3 Cardio-respiratory Dynamics and Physical Work

Meenal Dhall and Urvashi Gupta

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Contents:

    1.      Introduction

2.      Lung Volumes and Capacities

2.1 Static Lung Capacities

2.2 Dynamic Lung Capacities

3.      Cardiovascular Circulation

4.      Cardio-respiratory mechanism at rest and during physical activity/ work

Summary

 

Learning Outcomes:

  • To know more about the cardio-respiratory dynamics at rest and at work
  • To understand static and dynamic lung volumes and capacities
  • To appreciate the concept of respiratory mechanism during exercise or work
  • To gain insights about cardiovascular circulation and its regulation during exercise or work

    1.  Introduction

 

It is possible to live for several days without food or water but not so without fresh air. One cannot survive for more than a few minutes in the absence of oxygen. Oxygen is vital for the generation of a high energy compound called adenosine triphosphate (ATP) that is essential for the cellular activity. But the depots of oxygen and ATP in human body are very limited. Here, the role of physiological mechanisms is significant that makes both oxygen and ATP available to the body all the time without any delay or shortage. Molecular oxygen is made available to the body in three steps. In the first step, exchange of oxygen and carbon dioxide takes place between the body and the environment. Both these gases are then transported to and from the bodily tissues in the blood. Finally, the oxygen is utilized at the cellular level for accomplishing various essential functions.

 

Oxygen demands are fairly small when the body is resting. Based upon the size and body composition, it needs about one fourth of a litre of oxygen every minute. However, during exercise the amount of oxygen essential for maintaining adequate levels of ATP rises may folds, upto around twenty-folds. When the energy demands of cells and tissues are fulfilled by the breakdown of Creatinine Phosphate, without the involvement of oxygen, it is known as anaerobic metabolism. It happens in the case of extensive physical activity or exercise, where the muscle cell and tissues are utilized to the maximum. Similarly, when ATP is produced in the body by the breakdown of glucose releasing restricted amount of ATP and forming lactic acid, the oxygen- independent mechanism is also termed as anaerobic. Third type of metabolic state is known as aerobic, that uses oxygen for energy production. It includes all the daily activities such as working, eating, playing, etc. It involves the breakdown of carbohydrates and fats into carbon dioxide and water.

 

Exchange of oxygen and carbon dioxide between atmosphere and blood cells is the key role of lungs. The product of rate of breathing (or respiratory frequency) and mean volume per breadth (or tidal volume) gives the amount of air moving in and out of the lungs. This is the volume of air that ensures the gaseous exchange. On an average, 8-10 litres of air moves in and out of the lungs every minute, at rest. However, in the case of maximal oxygen uptake, this volume can rise upto about 100- 200 litres per minute. The volume of air increases to such an extent as in this case the sole aim of the physiological mechanisms is to flush out carbon dioxide and make the maximum amount of oxygen available to the cells and tissues.

 

The heart acts as the driving force for transporting oxygen through arteries and veins that direct the blood to move to the respective regions. During exercise the demand for oxygen by the working muscles increase which is met increased blood flow through the pulmonary circulation. At work or while exercising typical blood pressure might be 175/65 and the heart beats at a rate of about 50–90 beats per minute pumping more blood out to all parts of body especially to the working muscles.

 

2. Lung volumes and capacities

 

At birth, the average rate of breathing is 40 per minute with a standard deviation of 10. This frequency decreases gradually to 30 breaths per minute with the end of first year. At 5-6 years of age, the frequency of breathing further decreases to 22 breaths per minute. With a standard deviation of about 3, the breathing rate stabilizes to 16-17 per minute in adults. However, no significant differences are seen in breathing frequency among males and females. A limited proportion of lungs’ capacities are used at rest. However, while exercising or doing any other physical activity substantial reserves, that are stored for every physiological mechanism, are utilized then.

