Exercise 23 Review Sheet Cardiovascular Physiology

The cardiovascular system, a complex network of organs and vessels, is essential for delivering oxygen and nutrients throughout the body while removing metabolic waste. Understanding its physiology is crucial for healthcare professionals and anyone interested in optimizing their health. Exercise, a potent stimulus for cardiovascular adaptation, provides a unique window into the system's remarkable capabilities. This review sheet will delve into the key concepts covered in Exercise 23, exploring the intricate mechanisms governing cardiovascular function and its response to physical activity.

Cardiovascular Physiology: An Overview

The cardiovascular system comprises the heart, blood vessels (arteries, veins, and capillaries), and blood. Its primary function is to maintain a stable internal environment, or homeostasis, by regulating blood pressure, cardiac output, and blood distribution to various tissues. This is achieved through a complex interplay of neural, hormonal, and local control mechanisms.

The heart, a muscular pump, generates the pressure needed to circulate blood. The rhythmic contraction and relaxation of the heart, known as the cardiac cycle, propels blood through the pulmonary and systemic circuits. The pulmonary circuit carries blood to the lungs for oxygenation, while the systemic circuit delivers oxygenated blood to the rest of the body.

Blood vessels act as conduits for blood flow. Arteries carry blood away from the heart, branching into smaller arterioles that lead to capillaries. Capillaries are tiny vessels with thin walls, allowing for the exchange of oxygen, nutrients, and waste products between the blood and tissues. Venules collect blood from the capillaries, merging into larger veins that return blood to the heart.

Blood, the fluid medium of the cardiovascular system, carries oxygen, nutrients, hormones, and immune cells. It also transports carbon dioxide and other waste products to the lungs and kidneys for elimination.

Key Concepts in Exercise 23

Exercise 23 typically covers the following key concepts in cardiovascular physiology:

• Cardiac Output (Q): The volume of blood pumped by the heart per minute. It is the product of heart rate (HR) and stroke volume (SV).
• Heart Rate (HR): The number of times the heart beats per minute.
• Stroke Volume (SV): The volume of blood ejected from the heart with each beat.
• Blood Pressure (BP): The force exerted by the blood against the walls of the arteries. It is typically measured as systolic blood pressure (SBP) over diastolic blood pressure (DBP).
• Total Peripheral Resistance (TPR): The resistance to blood flow in the systemic circulation.
• Oxygen Consumption (VO2): The volume of oxygen consumed by the body per minute.
• Fick Equation: A fundamental equation that relates VO2 to cardiac output and the arteriovenous oxygen difference (a-vO2 difference).
• Cardiovascular Adaptations to Exercise: The long-term changes in the cardiovascular system that occur as a result of regular exercise training.

The Cardiac Cycle: Systole and Diastole

The cardiac cycle is the sequence of events that occur during one heartbeat. It consists of two main phases: systole and diastole.

• Systole: The phase of ventricular contraction. During systole, the ventricles contract, increasing the pressure inside the ventricles. This pressure forces the aortic and pulmonary valves open, allowing blood to be ejected into the aorta and pulmonary artery, respectively.
• Diastole: The phase of ventricular relaxation. During diastole, the ventricles relax, decreasing the pressure inside the ventricles. This allows the aortic and pulmonary valves to close, preventing backflow of blood into the ventricles. The mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles.

The duration of systole and diastole varies with heart rate. At rest, diastole is longer than systole, allowing sufficient time for ventricular filling. During exercise, heart rate increases, shortening both systole and diastole. However, the reduction in diastole is more pronounced, which can potentially limit ventricular filling at very high heart rates.

Cardiac Output: Heart Rate and Stroke Volume

Cardiac output (Q) is a crucial determinant of oxygen delivery to the tissues. It is calculated as:

Q = HR x SV

Where:

• Q = Cardiac output (L/min)
• HR = Heart rate (beats/min)
• SV = Stroke volume (mL/beat)

Heart Rate Regulation

Heart rate is primarily regulated by the autonomic nervous system.

• Sympathetic Nervous System: Increases heart rate through the release of norepinephrine, which acts on beta-adrenergic receptors in the heart.
• Parasympathetic Nervous System: Decreases heart rate through the release of acetylcholine, which acts on muscarinic receptors in the heart.

At rest, the parasympathetic nervous system dominates, resulting in a lower heart rate. During exercise, sympathetic activity increases and parasympathetic activity decreases, leading to an increase in heart rate.

Other factors that can influence heart rate include:

• Hormones: Epinephrine and thyroid hormones can increase heart rate.
• Body Temperature: An increase in body temperature can increase heart rate.
• Emotions: Stress and anxiety can increase heart rate.

