Together, the heart, blood vessels and blood are referred to as the cardiovascular system. This article looks at the cardiovascular system, and how it is changed during exercise, or as a result of increased fitness levels.

The Heart

The heart is a muscular pump, designed with the sole function of transporting blood around our circulatory system via a complex sequence of electrical contractions.

A healthy adult may have a heart rate averaging 60-70 BPM, a trained athlete could be half that rate. At 70 BPM, the heart will beat over 100,000 times a day or almost 37,000,000 beats a year. In one minute, the human heart will pump more fluid than a kitchen tap that is wide open.

A simplistic description of the heart is as follows. It is a hollow muscular pump, about the size of a fist, located slightly to the left side of the chest. It is divided into four chambers, and into two distinct halves. The upper two chambers are called atria, and the lower two chambers are called ventricles. The left side of the heart collects oxygenated blood from the lungs and pumps it around the body, while the right side collects deoxygenated blood from the body and pumps it through the lungs, where carbon dioxide is expelled and oxygen is absorbed into the blood.

A more complex analysis of the heart could result in it being described as four separate pumps within one vessel. The atria are priming pumps, ensuring a constant blood flow is available for the more powerful ventricles prior to their contraction and ejection of the blood.

An arterial pulse is feeling the pump of blood through the arteries during the momentary contraction phase of the heart muscle. This is known as the systole period of the cardiac cycle. The resting phase is called the diastole period. With each beat, the heart pumps the blood through two systems – the systemic circulation supplying the body, and the pulmonary circulation supplying the lungs.

The Cardiac Cycle (heartbeat)

The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:

• Diastole – Ventricles are relaxed.
• Systole – Ventricles contract.

During the diastole phase the atria and ventricles are relaxed and the atrioventricular valves are open. De-oxygenated blood from the superior and inferior vena cava flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The Sino Atrail node (pacemaker) contracts triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.

During the systole phase the right ventricle receives impulses from the Purkinje fibres and contracts. The atrioventricular valves close and the semilunar valves open. The de-oxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle.

The pulmonary artery carries the blood to the lungs. There the blood picks up oxygen via a process known as gaseous exchange and is returned to the left atrium of the heart by the pulmonary veins.

In the next diastole period, the semilunar valves close and the atrioventricular valves open. Blood from the pulmonary veins fills the left atrium. (Blood from the vena cava is also filling the right atrium.) The SA node contracts again triggering the atria to contract. The left atrium empties its contents into the left ventricle. The mitral valve prevents the oxygenated blood from flowing back into the left atrium.

During the systole phase the atrioventricular valves close and the semilunar valves open. The left ventricle receives impulses from the Purkinje fibres and contracts. Oxygenated blood is pumped into the aorta. The aortic valve prevents the oxygenated blood from flowing back into the left ventricle.

The aorta branches out to provide oxygenated blood to all parts of the body. The oxygen depleted blood is returned to the heart via the vena cava

The blood vessels

An adult human body contains around 100,000 km of blood vessels. The vessels responsible for transporting oxygenated blood from the heart are called arteries. The main artery leaving the heart into the systemic circulation is called the aorta.

These arteries sub-divide into arterioles and finally to capillaries. The walls of capillaries are so thin that they permit the exchange of gases within the working muscles. Oxygen is used to ‘burn’ the fuel in the body, and carbon dioxide is returned to the bloodstream.

Deoxygenated blood returns to the heart though veins. Veins have a series of one way valves to improve the flow back to the heart. The major veins returning to the heart are called the inferior and superior vena cava.

The pulmonary artery is the only artery that carries deoxygenated blood, when it transports deoxygenated blood from the heart to the lungs. Once the deoxygenated blood reaches the lungs, carbon dioxide is exchanged for oxygen and the oxygenated blood returns to the left atrium via the pulmonary vein.

The heart also requires its own elaborate circulation system, and this coronary circulation system requires around 5% of the total blood flow of the heart. Lack of exercise, perhaps coupled with a poor diet can result in lack of efficiency of the coronary circulation system, eventually leading to heart disease or heart failure.

Basic Heart Function Terminology

• Heart Rate is the number of times the heart beats in one minute.
• Stroke Volume is the quantity of blood expelled with every heartbeat.
• Cardiac output is the quantity of blood pumped through the aorta in one minute. Cardiac output is calculated by multiplying stroke volume by the number of beats per minute.

