Section 3: Cardiovascular and Respiratory Systems

The cardiovascular and respiratory systems combine to draw oxygen into the body, transport it to the tissues that need it and then remove any waste products such as carbon dioxide. This process is vital for the survival, maintenance and functioning of all the body’s tissues.

The cardiovascular system

The cardiovascular system (sometimes called the circulatory system) is comprised of the heart, blood vessels and blood. It is responsible for transporting oxygen and other important nutrients to the body’s tissues, including the working muscles, and for removing waste products, such as carbon dioxide.

Structure and function of the heart

The heart is essentially a muscular pump which pushes oxygen and nutrients around the body to the tissues. It is about the size of a man’s clenched fist and lies behind the sternum, just left of the centre. It is positioned between the right and left lungs.

The heart is a cardiac muscle. It is made up of thick muscular walls (myocardium) and divided into halves:

  • The right half receives blood from the body and pumps it to the lungs.
  • The left half receives blood from the lungs and pumps it to the body.

There are four heart chambers in total:

  • The two upper chambers (atria) receive blood from the veins.
  • The two lower chambers (ventricles) pump blood into the arteries.

Internal anatomy of the Heart

Heart Valves

There are a number of different valves around the heart, which all perform slightly different tasks.

The atrioventricular (AV) valves separate the atria and ventricles and prevent the flow of blood back into the atria during ventricular contraction.

The semilunar valves prevent the flow of blood back into the right (pulmonary valve) and left ventricles (aortic valve) during the ventricular relaxation.

Heart Rate

The heart is stimulated to contract by a complex series of integrated systems. The heart’s pacemaker – the sinoatrial node (SAN) – initiates cardiac muscle contraction. The SAN is located in the wall of the right atrium. The myocardium (heart muscle) is stimulated to contract about 72 times per minute by the SAN as part of the autonomic nervous system.

Blood Pressure

Blood pressure is a measure of the force that the blood applies to the walls of the arteries as it flows through them.

Blood pressure is measured in millimetres of mercury (mmHg) and is expressed using two numerical readings. The optimal blood pressure reading is written as 120/80mmHg. These two numbers represent the systolic and diastolic blood pressures, respectively.

Systolic blood pressure (SBP) is the pressure exerted on the artery walls when the cardiac muscle is contracting and pumping blood. This is the higher of the two numbers and is usually noted first. It is caused by the rise in volume of blood flowing through the arteries with each beat, which increases the pressure within the arteries.

Diastolic blood pressure (DBP) is the pressure exerted on the artery walls when the heart is in a relaxed state. The heart goes through this period of relaxation (or diastole) to allow the chambers to fill with blood prior to contraction. The diastolic blood pressure is always the lower of the two readings.

Control of Circulatory Blood Flow

The blood vessels are able to narrow (vasoconstrict) or widen (vasodilate) because of the smooth muscle found in their walls. More or less blood will flow through them as a result. This enables the body to direct the flow of blood to different tissues, depending on where the oxygen and nutrients are required. It also plays a part in the regulation of blood pressure.

After a meal, the blood vessels that feed the digestive system are vasodilated and blood flow is increased to assisted digestion, while blood vessels feeding the muscles are vasoconstricted, reducing local blood flow. During exercise, the opposite happens; more blood is routed to the muscles and less is available to the digestive organs and smooth muscles of the digestive tracts.

Eating a large meal too close to a training session or match will not allow enough time for the food to be digested in the stomach, causing cramp and sometimes vomiting.

Blood pressure is an expression of the arterial blood flow and the peripheral resistance the blood encounters as it flows around the body. It can therefore be expressed in the following equation:

Blood pressure = Cardiac output X Total peripheral resistance

Cardiac output is the volume of blood pumped out by the heart in one minute (ml/min). The greater the cardiac output, the higher the blood pressure.

Total peripheral resistance is the resistance the blood vessels offer to blood flow. The greater the resistance, the higher the blood pressure. Peripheral resistance is increased by constriction or decreased by dilation of the blood vessels (arterioles).

The blood circulation is a closed system in which the pressure varies constantly. It rises to a peak at the height of the contraction of each heartbeat as the heart pumps blood out. It then falls to a lower level, which it reaches just before each heartbeat.

Optimal blood pressure for reducing the risk of cardiovascular disease (CVD) is below 120mmHg for systolic and 80mmHg for diastolic pressure (ACSM, 2017). Readings much lower than this may have some clinical significance but are not associated with increased CVD risk.

Readings above optimal level pose an increased cardiovascular risk. Readings above 180mmHg systolic and 110mmHg diastolic are a contraindication for exercise.

Category Systolic (mmHg) Diastolic (mmHg)
Low <100 <60
Optimal <120 <80
Normal <130 <85
High normal – Pre-hypertension 130-139 85-89
Stage 1 hypertension 140-159 90-99
Stage 2 hypertension 160-179 100-109
Stage 3 hypertension >180 >110

Circulation of blood

All mammals, including humans, have a double circulatory system. Blood passes through the heart on two occasions, once through the pulmonary circulation and once through the systemic circulation.

Pulmonary and systemic circulation

Pulmonary circulation is the circulation between the heart and lungs.