 

2.1  Static Lung Capacities

 

Different measures for static lung volumes and capacities assess the dimensional component of air movement within the pulmonary tract. They do not impose any time limitation on the subject. They also measure the change in lung volume for any given/ applied pressure. Different static lung volume/ capacities are as follows:

  • Total lung capacity (TLC) is the volume of air present in the lungs after maximum inspiration.
  • Vital Capacity (VC) or forced vital capacity (FVC) is the maximum volume of air that can be expelled from the lungs after maximum inspiration. More precisely, vital capacity is evaluated when time factor is not associated with the duration of expiratory phase. When the same is assessed as hard and fast as possible, it is termed as forced vital capacity. Hence, it is rather considered as a dynamic lung capacity.
  • Inspiratory capacity (IC) is the maximum volume of air that is inspired followed by tidal expiration.
  • Tidal Volume (TV) is the volume of air inspired or expired per breath.
  • Inspiratory reserve volume (IRV) is the maximum volume of air inspired at the end of tidal inspiration.
  • Expiratory reserve volume (ERV) is the maximum volume of air expired at the end of tidal expiration.
  • Residual volume/ residual lung volume is the volume of air present in the lungs after maximum expiration.
  • Functional residual capacity (FRC) is the volume of air present in the lungs after tidal expiration. This air volume allows uninterrupted exchange of gases between the blood and alveoli to prevent fluctuations in blood gases during different phases of breathing.

   Even after forced expiration some volume of air remains in the lungs. Some volume of this air that does not participate in gaseous exchange is termed as dead space air volume. During inhalation, the dead space air is diluted by the alveolar air, while the dead space air volume dilutes the alveolar air after exhalation. In moderate exercise, consumption of oxygen and excretion of carbon dioxide increases. Ventilation process increases because of larger tidal volumes and not the rate of breathing that reduces oxygen concentration and increase in that of carbon dioxide. In case of severe exercise, an uneven increase in ventilation takes place. It happens due to increased breathing rate and reducing tidal volume. As a result more and larger expired dead space volumes are formed. The total dead space volume is enough to dilute the alveolar gas. This increases oxygen concentration and fall in carbon dioxide proportions.

 

2.2     Dynamic Lung Capacities

 

Various measures for dynamic lung volumes and capacities appraise the power component of pulmonary performance during different phases of ventilatory digression. They also take into account the compliance of lungs at any given time during actual movement of air. These are:

  • Forced expiratory volume (in one second) (FEV1.0) refers to the volume of air expired in the first second of forced manoeuvre of vital capacity test. It is described in L/second. It reflects pulmonary expiratory power.
  • Maximum voluntary ventilation (MVV) is the maximum volume gas exhaled as deeply and rapidly as possible for 15 seconds while breathing. This volume is extrapolated for 1 minute and reported in L/ minute.
  • orced mid-expiratory flow 25-75% (FEF 25-75%) is the maximum rate of flow of air during the mid-expiratory part of forced manoeuvre of vital capacity test. It is measured in L/second. It represents effort independent expiration.

    The strength and endurance of the ventilatory muscles could be further developed through specific exercise training that also improves both inspiratory muscle function and maximal voluntary ventilation. Ventilatory training enhances exercise capacity and reduces physiologic exertion.

 

3.  Cardiovascular circulation

 

The RBCs are now laden with oxygen and require a distribution network that connects the lungs with other parts of body and a means of delivery around the body. The heart acts as the driving force for transporting oxygen through arteries and veins that direct the blood to move to the respective regions. After gaseous exchange that takes place in lungs, oxygen molecules diffuse to the pulmonary blood from the lung alveoli. The pulmonary circulation uses 12 % of the total blood volume. There are four major blood vessels that supply blood to the lungs. The left and the right pulmonary arteries carry deoxygenated blood from the right atrium of the heart to the lungs, while the left and right pulmonary veins bring blood rich in oxygen from the lungs to the left side of the heart. The part of the circulatory system that feeds the lungs is called the pulmonary circulation; while the other one that feeds the rest of the body is called the systemic circulation. Following processes are of immense importance while studying the dynamics of circulatory mechanism:

 

Heart rate is the rate at which the heart pumps blood. It is recorded in beats per minute. On an average, heart rate at rest is accounted to be 72 beats per minute. Maximal heart rate is another variable, a near exact sore for which is given by subtracting age from 220. In certain cases, it is also referred to as pulse rate.