Exercise and the cardiovascular system

Progressive long-term training plans will force adaptations in the heart and related cardiovascular system. The nature of this adaption will be determined by the stresses under which the tissues have been placed. A few of the common and obvious adaptations are listed below…

Ventricular Hypertrophy

The heart will increase in size and mass with intensive exercise. This is known as ventricular hypertrophy and is a result of an increase in the thickness of the myocardium in the ventricles. The hypertrophy is an adaption to the increased frequency and force of ventricular contractions required during intensive exercise. Ventricular hypertrophy is particularly noticeable around the left ventricle chamber, which experiences the greatest pressure of the oxygenated blood being expelled around the body.

Stroke Volume

Stroke volume is increased through exercise as ventricular stretch (capability of the ventricle to enlarge) is increased. Once the ability of the ventricles to stretch is improved, it leads to a more powerful ventricular contraction due to a shift in the pressure changes in the ventricular chambers. This is also referred to as Preload or Frank-Starling mechanism. The ability to stretch in synergy with ventricular contractility increases both the amount of blood allowed into the ventricle and the force with which it is expelled – the heart is a muscle, and like all muscles that are trained has the capacity to contract with greater force (thus ejecting more contained blood). This increase in stroke volume leads to a reduction in heart rate as the efficiency of the heart has improved.

Bradycardia

The increase in stroke volume leads directly to a condition known bradycardia. In medical terms, bradycardia is a heart rate of less than 60 beats per minute and can be considered dangerous. This is not the case with exercise-induced bradycardia, where the cardiac muscle tissue has adapted over a long period to time to work more efficiently, resulting in a reduction in heart rate.

Peripheral Vascular Resistance

Aortic and pulmonary artery blood pressure is reduced as a result of exercise – healthy blood vessels have better vasodilation, reducing peripheral vascular resistance and allowing blood to travel much more efficiently through the systemic circuit. The further and more effectively the blood can travel through the circulatory system, the less work the heart has to do in order to upkeep oxygen and nutrient supply to the tissues. Trained individuals have enhanced blood return to the heart thanks to valve changes and a reduction in peripheral vascular resistance, which allows the heart to pump more blood with less beats.

Additional Exercise Benefits

The physiological adaptations to exercise are not restricted to the heart. The entire cardiovascular system evolves over time to become more efficient, including the muscular and valve actions in the venous system which returns the de-oxygenated blood to the heart. The reduction in peripheral vascular resistance has a positive knock-on effect on the rest of the cardiovascular and circulatory systems, easing the pressure on the heart. Included in circulatory adaptations is a process called capilliarisation, which is an increase in the amount of capillaries in the various body tissues. This process facilitates and improves the delivery of oxygen and nutrients across the body.

The amount of fluid which flows though a vessel is calculated by a complex formula which includes the pressure, the viscosity of the fluid and the diameter of the blood vessel. When an athlete is working hard, the capillaries can distend up to five fold. All of these increases together could lead to an increased blood flow through that capillary of up to an astonishing 625 times that of the flow at rest. Although the flow volume increases, the increased diameter of the capillary ensures that the velocity of the blood remains slow enough to permit gaseous exchange to take place.

In addition, there is a comparable increase in the respiratory exchange ratio, to expel the waste product of carbon dioxide. Another waste product – heat – is dispersed via the evaporation of perspiration.

Heart disease

A healthy heart will pump without fail until the moment of death. There are a number of factors which can lead to disease in the heart, resulting in problems, possibly heart attacks and premature death. Heart disease remains the biggest killer in the western world, with around 50% of dying due to heart disease.

The major risk factors are:

• Obesity
• Lack of exercise
• High cholesterol
• Smoking
• High blood pressure
• Diabetes mellitus
• Genetic defect
• Male (up until around the age of 70).

Genetic factors aside, heart our chances of developing heart disease can be hugely reduced by following a progressive exercise routine and a balanced diet. All of the exercise and dietary information you would need to keep your heart in good condition can be found on this website.

To summarise, the heart is a complex organ that silently performs a vital function in the maintenance of life. There are clear and important reasons for a regular training regimen to be adhered to in order to keep the heart functioning as effectively as possible. For all information on exercise selection and advice, refer to the training sections of this website.

Tweet This Post