Cardiorespiratory System and Cardiovascular Training

The Cardiorespiratory System: Introduction

The term 'cardiorespiratory' refers to the heart (cardio) and lungs (respiratory). This section explores the heart, lungs, and cardiovascular system (heart and blood vessels).

Objectives

By the end of this section, you should be able to:

• Describe the structure and function of the heart
• Describe the location, structure, and function of the heart valves
• Explain coronary circulation
• Describe the circulatory system
• Explain how disease affects the circulatory system
• Define blood pressure classifications and associated health risks
• Describe the respiratory system
• Explain long and short term effects of exercise on the circulatory and respiratory system
• Describe the benefits and risks associated with cardiovascular training

The cardiovascular and respiratory systems are responsible for the transport and uptake of oxygen $(O_2)$ in the body. The cardiovascular system refers to the heart and circulation. The respiratory system refers to the lungs.

Overview of the Cardiovascular System

The cardiovascular system consists of the heart, blood vessels, and blood. The heart is the pump which circulates blood around the body. The blood is circulated through a network of blood vessels called arteries, veins, and capillaries.

The process begins with the heart pumping blood to the lungs so that waste products such as carbon dioxide can be removed and oxygen $(O_2)$ can be collected and distributed around the body.

Once carbon dioxide has been removed and oxygen refuelling has occurred, the blood is carried from the lungs to the heart, ready to be pumped around the body.

The heart acts as a dual pump for two circulatory systems:

1. Pulmonary circulation (circulation in and out of the lungs)
2. Systemic circulation (circulation throughout the body)

The Heart

The heart is located posterior to the sternum, just left of centre in the chest. It is about the size of a clenched fist and is responsible for pumping blood around the body. The heart consists of four chambers: two upper atria and two lower ventricles. The walls of the four chambers are made of cardiac muscle, called the myocardium.

Coronary arteries, which surround and pierce the heart, deliver oxygen-rich blood to the myocardium, which is necessary for the muscular contraction of the heart.

The atria receive blood from the body or lungs and the ventricles pump blood to either the body or lungs.

The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The left ventricle pumps oxygenated blood through the aorta (the largest artery in the body) to the rest of the body. The walls of the left ventricle are thicker than those of the right, enabling it to contract more forcefully.

Two large veins (inferior and superior vena cava) return de-oxygenated blood from the body to the right atrium. When the right ventricle contracts, de-oxygenated blood is pumped to the lungs.

Heart valves between the atria and ventricles, aortic and pulmonary arteries determine the pathway of blood through the heart and stop the backward flow of the blood.

Heart Valves

The circulation of blood through the chambers of the heart is maintained as a one way system by a series of non-return valves which prevent any backflow of blood.

There are two valves between the atria and ventricles - the atrioventricular (AV) valves.

• The right AV valve is the tricuspid valve and prevents backflow of blood from the right ventricle to the right atrium
• The left AV valve is the bicuspid or mitral valve and prevents backflow of blood from the left ventricle to the left atrium.
• Both are attached to the ventricle walls by collagen cords, the cordae tendinae (the heart strings).
• As blood flows from the atria into the ventricles the valves are loose and fall open into the ventricle chambers. When the ventricles contract the pressure created forces the valves upwards and together to shut and stop any blood being pushed back into the atria.

The other two valves are located between the ventricles and the arteries into which they pump blood. These valves are known as the semilunar valves because they are composed of three moon shaped crescents.

• The pulmonary valve is located between the right ventricle and the pulmonary artery and prevents backflow of blood from the pulmonary artery into the right ventricle.
• The aortic valve is located between the left ventricle and the aorta and prevents backflow of blood from the aorta into the left ventricle.
• As the ventricles fill up, the semi lunar valves remain closed to prevent arterial backflow into the ventricles. As the ventricles pump, the pressure causes the semi lunar valves to flatten against the walls of the arteries opening to them allow blood to exit the heart.

Heart valves can become damaged, for example by infection, causing them to stiffen and narrow (stenosis). This leads to the heart working harder to get blood out and it can eventually fail. Advanced stenosis requires synthetic valve replacement.

