The Cardiorespiratory System: Structure, Function, and Exercise Application

Heart Location and Structure

The heart is a muscular organ located in the chest, or thoracic cavity, diagonally behind the breastbone, or sternum. Shaped and sized similarly to that of a clenched fist, this relatively small organ performs a tremendous amount of work to maintain life processes. Even at rest, the amount of blood pumped by the heart per minute (cardiac output) is an average of 5 liters. The structure of the heart (Figure 6-1) allows for an efficient mechanism to perform vital processes. The heart is divided, anatomically and functionally, into right and left sides by a partitioning wall, or septum. The right side of the heart receives de-oxygenated blood as it is returned from the body through the venous system and pumps the blood to the lungs, or pulmonary system. The left side of the heart receives the oxygenated blood from the lungs and pumps it, via the arterial system, throughout the body. Because the heart performs two distinct functions, this singular organ is often referred to as a double-pump.

Structure of the heart.

The heart is further broken down into upper and lower chambers called the atria (atrium, singular) and the ventricles, respectively. The superior atria (right and left) are the blood-receiving units of the heart. The blood is then forced through an efficient one-way system of valves, known as atrioventricular, or AV valves, to the inferior ventricles. The contraction of the ventricles forces blood through the semilunar valves and into the great arteries of either the pulmonary or systemic circulation. The familiar “lub-dub” sounds of the heart are produced by the closing of the atrioventricular and semilunar valves.

The heart is entirely contained within a loose yet protective sac, the pericardium, which prevents the beating heart from brushing against the chest wall. The heart itself is composed of three specialized layers of tissue: (a) the epicardium, (b) the myocardium, and (c) the endocardium. The epicardium is a thin membrane located on the outermost layer of the heart. The primary work of the heart is performed by the next layer of muscular tissue, the myocardium. The myocardium is thicker and stronger in the left ventricle as it is responsible for pumping blood into the systemic circulation, a much higher pressure system than the pulmonary circulation. The coronary arteries are the primary source of nourishment for the epicardium and myocardium, as only the endocardium is enriched by the blood-filled chambers of the heart. The term myocardial infarction, or heart attack, refers to a dysfunction of the tissues of the myocardium usually due to ischemia or a lack of blood flow to the myocardium. The endocardium is a smooth membrane that lines the cavities within the heart.

Conduction System

The conduction system of the heart (Figure 6-3) is “autorhythmic” in nature and controlled by a specialized nerve center in the brain. This center receives signals from the body and relays commands to the heart. Conduction begins with an electrical impulse of the sinoatrial (SA) node within the right atrium. The SA node, because of its rapid and spontaneous impulses, dictates regulation of contractions of the heart and is therefore referred to as the “pacemaker.” On an average, the adult’s heart, at rest, beats 60-80 times per minute. The electrical impulse of the SA node causes both atria to contract synchronously and, consequently, blood is forced into the ventricles. Almost immediately after the SA node fires, the electrical charge travels through specialized conduction tissue to reach the atrioventricular (AV) node. The AV node consists of slow-conducting muscle cells and delays the impulse before it excites the ventricular conductors, through the bundle of His (pronounced: hiss). The conduction system continues through the branches of the bundle of His into the Purkinje fibers. This chain of electrical events results in a simultaneous and powerful contraction of the ventricles. The blood is then forced from the ventricles into the major arteries. It is the contraction of the ventricles that constitutes a heartbeat.

Cardiac Cycle

The contraction/relaxation pattern produced in the heart is known as the cardiac cycle. The contraction phase is called systole and the relaxation phase is diastole. Generally, these terms refer to the contraction/relaxation of the ventricles. It is important to understand that the atria have a separate contraction/relaxation phase. Fundamentally, atrial contraction (systole) occurs during ventricular relaxation (diastole) and ventricular systole occurs as the atria relax. The time interval of the cardiac cycle decreases as the cardiac rate increases, as would occur with vigorous exercise.

The contraction/relaxation phases of the cardiac cycle create pressure changes adequate to produce blood flow through the arteries. Arterial blood pressure increases from 80 to 120 mmHg within the systemic circulatory system as a result of ventricular systole. The pressure in the pulmonary circulation is significantly lower. Simply stated, the contraction/relaxation of the ventricles causes a squeezing action that produces blood flow through the arteries.