Stroke Volume Regulation

Stroke volume is influenced by three main factors:

1. Preload: The volume of blood in the ventricles at the end of diastole (end-diastolic volume or EDV). An increase in preload increases stroke volume, according to the Frank-Starling mechanism. The Frank-Starling mechanism states that the force of ventricular contraction is proportional to the initial length of the muscle fibers. In other words, the more the ventricle is stretched during diastole, the more forceful the contraction will be during systole.
2. Afterload: The resistance against which the heart must pump blood. An increase in afterload decreases stroke volume. Afterload is primarily determined by arterial blood pressure.
3. Contractility: The force of ventricular contraction independent of preload and afterload. An increase in contractility increases stroke volume. Contractility is influenced by sympathetic nervous system activity and circulating catecholamines.

During exercise, stroke volume typically increases up to a certain point, after which it plateaus. This plateau is likely due to the reduced filling time at high heart rates.

Blood Pressure: Systolic, Diastolic, and Mean Arterial Pressure

Blood pressure (BP) is the force exerted by the blood against the walls of the arteries. It is typically measured as systolic blood pressure (SBP) over diastolic blood pressure (DBP).

• Systolic Blood Pressure (SBP): The peak pressure in the arteries during ventricular contraction (systole).
• Diastolic Blood Pressure (DBP): The minimum pressure in the arteries during ventricular relaxation (diastole).

Mean arterial pressure (MAP) represents the average pressure in the arteries during one cardiac cycle. It is a more accurate reflection of the driving force for blood flow than either SBP or DBP alone. MAP can be estimated using the following formula:

MAP = DBP + 1/3 (SBP - DBP)

Blood pressure is regulated by a variety of mechanisms, including:

• Autonomic Nervous System: Sympathetic nervous system activation increases blood pressure by increasing heart rate, stroke volume, and vasoconstriction.
• Renin-Angiotensin-Aldosterone System (RAAS): This hormonal system regulates blood volume and blood pressure.
• Baroreceptors: Pressure sensors in the arteries that detect changes in blood pressure and trigger compensatory responses.

During exercise, systolic blood pressure typically increases linearly with increasing exercise intensity, while diastolic blood pressure may remain relatively stable or even decrease slightly.

Total Peripheral Resistance: Vasoconstriction and Vasodilation

Total peripheral resistance (TPR) is the resistance to blood flow in the systemic circulation. It is determined by the diameter of the blood vessels, blood viscosity, and the length of the blood vessels. The most important factor regulating TPR is the diameter of the arterioles.

• Vasoconstriction: The narrowing of blood vessels, which increases TPR and decreases blood flow.
• Vasodilation: The widening of blood vessels, which decreases TPR and increases blood flow.

Vasoconstriction and vasodilation are regulated by a variety of factors, including:

• Autonomic Nervous System: Sympathetic nervous system activation causes vasoconstriction in most blood vessels, except those in the skeletal muscles and heart, where it causes vasodilation.
• Local Factors: Metabolic byproducts produced during exercise, such as carbon dioxide, lactic acid, and adenosine, cause vasodilation in the skeletal muscles.
• Hormones: Epinephrine can cause both vasoconstriction and vasodilation, depending on the receptor it binds to.

During exercise, TPR typically decreases due to vasodilation in the skeletal muscles, which allows for increased blood flow to meet the metabolic demands of the exercising muscles.

Oxygen Consumption and the Fick Equation

Oxygen consumption (VO2) is the volume of oxygen consumed by the body per minute. It is a measure of the body's metabolic rate and is often used to assess cardiorespiratory fitness.

The Fick equation relates VO2 to cardiac output (Q) and the arteriovenous oxygen difference (a-vO2 difference):

VO2 = Q x a-vO2 difference

Where:

• VO2 = Oxygen consumption (mL/min)
• Q = Cardiac output (L/min)
• a-vO2 difference = The difference in oxygen content between arterial and venous blood (mL O2/L blood)

The Fick equation demonstrates that oxygen consumption is determined by both the amount of blood delivered to the tissues (cardiac output) and the amount of oxygen extracted from the blood by the tissues (a-vO2 difference).

During exercise, both cardiac output and a-vO2 difference increase, leading to a substantial increase in oxygen consumption. The increase in cardiac output is primarily due to an increase in heart rate and stroke volume, while the increase in a-vO2 difference is due to increased oxygen extraction by the working muscles.

Cardiovascular Adaptations to Exercise Training

Regular exercise training leads to a number of beneficial adaptations in the cardiovascular system, including:

• Increased Stroke Volume: Endurance training increases stroke volume at rest and during exercise. This is due to an increase in left ventricular volume and contractility.
• Decreased Resting Heart Rate: Endurance training decreases resting heart rate. This is due to an increase in parasympathetic nervous system activity and a decrease in sympathetic nervous system activity.
• Increased Cardiac Output: Endurance training increases maximal cardiac output. This is due to an increase in both stroke volume and heart rate.
• Decreased Blood Pressure: Endurance training can lower blood pressure in individuals with hypertension.
• Increased Capillary Density: Endurance training increases the number of capillaries in the skeletal muscles. This allows for increased oxygen delivery to the muscles.
• Increased Blood Volume: Endurance training increases blood volume. This helps to maintain stroke volume during exercise.
• Improved a-vO2 Difference: Endurance training increases the ability of the muscles to extract oxygen from the blood. This is due to an increase in the number and size of mitochondria in the muscle cells.

