Heart Rate and Blood Flow Regulation

Circulatory Response to Exercise

• The circulatory system works with the pulmonary system to form the cardiopulmonary or cardiorespiratory system, which has three main purposes: transporting oxygen and nutrients to tissues, removing CO2 and wastes from tissues, and regulating body temperature.
• Two major adjustments of blood flow during exercise are: increased cardiac output and redistribution of blood flow from inactive organs to active muscle.

Cardiovascular Anatomy: Heart

• The heart has AV valves that prevent backflow of blood from the ventricles into the atria, and semilunar valves that prevent backflow of blood from the arteries into the ventricles.

Pulmonary and Systemic Circuits

• The pulmonary circuit involves the right side of the heart pumping deoxygenated blood to the lungs via pulmonary arteries, which then returns oxygenated blood to the left side of the heart via pulmonary veins.
• The pulmonary circuit provides oxygen to the blood.

Cardiac Cycle at Rest and During Exercise

• At rest, diastole is longer than systole, whereas during exercise, both systole and diastole are shorter.
• Heart rate increases during exercise are achieved primarily through a decrease in the time spent in diastole, and at high heart rates, the length of time spent in systole also decreases.

Cardiovascular Physiology

• A healthy 21-year-old female has an average resting heart rate of 75 beats per minute, with a total cardiac cycle lasting 0.8 seconds, spent in diastole (0.5 seconds) and systole (0.3 seconds).
• During heavy exercise, heart rate increases to 180 beats per minute, with a reduction in the time spent in both systole and diastole.

Factors that Influence Arterial Blood Pressure

• The determinants of mean arterial pressure (MAP) are cardiac output and total vascular resistance, represented by the equation: MAP = cardiac output (Q) x total vascular resistance.
• Cardiac output is the product of heart rate (HR) and stroke volume (SV), represented by the equation: Q = HR x SV.

Regulation of Blood Pressure

• Short-term regulation of blood pressure involves the sympathetic nervous system (SNS) and baroreceptors in the aorta and carotid arteries.
• Long-term regulation of blood pressure involves the kidneys, which control blood volume.

Normal Electrocardiogram

• The normal electrocardiogram during rest consists of a P-wave (atrial depolarization), QRS complex (ventricular depolarization and atrial repolarization), and ST/T-wave (ventricular repolarization).

Cardiac Output

• Cardiac output is the product of heart rate (HR) and stroke volume (SV), represented by the equation: Q = HR x SV.
• Cardiac output can be increased due to a rise in either heart rate or stroke volume.

Regulation of Heart Rate

• The parasympathetic nervous system acts as a braking system to slow down heart rate, while the sympathetic nervous system increases heart rate.
• Parasympathetic tone is present at rest, and changes in parasympathetic activity can cause heart rate to increase or decrease.
• During exercise, the quantity of blood pumped by the heart must change in accordance with the elevated skeletal muscle oxygen demand.

Parasympathetic and Sympathetic Nervous Systems

• The parasympathetic nervous system slows heart rate by inhibiting the SA and AV nodes via the vagus nerve.

• The sympathetic nervous system increases heart rate by stimulating the SA and AV nodes via cardiac accelerator nerves.### Blood Composition and Vascular System

• Blood is composed of two principal components: plasma and cells.

• The greatest vascular resistance to blood flow is offered in the arterioles.

Factors Affecting Blood Flow

• The relationship between vessel radius, vessel length, blood viscosity, and flow is directly proportional.
• The radius of the blood vessel is the most important factor determining resistance to blood flow.

Cardiac Output and Exercise

• Cardiac output increases during exercise in direct proportion to the metabolic rate required to perform the exercise task.
• This increase is achieved by an increase in both stroke volume and heart rate.
• Cardiac output is directly proportional to the percentage of maximal oxygen uptake (VO2 max).

Oxygen Delivery to Muscle

• During intense exercise, oxygen delivery to muscle is increased to meet the rise in oxygen demand.
• Blood flow to the contracting muscle must increase to meet this demand.
• This is accomplished via an increase in heart rate, stroke volume, and cardiac output.