The Vascular/Circulatory System

The vascular, or circulatory, system consists of the arteries and veins through which the heart pumps blood throughout the body.

The prime purpose of the vascular system is the transport and exchange of materials (oxygen, carbon dioxide, nutrients, hormones, heat, metabolic waste products and protective white cells) between the blood and tissues, which takes place in the capillaries.

Blood

Blood is a mixture of blood cells carried in a liquid called plasma. Two types of blood cells are particularly important. Red blood cells transport oxygen, and white blood cells are important to the immune system. Blood also carries nutrients, hormones and medicines.

Blood Vessels

Arteries

The arteries carry blood from the heart to the capillaries. Arteries become progressively smaller, becoming arterioles as they reach the tissues.

Arteries are characterised by their thick muscular walls, made up of smooth muscle and elastic connective tissue, which assist in the maintenance of blood pressure. Following ventricular contraction, the pressure and recoil of the artery wall contributes to blood pressure. The pressure in the large arteries is very high, but diminishes as the arteries become smaller and get further away from the heart. Without this pressure, blood would gravitate to the lowest parts of the body. Blood pressure enables the circulating blood to move upwards against gravity to supply blood to the brain, as well as forcing blood through the tiny capillaries.

In general, the ‘A’ in arteries is useful to remember as ‘Away’ from the heart. Arteries transport oxygenated blood away from the heart. The only exception to this rule is the pulmonary artery, which transports de-oxygenated blood to the lungs, but it still carries blood away from the heart.

Veins

Veins carry blood under low pressure from capillaries back to the heart. Smaller veins are called venules. The walls of veins are made up of smooth muscle and connective tissue. The key ways in which veins differ from arteries are that they have thin fibrous walls and one-way valves at regular intervals to prevent the backflow of blood.

The pulmonary vein transports oxygenated blood from the lungs to the heart; all other veins carry de-oxygenated blood.

Venous return is the amount of blood that is returned to the heart by the veins. The heart can only pump the blood that it receives. If venous return decreases, the heart contracts less forcefully, causing a decrease in blood pressure.

Capillaries

Capillaries carry blood from arterioles to venules. The capillary walls are only one cell thick. It is in the capillaries that the exchange of materials between the blood and tissues takes place. The process by which this exchange occurs is called diffusion.

With increasing age (as well as some illnesses and disabilities), blood vessels become less elastic and therefore less efficient carriers of blood. One example of this is when vessels become susceptible to atherosclerosis. This is when the inner walls of arteries become coated in places with atheromatic plaques: cholesterol-containing material that occludes the vessels, preventing efficient blood circulation. Since the functioning of all tissues is dependent on good blood circulation, these factors have widespread effects on this system.

Diffusion

Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration. Within the body, oxygen and carbon dioxide move by diffusion. An example of this is in the lungs, where there is a low concentration of oxygen in the blood of the pulmonary capillaries. The opposite is true for carbon dioxide. In this case, oxygen diffuses into the capillary and carbon dioxide diffuses across the capillary membrane and into the lungs. In each case, the gases move from where there is more to where there is less.

Diffusion that occurs in the exchange of gases (oxygen and carbon dioxide) within the lungs is also known as gaseous exchange.

In the capillaries within muscles, oxygen diffuses from the blood (high concentration) to the tissues (low concentration), while carbon dioxide diffuses from the tissues (high) to the blood (low) to be brought back to the lungs and exhaled.

Circulation

The vascular system has two major pathways of circulation. These are pulmonary (lungs) and systemic (body).

Pulmonary Circulation

Pulmonary circulation is the flow of blood from the right side of the heart to the lungs and then back to the left side of the heart.

The right ventricle pumps de-oxygenated blood via the pulmonary arteries to the lungs. Pulmonary diffusion then takes place in the lungs. Carbon dioxide is exhaled and oxygen is inhaled. This exchange takes place via the pulmonary capillaries. The capillaries unite to form venules and then veins and finally to form the two pulmonary veins from both lungs that return oxygenated blood to the left atrium.