 

Mean stroke volume is the volume of blood that is ejected by the heart with each stroke. It is measured in L per beat. In resting state, heart ejaculates about 70 ml of blood per beat. Stroke volume tends to increase with exercise intensity until about 120 mL.bt-1 after which it levels off, further after which it falls as the heart rate increase to the maximum.

 

Cardiac output is described as the total amount of blood driven out from the ventricles in one minute. It is calculated as the product of heart rate and stroke volume. Therefore, at rest cardiac output is found to be:

 

Cardiac Output = 72 beats/min x 0.07 L/beat = 5.0 L/min

 

The increased oxygen demand by the muscles during exercise is fulfilled by increased cardiac output resulting from a faster heart rate and larger stroke volume. In a normal young, untrained adult, the maximal cardiac output is about 15-20 L.min-1. In well trained athletes, it may reach up to 35 L/min.

 

Systolic and diastolic pressure are measured by using a reference point generally over the brachial artery at the level of the right atrium. Blood pressure gives an estimate of the work done by heart and the force that blood exerts against the walls of heart during different ventricular/ arterial states (contraction or relaxation). Systolic blood pressure is the resistance experienced by the blood against the walls during ventricular systole/ contraction. The similar situation in case of arterial diastole/ contraction is called the diastolic blood pressure. At rest in normotensive individuals, the average highest systolic/ diastolic blood pressure experienced is 120/ 80 mmHg.

 

4. Cardio-respiratory mechanism at rest and during physical activity/ work

 

During exercise the amount of oxygen rises may folds, upto around twenty-folds. Breathing patterns during physical activity or exercise advance in a highly economical and effective manner. Consumption of oxygen and release of carbon dioxide are influenced by physical activity the most than any other physiologic pressure. An increased gaseous exchange between the alveoli and the blood maintains proper gas concentrations. During light to moderate intensity exercises, ventilation increases linearly with carbon dioxide production and oxygen consumption. Here ventilation increases mainly through increased tidal volume. During strenuous exercises, rate of breathing increases to the highest, that is significant to provide good aeration of body tissues and blood. To oxygenate the increased blood flow that is required during physical work or exercise, the person starts taking deeper breaths and breathes faster, up to 40–60 breaths per minute.

 

When the normal metabolic needs of consumption of oxygen and removal of carbon dioxide in pulmonary ventilation get increased to an abnormal level leads to a very harmful condition termed as hyperventilation. Also known as overbreathing, it gradually stabilizes the normal rate of gaseous exchange in the body. However, few seconds of hyperventilation may cause excessive carbon dioxide unloading and lightheadedness.

 

Blood flow through the pulmonary circulation increases while exercising or doing any activity. Adrenaline that is also released into the blood during this state, acts as a vasodilator that leads to the arteriolar dilation that further increases blood supply to the lungs. During exercise the demand for oxygen by the working muscles increases. It is met by increased cardiac output, which in turn is achieved by a faster heart rate and larger stroke volume. It means that the heart rate increases and reaches up to 170–210 beats per minute during intense exercise, while heart pumps out more blood with each beat when the stroke volume become higher.

 

Cardiac output may increase from 5 L/min to the maximum of 35 L/min in trained athletes or those involved in high physical activity on a regular basis. During steady state physical activity, systolic blood pressure and heart rate both increase, while afterwards the systolic pressure plateaus. In this case, diastolic blood pressure hardly changes or may even fall. At rest, most of the total blood volume moves to the viscera- gut, liver, spleen, kidneys; and skin. While working or exercising, most of the blood is provided to the working skeletal muscles. There is also increase in the flow of blood to the skin needed for dissipation of heat.