Changes in Cardiac Output and Blood Pressure During Exercise

• Cardiac output increases during exercise, with a linear increase in proportion to VO2 max.
• Stroke volume and heart rate increase during exercise, but stroke volume reaches a plateau between 40% to 60% of VO2 max in untrained and moderately trained individuals.
• Blood pressure increases during exercise, with a linear increase in systolic and diastolic pressure.

Maximal Heart Rate and Stroke Volume

• For adults, maximum heart rate (HR max) is equal to 220 - age.

• For children, HR max is equal to 208 - 0.7 x age.

• Maximum stroke volume (SV max) is approximately 140 ml/beat for a 70-kg, active male.### Arteriovenous Mechanisms

• Increased cardiac output and O2 difference during exercise

• Redistribution of blood flow from inactive organs to working skeletal muscle

Changes in Cardiac Output During Exercise

• Cardiac output increases in direct proportion to the metabolic rate required to perform the exercise task
• Relationship between cardiac output and percent max O2 uptake is essentially linear
• Increase in cardiac output during exercise is achieved by an increase in both stroke volume and heart rate
• Stroke volume does not increase beyond a workload of 40% to 60% of V̇O2 max in untrained or moderately trained subjects
• Rise in cardiac output in these individuals is achieved by increases in heart rate alone

Heart Rate Variability (HRV)

• Time between heartbeats
• Balance between SNS and PNS (sympathovagal balance)
• High variation in HRV is considered "healthy"
• Exercise training promotes high levels of HRV
• Low HRV is a predictor of cardiovascular morbidity and mortality

End-Diastolic Volume

• Frank-Starling mechanism: greater EDV results in a more forceful contraction due to stretch of ventricles
• Dependent on venous return (blood flow back to the heart)
• Venous return increased by venoconstriction and skeletal muscle pump

Prolonged Exercise

• Cardiac output is maintained
• Gradual decrease in stroke volume due to dehydration and reduced plasma volume
• Gradual increase in heart rate, particularly in heat
• Cardiovascular drift

Function of Respiratory System

• Means of gas exchange between the external environment and the body
• Replacing O2, removing CO2, and regulating acid-base balance
• Ventilation: mechanical process of moving air into and out of lungs
• Diffusion: random movement of molecules from an area of high concentration to an area of lower concentration

Bronchial Tree

• Consists of the passageways that connect the trachea and the alveoli
• Functions to filter and humidify the air as it moves toward the respiratory zone of the lung
• Divided into two functional zones: conducting zone and respiratory zone

Gas Exchange in the Lungs

• Occurs across about 300 million alveoli
• Enormous number of alveoli provides the lung with a large surface area for diffusion
• Rate of gas diffusion is further assisted by the fact that each alveolus is only one cell layer thick

Alveolar Cells

• Three types of alveolar cells: Type I, Type II, and alveolar macrophage
• Type II cells release pulmonary surfactant, which lowers the surface tension of the alveoli and prevents their collapse

Mechanics of Inspiration

• Movement of air into the lungs
• Diaphragm pushes downward, ribs lift outward, and volume of lungs increases
• Intrapulmonary pressure is lowered

Airway Resistance

• Airflow depends on pressure difference between two ends of airway and resistance of airways
• Airway resistance depends on the diameter of the airway
• Chronic obstructive pulmonary disease (COPD) and asthma increase airway resistance

Exercise and Chronic Obstructive Pulmonary Disease

• Increased airway resistance due to constant airway narrowing
• Decreased expiratory airflow
• Includes two lung diseases: chronic bronchitis and emphysema
• Increased work of breathing leads to shortness of breath

Pulmonary Ventilation

• Alveolar ventilation (VA): volume of air that reaches the respiratory zone
• Dead-space ventilation (VD): volume of air remaining in conducting airways
• V = VA + VD

Spirometry

• Measurement of pulmonary volumes and rate of expired airflow
• Useful for diagnosing lung diseases, such as COPD
• Spirometric tests: vital capacity (VC), forced expiratory volume (FEV1), and FEV1/VC ratio