Systemic Circulation

Systemic circulation is the flow of blood from the left side of the heart to all parts of the body and then back to the right side of the heart.

The left ventricle pumps blood via the aorta into the arteries, arterioles and capillary networks throughout the body. The capillaries merge to form venules and veins. The veins return blood to the right atrium.

Key Points of the Vascular System

• The vascular system transports and exchanges materials (oxygen, carbon dioxide, nutrients, hormones, heat, etc.) between blood and tissues
• Arteries have thick muscular walls and, in general, transport oxygenated blood from the heart to the capillaries
• Veins have thin muscular walls and, in general, transport de-oxygenated blood, under low pressure, from capillaries back to the heart
• Oxygen and carbon dioxide transfer occurs in the lungs and tissues by diffusion
• Pulmonary circulation is the movement of blood from the right side of the heart to the lungs and then back to the left side of the heart
• Systemic circulation is the movement of blood from the left side of the heart to the body and then back to the right side of the heart

The Coronary Arteries

The heart is a muscle and it requires a blood supply that is rich in oxygen in order for it to contract. The blood supply for the heart is supplied by the coronary arteries. These arteries keep the myocardium supplied with oxygen.

There are two coronary arteries; one on the left side of the heart and one on the right side.

The left coronary artery has two main branches:

• The left anterior descending artery supplies the anterior portion of the left atrium and ventricle with oxygenated blood.
• The left circumflex artery supplies the posterior portion of the left atrium and left ventricle with oxygenated blood.

The right coronary artery supplies the right atrium and right ventricle with oxygenated blood. Each of these arteries has many other branches.

The myocardium has virtually no anaerobic capacity so it relies on the oxygen-rich blood supply from the coronary arteries to function. The diastole phase (recovery phase of the cardiac cycle) is when the arteries are able to fill with oxygen-rich blood most effectively. The time for this phase decreases with higher heart rates.

During the pumping phase (cardiac systole), the coronary arteries are compressed, restricting blood flow to them.

Once the blood has passed through the capillary beds of the myocardium, the blood flows into the coronary veins, which join together to form the coronary sinus. This blood vessel empties blood into the right atrium, where it joins the deoxygenated blood returning to the lungs for re-oxygenation.

With regular cardiovascular activity, both resting and working heart rates decrease. This leads to increased time for diastole and allows for greater filling of the coronary arteries. This improves coronary blood flow to the myocardium both at rest and during activity.

The Cardiac Cycle

The cardiac cycle is the sequence of events in one beat of the heart (the alternate contraction and relaxation of the heart). The contraction phase is called systole, which causes a volume of blood to be pumped to the arteries. When the heart relaxes, this is called the diastole and blood flows into the heart from the veins. This cycle of events helps to keep the blood moving from the veins through the heart and to the arteries.

Heart Rate

A healthy adult has a resting heart rate of between 60–80 beats per minute (bpm), whereas a well-conditioned individual tends to have a resting heart rate of around 35–50 bpm. The heart is a muscle and, just as skeletal muscles get stronger when exercised, so does the heart. The muscular walls of the left ventricle can sometimes increase in size, this is called cardiac hypertrophy.

Stroke Volume

The stroke volume is the amount of blood pumped by a ventricle per heartbeat. A normal stroke volume at rest is between 70–80ml per beat. In well-trained individuals, the stroke volume at rest is 100–110ml per beat. During exercise, the stroke volume increases. In well-trained individuals, values as high as 200ml per beat have been recorded.

Cardiac Output

Cardiac output is the amount of blood pumped out by the ventricles in one minute. During exercise, cardiac output must increase to meet the body’s demand for oxygen.