 

As the extent of any physical activity increases, oxygen demand also rises, until a point is reached when it fails to boost further inspite of additional increment in work load. This is termed as maximal oxygen consumption also known as VO2 max. After VO2 max has been achieved an increased work load can be sustained only for a very short period, and by using the anaerobic metabolism in the working muscles. A person’s VO2 max level is not fixed at a given value. It could be altered by the level of physical activity done habitually.

 

All of these changes and mechanisms are coordinated and controlled by special centers in the brain. It uses the nervous system and chemical messengers to control all of these processes.

 

Summary

 

One cannot survive for more than a few minutes in the absence of oxygen. Oxygen demands are fairly small when the body is resting. Based upon the size and body composition, it needs about one fourth of a litre of oxygen every minute. However, during exercise the amount of oxygen rises may folds, upto around twenty-folds. Exchange of oxygen and carbon dioxide between atmosphere and blood cells is the key role of lungs.

 

A limited proportion of lungs’ capacities are used at rest. However, while exercising or doing any other physical activity substantial reserves, that are stored for every physiological mechanism, are utilized then. Breathing patterns during physical activity or exercise advance in a highly economical and effective manner. During light to moderate intensity exercises, ventilation increases linearly with carbon dioxide production and oxygen consumption. During strenuous exercises, rate of breathing increases to the highest, that is significant to provide good aeration of body tissues and blood.

 

The RBCs are now laden with oxygen and require a distribution network that connects the lungs with other parts of body and a means of delivery around the body. The heart acts as the driving force for transporting oxygen through arteries and veins that direct the blood to move to the respective regions. After gaseous exchange that takes place in lungs, oxygen molecules diffuse to the pulmonary blood from the lung alveoli. During steady state physical activity, systolic blood pressure and heart rate both increase, while afterwards the systolic pressure plateaus. In this case, diastolic blood pressure hardly changes or may even fall. At rest, most of the total blood volume moves to the viscera- gut, liver, spleen, kidneys; and skin. While working or exercising, most of the blood is provided to the working skeletal muscles. There is also increase in the flow of blood to the skin needed for dissipation of heat.

you can view video on Cardio-respiratory Dynamics and Physical Work

 

Key words

  • Cardio-respiratory system: The system that circulates blood through the body; consists of the heart, blood vessels, and respiratory system.
  • Pulmonary circulation: The part of the circulatory system that moves blood between the heart and the lungs; controlled by the right side of the heart.
  • Systemic circulation: The part of the circulatory system that moves blood between the heart and the rest of the body; controlled by the left side of the heart.
  • Venae cavae: The large veins through which blood is returned to the right atrium of the heart.
  • Atria: The two upper chambers of the heart in which blood collects before passing to the ventricles; also called auricles.
  • Ventricles: The two lower chambers of the heart from which blood flows through arteries to the lungs and other parts of the body.
  • Aorta: The large artery that receives blood from the left ventricle and distributes it to the body.
  • Systole: Contraction of the heart.
  • Diastole: Relaxation of the heart.
  • Blood pressure: The force exerted by the blood on the walls of the blood vessels; created by the pumping action of the heart.
  • Veins: Vessels that carry blood to the heart.
  • Arteries: Vessels that carry blood away from the heart.

 

References

  1. McArdle, W.D., Katch, F.I. and Katch, V.L. (2010). Essentials of Exercise Physiology. Lippincott Williams and Wilkins. 7th ed.
  2. Hale, T. (2003). Exercise Physiology A Thematic Approach. John Wiley and sons Ltd.
  3. Vander, A.J., Sherman, J.H. and Luciano, D.S. (1994). Human Physiology The Mechanics of Body Function. WCB/ McGraw- Hill.
  4. Cardiorespiratory Endurance. Basic Physiology of Cardiorespiratory Endurance Exercise. Assessed from www.mhhe.com/fahey

 

Suggested Readings

  1. Malina, R., Bouchard, C. and Bar-Or, O. (2004). Growth, maturation and Physical Activity. Human Kinetics. 2nd ed.
  2. McCarthy, K. and Gildea, T.R. (2010). Pulmonary Function Testing.