These cardiovascular adaptations result in improved cardiorespiratory fitness, reduced risk of cardiovascular disease, and improved overall health.

Exercise 23: Practical Applications and Examples

Exercise 23 often involves practical applications of these concepts, such as calculating cardiac output, blood pressure, and oxygen consumption during various exercise intensities. You might be asked to analyze data from exercise tests and interpret the cardiovascular responses to exercise.

Example 1: Calculating Cardiac Output

A person has a heart rate of 70 beats/min and a stroke volume of 70 mL/beat. Calculate their cardiac output.

Solution:

Q = HR x SV

Q = 70 beats/min x 70 mL/beat

Q = 4900 mL/min

Q = 4.9 L/min

Example 2: Analyzing Blood Pressure Response to Exercise

A person's blood pressure at rest is 120/80 mmHg. During exercise at a moderate intensity, their blood pressure increases to 160/85 mmHg. Explain the changes in systolic and diastolic blood pressure.

Solution:

Systolic blood pressure increased from 120 mmHg to 160 mmHg due to the increased force of ventricular contraction and the increased cardiac output. Diastolic blood pressure increased slightly from 80 mmHg to 85 mmHg, which is a normal response to exercise. It indicates some vasoconstriction in non-working muscles to maintain blood flow to the active muscles.

Example 3: Understanding the Fick Equation

A person has a cardiac output of 20 L/min and an a-vO2 difference of 150 mL O2/L blood during maximal exercise. Calculate their oxygen consumption.

Solution:

VO2 = Q x a-vO2 difference

VO2 = 20 L/min x 150 mL O2/L blood

VO2 = 3000 mL O2/min

VO2 = 3.0 L/min

Common Misconceptions and Pitfalls

• Confusing Heart Rate and Cardiac Output: It's important to remember that heart rate is only one component of cardiac output. Stroke volume also plays a crucial role, and changes in either heart rate or stroke volume can affect cardiac output.
• Ignoring the Role of the Autonomic Nervous System: The autonomic nervous system plays a vital role in regulating cardiovascular function. Understanding the effects of sympathetic and parasympathetic activity is essential for understanding the cardiovascular response to exercise.
• Overlooking the Importance of Preload, Afterload, and Contractility: These three factors are key determinants of stroke volume, and understanding how they are influenced by exercise is important for understanding the cardiovascular response to exercise.
• Failing to Appreciate the Complexity of Blood Pressure Regulation: Blood pressure is regulated by a complex interplay of neural, hormonal, and local control mechanisms. Understanding these mechanisms is essential for understanding the cardiovascular response to exercise and the effects of exercise on blood pressure.
• Not Connecting Theory to Practical Applications: It's important to be able to apply the theoretical concepts of cardiovascular physiology to real-world scenarios, such as analyzing exercise test data and interpreting cardiovascular responses to exercise.

Frequently Asked Questions (FAQ)

Q: Why does heart rate increase during exercise?

A: Heart rate increases during exercise due to increased sympathetic nervous system activity and decreased parasympathetic nervous system activity. These changes are triggered by signals from the brain and muscles, as well as by hormonal factors such as epinephrine.

Q: Why does stroke volume increase during exercise?

A: Stroke volume increases during exercise due to increased preload, increased contractility, and decreased afterload. Preload increases due to increased venous return, contractility increases due to sympathetic nervous system activity, and afterload decreases due to vasodilation in the skeletal muscles.

Q: Why does systolic blood pressure increase during exercise?

A: Systolic blood pressure increases during exercise due to the increased force of ventricular contraction and the increased cardiac output.

Q: Why does diastolic blood pressure remain relatively stable or decrease slightly during exercise?

A: Diastolic blood pressure remains relatively stable or decreases slightly during exercise due to vasodilation in the skeletal muscles, which reduces total peripheral resistance.

Q: What are the benefits of exercise for the cardiovascular system?

A: Exercise leads to a number of beneficial adaptations in the cardiovascular system, including increased stroke volume, decreased resting heart rate, increased cardiac output, decreased blood pressure, increased capillary density, increased blood volume, and improved a-vO2 difference.

Conclusion

Understanding cardiovascular physiology is fundamental to comprehending the body's response to exercise and the long-term benefits of physical activity. Exercise 23 provides a crucial foundation for exploring the intricate mechanisms governing heart rate, stroke volume, blood pressure, and oxygen consumption. By mastering these concepts and their practical applications, you will be well-equipped to appreciate the remarkable adaptability of the cardiovascular system and its vital role in maintaining health and performance. Remember to connect the theoretical concepts to real-world scenarios and avoid common misconceptions to gain a deeper understanding of this essential physiological system.