Blood Pressure

Measurement of arterial blood pressure provides significant information concerning heart function. With the use of a device called a sphygmomanometer, the qualified technician can easily record blood pressure. First, an inflatable cuff is fastened around the arm, just above the elbow. Air is then pumped into the cuff to apply pressure sufficient enough to cut off blood circulation. As the air is slowly released from the cuff, the technician listens for the first pulsing sounds with a stethoscope. When the first sounds are heard, the pressure within the arteries can be determined by the reading shown on the dial attached to the cuff. This is the systolic blood pressure. The diastolic blood pressure is viewed on the dial when the technician can no longer hear the pulsing sound with the stethoscope. These figures reflect the pressures being exerted on the arterial walls during the contraction and relaxation of the heart. Resting blood pressure of the healthy person averages around 120/80 mmHg.

A resting blood pressure of 140/90 is considered by health care professionals to be high blood pressure, or hypertension, and often relates to serious health problems, such as cardiac arrest, stroke, aneurysm, and kidney failure.

The Circulatory System

The system that allows the blood to flow through the heart, lungs, and body is called the circulatory system. The circulatory system consists of the blood-carrying vessels: (a) the arteries, (b) capillaries, and (c) veins. As a unit, these vessels produce a circuit of blood flow throughout the body. That is, they work together as a closed system to provide a specific function in circulation. Arteries carry blood away from the heart to capillaries, which work as exchange vessels for nutrients and gasses, and veins transport blood from the capillaries back to the heart. Figure 6-4 illustrates the blood flow pattern of the circulatory vessels.

Pulmonary Circulation

As previously stated, arteries constitute a major part of our blood transportation system and function to direct blood flow away from the heart. With each ventricular contraction, blood is pumped into the largest arteries (the aorta and pulmonary arteries). The pulmonary arteries receive deoxygenated blood from the right ventricle and direct it to the lungs where carbon dioxide is exchanged for fresh oxygen. This newly oxygenated blood leaves the pulmonary circulation, returns to the heart (left atrium and ventricle), and is then forced through the aorta, to the body, by a network of arteries. The primary arteries of this network are the carotid arteries in the neck and head, the abdominal arteries, and the auxillary and iliac arteries of the arms and legs, respectively. A great number of smaller arteries branch off the large arteries, each yielding smaller and smaller units until the blood passes into the smallest arteries, called arterioles. The elastic, muscular structure of arterial walls is capable of expansion and contrac­tion to regulate a smooth continued blood flow.

Exchange Vessels

Once again, the purpose of the pumping action of the heart and transportation mechanisms of the circulatory system is to exchange oxygen and other nutrients for waste products. This exchange takes place in microscopic vessels, called capillaries, which connect the arterioles to the smallest branches of the veins, the venules.

The structure of the capillaries is significant in that the single-celled composition of their walls allows for an easy transfer of materials to nearby tissue cells. It is here at the capillary level that the blood gives up its oxygen, food, and fluids to the tissues and the tissues give up carbon dioxide and fluid wastes to the blood. The blood leaving the capillaries, now laden with waste products and oxygen poor, returns to the heart through the venous system.

Venous System

The veins complement the arteries in function. Structurally, veins resemble the arteries, however, they are thinner walled and less muscular. The venous blood is returned to the heart under low pressure and is often forced to move against gravity. To keep a steady blood flow to the heart, veins contain a one-way system of valves that prevent backflow of blood. Additionally, the massaging action of the muscles in the legs and arms helps move blood back to the heart (venous return). This is why we must include a cool-down period after vigorous exercise. The muscular action will help prevent the blood from pooling in the extremities. Blood return begins in the venules, the smallest venous unit. The venules combine to form larger structures facilitating blood return to the right atrium through the largest veins, the superior and inferior vena cava. The blood is now returned to the heart and the circulatory process repeats itself.

Pulmonary system

Of all systems required to maintain life, the pulmonary system is one of the most crucial. We can survive for weeks without food, days without water, but live only a few minutes without oxygen. The exchange of gasses, such as oxygen and carbon dioxide, is of the utmost importance. It is the pulmonary, or respiratory, system that provides these life-sustaining processes. Assisted by numerous integrated body systems, the pulmonary system is responsible for providing two major functions: (a) air distribution, and (b) gas exchange. Additionally, the pulmonary system effectively filters, warms, and humidifies the inhaled air. Organs associated with the pulmonary system also produce sound, speech, and provide us with a sense of smell.