A simple formula enables cardiac output to be determined:

$\text{Cardiac output} = \text{stroke volume} \times \text{heart rate}$

If an average stroke volume is 71ml, and an average resting heart rate is 70 bpm, then: Cardiac output = 71ml x 70 bpm = 4,970ml per minute, or nearly 5 litres of blood

As previously indicated, a key adaptation to a regular endurance exercise programme is that the resting heart rate decreases and stroke volume increases. For example, a well-trained individual whose stroke volume is 100ml and heart rate is 50 bpm has a cardiac output of:

$100 \text{ml} \times 50 \text{bpm} = 5,000 \text{ml}$

Cardiac output can be maintained with fewer beats in such individuals.

Circulatory Disease

There are two main forms of disease that can affect the arteries.

• Arteriosclerosis - This is a progressive degeneration of the arterial walls that leads to hardening and loss of elasticity. The ability of arteries to relax and allow more blood to pass through is decreased and often leads to high blood pressure. It is associated with ageing and is sometimes known as hardening of the arteries.
• Atherosclerosis - The progressive narrowing of arteries caused by circulating fats (lipids) being deposited in the lining of the artery walls that have become rough. These plaque/fatty deposits are known as atheroma. Calcium is also laid down around these deposits further leading to arteriosclerosis.

As the arteries narrow it becomes increasingly difficult to meet the oxygen demand of the organs and ]muscles. When the demand for oxygen outweighs the supply- for example when an individual exercises - the muscle or organ is deprived of oxygen and this results in pain.

When the coronary arteries are affected by arteriosclerosis and atherosclerosis it is known as coronary heart disease (CHD). The imbalance of oxygen demand and supply is known as myocardial ischaemia and typically results in chest pain commonly known as angina.

Risk Factors

There are several risk factors that will affect an individual's risk of developing circulatory disease.

Risk factors include:

Non-Lifestyle Factors

• family history
• age

Lifestyle Factors

• smoking
• environment
• physical inactivity - sedentary lifestyle
• diet e.g., high levels of saturated fats and cholesterol
• hypertension
• type 2 diabetes
• obesity/overweight
• high blood cholesterol (blood lipid profile ratio of high density lipoproteins and low density lipoproteins)
• stress
• hormone replacement therapy

Simple lifestyle changes can help decrease the risk of disease, for example increasing physical activity. A long- term adaptation to regular endurance exercise is a decrease in resting and exercising heart rates. This increases the time for diastole and improves coronary blood flow allowing oxygen demands of the myocardium to be met more effectively.

Controlling body weight, giving up smoking, eating a healthy diet low in saturated fats and managing stress levels are all ways to help reduce the risk of circulatory disease.

Blood Pressure

Blood pressure is the pressure exerted by the blood on the walls of the arteries. It is measured in millimetres of mercury (mmHg) and is expressed with two different readings, for example, 120/80 mmHg:

• Systolic pressure - The contraction phase of the myocardium
• Diastolic pressure - The relaxation phase of the myocardium

Classifications of Blood Pressure Measurements:

Hypotension

This is low blood pressure and is a reading of <90 systolic over< 60 diastolic.

Hypotension is not generally considered a health risk but could be indicative of other health issues. It is most common in those who are young, female, of slight build, fit and vegetarian.

Normal

90-120 Systolic over 60-80 Diastolic

Normal blood pressure indicates a healthy cardiovascular system, with sufficient pressure to supply all of the organs with blood, but not too high to cause damage.

Pre High Blood Pressure

120-140 Systolic over 80-90 Diastolic

Pre high blood pressure is not a contraindication to exercise, but does suggest that lifestyle changes should be advised to prevent the condition worsening.

Hypertension

140 Systolic over >90 Diastolic

Hypertension is very common. It is most likely in those who are older males, overweight, unfit, stressed and have a poor diet. Chronic hypertension results in a constant strain and battering of the smooth inner artery walls, which is associated with an increased risk of coronary heart disease, stroke, kidney dysfunction and some forms of dementia. High blood pressure often accompanies atherosclerosis and arteriosclerosis.

By managing blood pressure with a healthy lifestyle - and in some cases medication - the risk of associated disease can be decreased.

