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Questions and Answers
What is the contraction phase of the cardiac cycle called?
During exercise, what happens to the stroke volume of an untrained person past 40% of VO2 max?
What average stroke volume is expected for a male at rest?
What role does preload play in stroke volume?
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What happens to diastole during intense exercise?
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What is the primary factor opposing stroke volume during heart contraction?
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How does exercise affect the contractility of the heart?
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What is the average stroke volume for females during exercise?
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What is the primary effect of mitochondrial biogenesis on oxygen extraction?
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Which process describes the formation of new capillaries from existing ones?
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What type of angiogenesis involves a bud growing from the side of a capillary?
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What primarily stimulates angiogenesis during exercise?
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How does increasing capillary numbers benefit oxygen diffusion during exercise?
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What is a primary reason resistance training does not increase angiogenesis?
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How does resistance training affect mitochondrial biogenesis?
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What role do vascular endothelial growth factors (VEGF) play in angiogenesis?
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What initiates the process of muscle contraction?
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What occurs during the sliding filament theory?
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How is muscle relaxation related to myosin binding?
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Where is calcium stored in skeletal muscle cells?
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What is the optimal sarcomere length for generating force?
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Which type of skeletal muscle fiber is classified as fast?
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What is the role of tropomyosin in muscle contraction?
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What percentage of total body weight is made up of skeletal muscle?
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What characteristic distinguishes type I muscle fibers from type II muscle fibers?
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Which of the following describes the primary energy source for type IIx muscle fibers?
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What is the role of myoglobin in muscle fibers?
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Which muscle fiber type has the highest force generation?
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What effect does resistance exercise have on muscle fiber types?
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Which muscle fiber type would be best suited for long-distance running?
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What type of muscle fiber would a powerlifter primarily rely on?
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Which of the following characteristics is typically associated with type IIa muscle fibers?
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What occurs to systolic blood pressure during exercise?
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Which physiological mechanism is primarily responsible for blood pressure regulation during rest?
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Why is calculating mean blood pressure simply by averaging systolic and diastolic measures inappropriate?
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How do baroreceptors respond when blood pressure is elevated?
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During exercise, how does the body react differently between arm and leg activities?
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What happens to diastolic blood pressure during exercise?
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What components are considered when blood pressure increases when it is too low?
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Where are baroreceptors located in the human body?
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Study Notes
Heart Rate and Stroke Volume
- Heart rate starts increasing above 100 bpm, driven by the cardiac accelerator nerve
- Cardiac accelerator nerve interacts with the SA node, engaging sympathetic and cardiac accelerator pathways.
- Stroke volume is the amount of blood ejected from the heart in one contraction.
- Stroke volume increases when transitioning from rest to exercise.
- Stroke volume peaks and plateaus at 40% of VO2 max in untrained individuals.
- Beyond 40% VO2 max, heart rate becomes the primary driver of increased cardiac output, not stroke volume.
- The time for systole (contraction) and diastole (relaxation) decreases during exercise, especially diastole, reducing the time for ventricular filling.
- Average stroke volume at rest:
- Male: 70 ml per beat
- Female: 60 ml per beat
- Average stroke volume during exercise:
- Male: 110 ml per beat
- Female: 90 ml per beat
- Variables that increase stroke volume during exercise:
- Preload: Amount of blood in the ventricle before contraction.
- Contractility: Strength of ventricular contraction.
- Afterload: Aortic blood pressure, the pressure the heart must overcome to eject blood.
Preload, Contractility, and Afterload
- Preload is increased by:
- Mitochondrial biogenesis (more mitochondria for oxygen utilization)
- Angiogenesis (formation of new capillaries).
- Increased preload leads to more blood filling the ventricle, resulting in greater stroke volume.
- Contractility is enhanced by increased force of myosin and actin interaction, leading to greater ejection volume despite the same initial blood volume.
- Afterload is the pressure in the systemic circulation that the heart must overcome.
- High blood pressure acts as an opposing force to stroke volume.
Angiogenesis
- Angiogenesis is the formation of new capillaries from existing ones.
- Two types of angiogenesis:
- Capillary intussusception: Splitting existing capillaries into two by forming a wall in between.
- Sprouting angiogenesis: New capillaries budding from the side of existing capillaries.