The pulmonary system  is divided into two major components: (a) the conducting airways, and (b) the functional unit. The elements of these systems will be examined as we trace an inhalation through the pulmonary system. The conducting airways consist of the mouth, nose, pharynx, larynx, and the primary branches of the bronchial tubes.

Air enters the body through the mouth or nostrils into the nasal cavity where it is warmed and humidified.

Coduction system of the heart.

The oral and nasal passages lead to the throat, or pharynx. The pharynx allows for passage of air into the lungs. After passing through the pharynx, the inspired air enters the larynx. The larynx is composed of pieces of cartilage, the largest of which is known as the Adam’s apple. The larynx is often referred to as the voice box because the production of sound occurs as air passes the vocal cords which stretch across the interior of the larynx. Another cartilaginous structure found in the larynx is the epiglottis. The epiglottis partially covers the opening in the larynx and closes during swallowing to prevent food passage into the trachea. The epiglottis, or glottis, is the structure involved in the potentially dangerous Valsalva maneuver.

Valsalva Maneuver

The Valsalva maneuver occurs when a person deeply inhales and then holds his/her breath during strenuous activity, as in lifting weights or shoveling snow. The glottis is closed against pressure. This causes an increased thoracic pressure which interrupts venous return to the heart, blood flow to the coronary arteries, and oxygen supply to the brain. In healthy individuals, this may result in dizziness, slowing of the heart beat, or a temporary loss of consciousness. For the individual predisposed to cardiovascular disease, the Valsalva maneuver could trigger cardiac arrest and result in death. Therefore, proper breathing techniques are essential during heavy exercise.

Passage of Air

The air flows from the larynx into the trachea, or windpipe, which connects the larynx to the bronchi in the chest cavity. The trachea is structurally protected by cartilaginous rings. Sometimes, however, a blockage of the trachea occurs, such as in choking. The lifesaving Heimlich maneuver can be used to free the trachea of obstructions caused by food or other foreign bodies. The Heimlich maneuver is an easily acquired skill and, like CPR, should be learned by all professionals in the health/fitness field.

In the chest cavity, the trachea branches into two main bronchi, the right and the left bronchus, which travel into the respective lungs. In the lungs, the bronchus develops into smaller passageways for air known as the bronchioles. At this site, the functional unit of the pulmonary system begins. The functional unit of the pulmonary system includes the bronchioles and alveoli, alveolar sacs within the lungs. The structure of this unit, as seen in Figure 6-5, resembles that of an upside down tree as it branches out into the terminal structures.

Pulmonary Airways

The bronchioles, found within the lungs, consist primarily of smooth muscle and elastic tissue in the walls. Excessive spasm in the smooth muscles of bronchioles creates breathing difficulties and associated diseases, such as asthma. Decreasing into respiratory units, the bronchioles lead to tiny tubes, or alveolar ducts. The ducts attach to a cluster of grape-like structures called alveolar sacs. The alveolar sacs are composed of millions of alveoli. The alveoli cover a large surface area and are extremely thin walled. The number, structure, and proximity of the alveoli to the structurally similar pulmonary capillaries allow for an efficient diffusion of gasses between air and blood. It is in the lungs that inhaled oxygen passes through the alveoli and enters the blood in the nearby capillaries. Some of the oxygen is absorbed in the blood, but most of it combines with the protein molecule called hemoglobin of the red blood cell. Oxygen is carried within the red blood cells to the tissues. Hemoglobin then releases oxygen to the tissues in exchange for carbon dioxide. Carbon dioxide is transported back to the alveoli for removal during exhalation.

Blood flow pattern of the circulatory vessles.

The lungs are a pair of pine cone-shaped organs that lie within the chest cavity, one on either side of the heart. They are well protected by the surrounding structures: (a) the ribs, (b)intercostal muscles, (c) sternum, (d) spine, and (e) diaphragm. A thin layer of moist membranes, called pleura, covers the lungs and lines the chest cavity, which allows for smooth inflation/deflation of the lungs.The protective structures surrounding the lungs also provide the mechanics of breathing. The breathing process begins with the respiratory center of the brain. Nerve impulses signal the muscles of respiration, the diaphragm and the intercostal muscles, to contract. The diaphragm is the pair of tent-like muscles that separates the lung cavity from the abdominal cavity like a bellows used to fan the flames of a fire. As the diaphragm contracts, it moves downward and increases the volume within the chest cavity. Contraction of the intercostal muscles pulls the ribs outward, causing further enlargement of the cavity. A vacuum is then created within the space and the negative pressure draws in the outside air. Exhalation occurs as the muscles relax and reverse the process, causing the lungs to contract and force air out.