Blood Pressure Responses to Exercise

The Valsalva Effect

The Valsalva effect/manoeuvre is the holding of breath during exertion. It has been associated with heavy weight lifting. It is performed by moderately forceful attempted exhalation against a closed airway, usually done by closing one's mouth, pinching one's nose shut while pressing out as if blowing up a balloon. This can cause undesirable fluctuations in blood pressure which can be dangerous for those with cardiovascular disease. It should also be avoided during pregnancy.

Short Term (During Exercise)

• Systolic blood pressure increases proportionately with exercise intensity. Heavy resistance training will increase it most and therefore is not recommended for those with high blood pressure.
• Diastolic blood pressure remains largely unchanged.

Long Term (After Weeks/Months)

• In those with mild hypertension, resting and exercising blood pressures can be reduced by up to 10 mmHg with endurance-type cardiovascular activities.
• The time for diastole is increased improving coronary blood flow.

Cardiovascular Responses to Exercise

The adaptations that occur in the cardiovascular system all result in an increased efficiency of oxygen and blood supply at rest and during exercise.

Short Term (During Exercise)

• Heart rate increases in proportion to intensity; this enables higher levels of oxygen to be delivered to working muscles and more carbon dioxide to be removed. When exercise progresses past 30 minutes the heat related loss of water and electrolytes from the blood results in a steady upward drift of heart rate (cardiac drift). Adequate hydration before and during exercise can help minimise cardiac drift.
• Stroke volume increases
• Cardiac output increases by as much as 20 l/min in a sedentary individual and 40 l/min for trained individuals
• Blood flow is diverted away from areas such as the intestines by vasoconstriction (narrowing) of the arteries to divert blood to the working muscles to meet the increased oxygen demands of activity.
• Vasodilation widens the arteries and arterioles supplying the muscles to enable an increased blood flow. Vasoconstriction narrows the arteries, decreasing the blood supply to the intestines. Sphincter valves open and close capillary beds in the muscles.
• Increased oxygen supply to meet demands of exercise
• Improved 'tone' in the smooth muscle of the artery walls increasing their ability to expand and contract to move blood
• More efficient circulation

Long Term (After Weeks/Months)

• Heart rate decreases and post-exercise recovery times improve. A trained person's resting heart rate is typically 52 beats per minute, whereas an untrained average resting heart rate is around 72 beats per minute.
• Stroke volume increase due to stronger myocardium
• Increased cardiac output at a lower heart rate
• Improved blood supply to the myocardium due to increased time for diastole as a result of reduced heart rates at work and rest
• Increased blood volume
• Increased red blood cell count
• Increased coronary blood flow from approximately 250 cubic centimetres per minute to approximately 1000 cubic centimetres per minute
• Increased haemoglobin levels improving oxygen carrying capacity of the red blood cells
• Hypertrophy of the myocardium especially the left ventricle
• Improved blood cholesterol profile reducing the risk of CHD
• Increased capillarisation allowing for improved gaseous exchange in the muscles and lungs
• Decreased risk of coronary heart disease and other diseases such as type II diabetes

Benefits and Risks of Cardiovascular Exercise

Regular cardiovascular endurance exercise will bring about both long- and short-term changes in the heart, lungs and circulatory system.

Other benefits of cardiovascular exercise include:

• improvements in everyday function
• increases in bone density especially in areas placed under load from impact activities
• reduction of body fat and maintenance of body fat levels within healthy ranges
• decreased risk of CHD, type 2 diabetes and other diseases

While the benefits of activity are numerous, any mode of exercise carries with it associated risks. While careful programming and health screening can help to minimise these risks, they cannot always be avoided altogether. It is important to inform those considering participation in an exercise programme of both the benefits and risks involved.

These risks include:

• increased risk of muscle strain
• increased risk of skeletal/joint injury
• increased risk of connective tissue damage
• increased risk of muscle imbalances in certain modes of exercise e.g., tight hamstrings in runners and cyclists
• increased workload on the heart, which for some individuals is dangerous
• increased levels of carbon dioxide production
• increased lactic acid production
• decreased levels of body fat below recommended levels for some athletes, which can increase the risk of osteoporosis