- Angiogenesis is stimulated by mechanical forces on capillaries during exercise.
- Shear forces: Friction of red blood cells against capillary walls.
- Compression: Contraction of skeletal muscle fibers compressing blood vessels.
- Stretch: Increased blood pressure stretching the vessels.
- VEGF (vascular endothelial growth factor) is released during muscle contraction and acts as a signaling molecule for angiogenesis.
Capillary Density and Oxygen Delivery
- Increased capillary numbers during exercise increase oxygen delivery to contracting skeletal muscle by:
- Opening more capillaries and increasing transit time for red blood cells.
- Reducing the diffusion distance between red blood cells and mitochondria.
- Enhanced oxygen delivery improves ATP production, allowing for longer exercise duration.
Resistance Training and Angiogenesis/Mitochondrial Biogenesis
- Resistance training does not significantly increase angiogenesis.
- Skeletal hypertrophy (muscle growth) can push capillaries out of the muscle area, even preventing an increase in capillary density.
- Resistance training leads to increased density and expansion of the mitochondrial network.
Skeletal Muscle Contraction: Sliding Filament Theory
- Actin and myosin filaments slide past each other to shorten muscle fibers, according to the sliding filament theory.
- The distance between Z discs decreases.
- The length of the A band (containing myosin) remains constant.
- Actin filaments slide inwards, narrowing the H and I bands.
Fiber Types
- Humans have three main fiber types:
- Slow fibers (Type I): High capillary density, high myoglobin, high mitochondria, high fatigue resistance, aerobic metabolism, low contraction rate, moderate force.
- Fast fibers (Type IIa): Intermediate in all characteristics, both anaerobic and aerobic metabolism.
- Fast fibers (Type IIx): Low capillary density, low myoglobin, low mitochondria, low fatigue resistance, anaerobic metabolism, high contraction rate and force.
Muscle Fiber Contraction: Steps
- Signal from the brain travels down the spinal cord, triggering an action potential in the alpha motor neuron.
- Action potential reaches the axon terminal and releases acetylcholine (ACh).
- ACh binds to nicotinic receptors on the muscle fiber, causing depolarization at the endplate.
- Depolarization travels through the sarcolemma and T-tubules.
- T-tubule depolarization triggers calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, revealing myosin-binding sites on actin.
- Myosin heads bind to actin, releasing ADP+Pi and initiating the power stroke.
- ATP binds to myosin, breaking the cross-bridge, and the myosin head is cocked back.
- The cycle continues as long as calcium is present.
Blood Pressure Regulation
- Blood pressure regulation is crucial for maintaining adequate oxygen and nutrient delivery to tissues.
- Two main regulators:
- Slow acting: Kidney through the renin-angiotensin system.
- Fast acting: Baroreceptors.
- Baroreceptors are located in the carotid arteries and aortic arch, sensing blood pressure.
- When blood pressure increases:
- Baroreceptors send signals to the brain.
- Parasympathetic nerve activity increases, and sympathetic activity decreases.
- Heart rate slows, and arteries dilate.
- When blood pressure decreases:
- Baroreceptors detect the decrease.
- Sympathetic nerve activity increases, and parasympathetic activity decreases.
- Heart rate increases, and arteries constrict.
- During exercise, baroreceptors reset to a higher set point, allowing for increased blood pressure.
- Mean arterial blood pressure (MAP) is not simply the average of systolic and diastolic due to the unequal duration of systole and diastole.
Blood Pressure Changes During Exercise
- Systolic blood pressure increases during exercise.
- Diastolic blood pressure remains relatively unchanged or may slightly decrease.
- MAP also increases during exercise.
Blood Pressure Response to Exercise Location
- Arm exercise, with the same workload as leg exercise, results in higher blood pressure due to:
- Vasodilation in the arms, but constriction in the legs.
- Only one-third of the body is vasodilated, leading to higher resistance and blood pressure.
- Leg exercise dilates a larger portion of the body, leading to lower blood pressure with similar workload.
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Description
This quiz explores the relationship between heart rate and stroke volume, focusing on how these cardiovascular parameters change during exercise. Understand how the cardiac accelerator nerve and other variables influence stroke volume and overall cardiac output. Ideal for students studying physiology and exercise science.