The pulmonary system.

The total amount of air exchanged between the body and the atmosphere per minute is referred to as minute ventilation. At an average of 12 ventilations per minute, approximately 6 liters of air are exchanged per minute. During exercise, the demand for oxygen and amount of carbon dioxide to be removed increases. The respiratory center in the brain responds to the stimuli, and consequently increases the rate and depth of ventilations accordingly.

After a thorough analysis of the components of the cardiopulmonary system, we can better comprehend how the system functions collectively. In summary, air is inhaled from the atmosphere, through the conducting airways to the functional unit of the lungs. A gaseous exchange occurs between the alveoli and capillaries of the lungs. Carbon dioxide returns to the lungs and is exhaled. The newly inspired oxygen travels within the red blood cells from the lungs to the left side of the heart and is pumped throughout the body via the arterial circulation. Exchange of oxygen, carbon dioxide, and nutrients in the tissues occurs throughout the body at the capillary level. Oxygen-poor blood is then returned through the venous system to the right side of the heart. The heart contracts and forces blood back to the lungs whereby the process repeats itself.

How the Cardiopulmonary System Meets the Demands of Exercise

Increased Heart Rate

Given an average resting heart rate (RHR) of 70 beats per minute (bpm), the heart can comfortably (assuming average fitness and without disease) perform at least twice its resting values. Aerobic exercise is generally performed between 40-85% of heart rate reserve (HRR). This is determined by subtracting your age from 220 minus RHR x % HRR plus RHR. The average adult at 20 years of age, for example, can comfortably train between 122-181 bpm.This is a remarkable performance for such a small organ.

Increased Stroke Volume

The amount of blood pumped by the heart per beat can increase as much as 50-60% above resting values to meet the physiological demands of exercise. The tremendous increases in stroke volume are a result of fitness (adaptation) and are less significant in the untrained exerciser. That is, increased stroke volume is a training effect of aerobic exercise and allows the fit individual to pump more blood per beat, resulting in a lower heart rate for a given workload.

Increased Cardiac Output

Cardiac output (Q), the amount of blood pumped by the heart per minute, is a product of heart rate (HR) times stroke volume (SV), that is, Q =HR x SV. The average adult heart at rest pumps approximately 5 liters of blood per minute. The cardiac output, in response to an exercise stimulus, can increase to almost eight times its resting values. The increases are found within both the heart rate and stroke volume and depend tremendously on the efficiency of the system, that is to say, fitness. The sedentary individual will typically exhibit a cardiac output of 20-22 liters per minute during maximal exercise (four times resting values), whereas the elite athlete is capable of increases almost eight times the resting values, or 35-40 liters per minute.

 Vasodilation and Vasoconstriction

An extraordinary physiological adaptation of the circulatory system is the ability to regulate direction of blood flow. The vessels have the capacity to constrict or dilate in order to redistribute blood flow to meet the physiological demands. During exercise, blood flow is diverted away from tissues that are less metabolically active, such as internal organs, and redirected to the active muscles. In fact, depending on the intensity of the exercise, as much as 88% of the blood flow is directed to the muscles during exercise.

Increased Extraction of Oxygen

Not only is the body capable of directing blood flow to the active tissues, but the ability to extract oxygen from the blood increases with exercise. Oxygen extraction at the capillary level increases from an average of 25% at rest to as much as 85% during exercise-another amazing adaptation.

Vital Capacity

Vital capacity is defined as the greatest volume of air voluntarily moved in one breath, either during inhalation or exhalation, and the sum of the tidal volume and inspiratory/expiratory reserve volumes. Tidal volume is the amount of air inhaled or exhaled in an average breath. Reserve volume refers to the excess volumes that may be used in forceful inspiration or expiration. Research suggests that vital capacity is primarily based on body size and is not significantly influenced by training. (I,S) We do, however, increase the percentage of the vital capacity used during exercise. Figure 6-6 demonstrates the various lung capacities.

Lung volumes.

Increase in Respiratory Rate

The rate of breathing plays a crucial role in delivery of oxygen during exercise. At rest, the adult averages 12 breaths per minute, compared to an exercise ventilatory rate of 35-40 breaths per minute, or an unbelievable rate of 60-70 breaths per minute of the elite athlete. Considering both volume of air and ventilatory rate, we can observe increases in minute ventilation from 6 liters/min to 100 liters/min with exercise (or more).


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