Chapter 8

Cardiorespiratory Responses to Acute Exercise

Numerous interrelated cardiovascular changes occur during dynamic exercise. The primary goal of these adjustments is to increase blood flow to working muscle; however, cardiovascular control of virtually every tissue and organ in the body is also altered. To better understand the changes that occur, we must examine the function of both the heart and the peripheral circulation. In this section we examine changes in all components of the cardiovascular system from rest to acute exercise, looking specifically at the following:

 Heart rate

 Stroke volume

 Cardiac output

 Blood pressure

 Blood flow

 The blood

We then see how these changes are integrated to maintain adequate blood pressure and provide for the exercising body s needs.

In Review

 Mean arterial blood pressure increases immediately in response to exercise, and the magnitude of the increase is proportional to the intensity of exercise. During whole-body endurance exercise, this is accomplished primarily by an increase in systolic blood pressure, with minimal changes in diastolic pressure.

 Systolic blood pressure can exceed 200 to 250 mmHg at maximal exercise intensity, the result of increases in cardiac output. Upper body exercise causes a greater blood pressure response than leg exercise at the same absolute rate of energy expenditure, likely due to the smaller muscle mass involved and the need to stabilize the trunk during dynamic arm exercise.

 Blood flow is redistributed during exercise from inactive or low. -activity tissues of the body like the liver and kidneys to meet the increased metabolic needs of exercising muscles.

 With prolonged aerobic exercise, or aerobic exercise in the heat, sv gradually decreases and HR increases proportionately to maintain cardiac output. This is referred to as cardiovascular drift and is associated with a progressive increase in blood flow to the vasodilated skin and losses of fluid from the vascular space.

 The changes that occur in the blood during exercise include the following:

1. The (a- V)O2 difference increases, as venous oxygen concentration decreases, reflecting increased extraction of oxygen from the blood for use by the active tissues.

2. Plasma volume decreases. Plasma is pushed out of the capillaries by increased hydrostatic pressure as blood pressure increases, and fluid is drawn into themuscles by the increased oncotic and osmotic pressures in the muscle tissues, a by-product of metabolism. With prolonged exercise or exercise in hot environments, increasingly more plasma volume is lost through sweating.

3. Hemoconcentration occurs as plasma volume (water) decreases. Although the actual number of red blood cells stays relatively constant, the relative number of red blood cells per unit of blood increases, which increases oxygen-carrying capacity.

Respiratory Responses to Acute Exercise

Now that we have discussed the role of the cardiovascular system in delivering oxygen to the exercising muscle, we examine how the respiratory system responds to acute dynamic exercise.

In Review

 During exercise, ventilation shows an almost immediate increase due to increased inspiratory center stimulation. This is caused by both central command and neural feedback from muscle activity itself. This phase is followed by a plateau (during light exercise) or a much more gradual increase in respiration (during heavy exercise) that results from chemical changes in the arterial blood resulting from exercise metabolism.

 Altered breathing patterns and sensations associated with exercise include dyspnea, hyperventilation, and performance of the V alsalva maneuver.

 During mild, steady-state exercise, ventilation increases to match the rate of energy metabolism; that is, ventilation parallels oxygen uptake. The ratio of air ventilated to oxygen consumed is the ventilatory equivalent for oxygen (ѶE/ѶСО2)·

 At low exercise intensities, increased ventilation is accomplished by increases in tidal volume (the amount of air moved in and out of the lungs during regular breathing). At higher intensities, the rate of respiration also increases.

 Maximal rates of pulmonary ventilation depend on body size. Maximal ventilation rates of approximately 100 L/min are common for smaller individuals but may exceed 200 L/min in larger individuals.

 The ventilatory threshold is the point at which ventilation begins to increase disproportionately to the increase in oxygen consumption. This increase in VE reflects the need to remove excess carbon dioxide.

 We can estimate lactate threshold with reasonable accuracy by identifying that point аt whісh ѶЕ/ѶО2 ѕtаrtѕ tо іnсrеаѕе whіlе ѶE/ѶСО2 соntіnuеѕ tо dесlіnе.

Respiratory Limitations to Performance

In Review

 Respiratory muscles can account for up to 10% of the body's total oxygen consumption and 15% of the cardiac output during heavy exercise.

 Pulmonary ventilation is usually not a limiting factor for performance even during maximal effort, although it can limit performance in some elite endurance athletes.

 The respiratory muscles are well designed to avoid fatigue during long term activity.

 Airway resistance and gas diffusion usually do not limit performance in normal, healthy individuals exercising at sea level.

 The respiratory system can, and often does, limit performance in people with various types of restrictive or obstructive respiratory disorders.

Respiratory Regulation of Acid- Base Balance

In Review

 Excess H+ (decreased pH) impairs muscle contractility and ATP generation.

 The respiratory and renal systems play integral roles in maintaining acid-base balance. The renal system is involved in more long- term maintenance of acid- base balance through the secretion of H+.

 Whenever H+ concentration starts to increase, the inspiratory center responds by increasing the rate and depth of respiration. Removing carbon dioxide is an essential means of reducing H+ concentrations.

 Carbon dioxide is transported in the blood primarily bound to bicarbonate. Once it reaches the lungs, carbon dioxide is formed again and exhaled.

 Whenever H+ concentration begins to increase, whether from carbon dioxide or lactate accumulation, bicarbonate ion can buffer the H+ to prevent acidosis.

In Closing

In this chapter, we discussed the responses of the cardiovascular and respiratory systems to exercise. We also considered the limitations that these systems can impose on abilities to perform sustained aerobic exercise. The next chapter presents basic principles of exercise training, allowing us to better understand in the subsequent chapters how the body adapts to resistance training as well as aerobic and anaerobic training.

Study Questions

1.Describe how heart rate, stroke volume, and cardiac output respond to increasing rates of work. Illustrate how these three variables are interrelated.

2.How do we determine HRmax? What are alternative methods using indirect estimates? What are the major limitations of these indirect estimates?

3.Describe two important mechanisms for returning blood back to the heart during exercise in an upright position.

4.What is the Fick principle, and how does this apply to our understanding of the relationship between metabolism and cardiovascular function?

5.Define the Frank- -Starling mechanism. How does this work during exercise?

6.How does blood pressure respond to exercise?

7.What are the major cardiovascular adjustments that the body makes when someone is overheated during exercise?

8.What is cardiovascular drift? What two theories have been proposed to explain this phenomenon?

9.Describe the primary functions of blood.

10.What changes occur in the plasma volume and red blood cells with increasing levels of exercise? With prolonged exercise in the heat?

11.How does pulmonary ventilation respond to increasing intensities of exercise?

12.Define the terms dyspnea, byperventilation, Valsalva maneuver, and ventilatory thresbold.

13.What role does the respiratory system play in acid-base balance?

14.What is the normal resting pH for arterial blood? For muscle? How are these values changed as a result of exhaustive sprint exercise?

15.What are the primary buffers in the blood? In muscles?