to eject blood. Increased blood pressure, hypertension, or the Valsalva maneuver can increase afterload and make it harder for the heart to eject blood.

3. Contractility is the strength of ventricular contraction at a given preload and heart rate. Sympathetic stimulation increases contractility, while hypoxia, acidosis, and tissue damage can decrease it.

4. Heart rate affects how much time the ventricles have to fill between beats. When heart rate becomes very high, filling time decreases, which can reduce preload and limit stroke volume

Outline the changes that occur in blood distribution when we go from rest to exercise. Discuss the component(s)/structures(s) in the vascular system that make the changes in blood distribution possible. Discuss the input/mechanism(s) that signal for the changes in distribution to occur.

At rest, a large portion of blood flow goes to organs such as the liver and kidneys, while skeletal muscles receive about 15% of cardiac output.

During exercise, blood flow is redistributed so active skeletal muscles can receive 80% or more of cardiac output during intense exercise.

Blood flow to the heart and skin also increases, while blood flow to organs like the kidneys and digestive system decreases.

This redistribution is mainly controlled by the arterioles, which contain smooth muscle that allows them to dilate or constrict. Local factors in working muscles, such as increased H⁺, K⁺, metabolic activity, and nitric oxide, cause vasodilation. At the same time, sympathetic nervous system activity causes vasoconstriction in less active tissues, helping redirect blood to the areas that need it most.

Graph the expected response of heart rate, stroke volume, and A-V O2 difference during a maximal exercise test. Explain the factor(s) that contribute the change in each variable. Which variable is the most important in determining VO2 max?

During a maximal exercise test, heart rate increases in a fairly linear pattern as exercise intensity increases. This occurs because parasympathetic activity decreases and sympathetic stimulation increases.

Stroke volume increases during lower intensities because of increased venous return, preload, and contractility, but it usually plateaus around 40–60% of maximal intensity due to reduced ventricular filling time.

The a-vO₂ difference continues increasing throughout exercise because working muscles extract more oxygen from the blood. Although heart rate increases greatly during exercise, maximal stroke volume is especially important for determining VO₂ max, since it strongly influences maximal cardiac output.

The Fick equation shows that both cardiac output and a-vO₂ difference determine oxygen consumption

You are riding a specially designed cycle ergometer whereby you are suspended up-side-down from the ceiling by a harness. By changing your posture in this manner gravity aids venous return and thus increases preload. However, when stroke volume is measured, you find that it is unchanged. Propose a mechanism for this observation

Even though being upside-down allows gravity to increase venous return and preload, stroke volume may not increase if another factor is limiting it. According to the Frank-Starling mechanism, increased preload normally causes a stronger ventricular contraction. However, if the ventricle is already near the upper, flatter portion of its Frank-Starling relationship, additional filling may not produce much more force or stroke volume. Stroke volume could also remain unchanged if increased afterload or decreased contractility offsets the increase in preload.

Why does VO2 max need to be normalized to body weight?

VO₂ max is often normalized to body weight so that aerobic capacity can be compared more fairly between people of different sizes.

Absolute VO₂ max is measured in liters of oxygen per minute, but larger individuals may consume more oxygen simply because they have more body mass. Dividing VO₂ max by body weight gives relative VO₂ max, which is typically expressed in mL/kg/min. This makes it easier to compare aerobic capacity across individuals, although body-mass normalization does not eliminate every difference related to body size and composition.

Outline where insulin, glucagon, T3/T4, Epi/Norepi, Cortisol, and growth hormone come from, how they change during exercise and why their levels need to change in order to facilitate exercise.

Overall, these changes help maintain blood glucose, increase the availability of fat as fuel, and support cardiovascular function during exercise. Insulin generally decreases, while glucagon, catecholamines, cortisol, and growth hormone increase during prolonged exercise.