  • The pulmonary heart chambers are the left atrium and right ventricle.
  • The pulmonary blood vessels are the pulmonary artery and the pulmonary vein.

Systemic circulation is the circulation between the heart and the body.

  • The systemic heart chambers are the left ventricle and right atrium.
  • The systemic blood vessels are the aorta and the inferior and superior vena cava. The inferior vena cava carries blood from the lower body. The superior vena cava carries blood from the upper body.

Structure and function of blood vessels

As the name suggests, blood vessels are responsible for carrying blood around the body. There are various types of blood vessels, which are differentiated by their shape, size and function.

Blood vessel Structure Function
  • Thick, muscular walls.
  • Subdivide into smaller blood vessels called arterioles.
  • The largest artery is the aorta, which leaves the left ventricle carrying blood under the highest pressure.
  • Carry blood under high pressure away from the heart.
  • All carry oxygenated blood except the pulmonary artery.
  • Thinner walls than arteries with little muscle.
  • Subdivide into smaller blood vessels called venules.
  • Contain one-way valves to prevent blood from flowing in the wrong direction.
  • Carry blood towards the heart under low-to-moderate pressure.
  • All carry deoxygenated blood except the pulmonary vein.
  • Extremely thin walls (approximately one cell thick).
  • Link arteries to veins.
  • Significantly higher in number than arteries and veins.
Allow for diffusion of gases and nutrients throughout the body, including muscle tissues.

Components of blood and their function

Blood is the substance that carries nutrients and oxygen to all structures of the body and removes waste products and carbon dioxide. Blood is composed of a number of cells suspended in a liquid called plasma. Blood consists of the following four components:

The Respiratory System

The respiratory system is responsible for the intake of oxygen from the air into the body and the removal of carbon dioxide from the body into the air. It consists of the lungs and respiratory muscles (the diaphragm and intercostal muscles).

The respiratory system works interdependently with the circulatory system, ensuring the supply of oxygen keeps the body alive and performing its daily functions. It is essential for aerobic energy production and muscle work.

Structure and function of the lungs

The lungs are sponge-like structures that fill up most of the thoracic cavity (thorax) and are protected by the ribs on either side. A large sheet of muscle at the bottom of the ribcage (the diaphragm) separates the thorax from the abdomen.

The primary function of the lungs is gaseous exchange, i.e. receiving vital oxygen and passing it through to the circulatory system, while ensuring potentially harmful waste products, such as carbon dioxide go in the opposite direction and are expelled from the body.

Carbon dioxide and oxygen leave the body in the reverse direction during exhalation.

Two mechanics of breathing

The two main mechanisms that trigger the human body to breathe are:

  • Rising levels of carbon dioxide in the blood.
  • Stretch receptors in the respiratory muscles (intercostal muscles) becoming stretched.

The main muscles involved in the action of breathing are the diaphragm and the internal and external intercostal muscles.

The main phases of the breathing cycle are:

  • Inspiration/inhalation – drawing air into the lungs.
  • Expiration/exhalation – expelling air from the lungs.

There is also a short pause before both inspiration and expiration.

During inspiration, the diaphragm muscle contracts, causing the normal ‘dome shape’ to flatten. The external intercostal muscles also contract, raising the ribcage. These actions increase the chest cavity volume. This increase in volume creates a negative pressure between the air in the lungs and air in the atmosphere. This is very much like a vacuum effect in which the negative pressure sucks air into the lungs until the two pressures are balanced.

During expiration, the diaphragm muscle relaxes and rises, returning upwards to its dome shape. The intercostal muscles also relax, decreasing the chest cavity volume. This creates a positive pressure, which ‘pushes’ some of the air out of the lungs.

During exercise, when breathing becomes more vigorous, the internal intercostal muscles become active. During expiration, they contract, forcing the ribs down and removing the air in the lungs.

Gaseous exchange

Gaseous exchange occurs in the lungs and the cells of the body. During gaseous exchange:

  • Oxygen (O2) in the alveoli (air sacs in the lungs) diffuses into the bloodstream (capillaries surrounding the alveoli) and travels to the heart where it is circulated around the body.
  • Carbon dioxide (CO2) is transported from the body via the blood. It diffuses into the alveoli where it is removed during expiration.
  • Oxygen in the blood (travelling from the heart) diffuses into the cells (mitochondria) for aerobic energy production.
  • Carbon dioxide from the cells diffuses into the blood where it is circulated back to the heart and then into the lungs for removal during expiration.


Gaseous exchange occurs through a process called diffusion, which is the movement of a gas from an area of high concentration to an area of low concentration. The concentration of oxygen decreases between the mouth and the lungs, therefore the gas flows in this direction. Carbon dioxide flows in the opposite direction.

Once the oxygen gets into the alveoli, it continues to follow this concentration gradient and diffuse into the bloodstream. The alveoli have minute capillaries running over and around them. The alveolar walls and the capillary walls are so thin that they allow gases to pass through them; oxygen passes into the blood and, at the same time, carbon dioxide passes back into the lungs to be exhaled.

The combined work of the cardiovascular and respiratory systems

Haemoglobin (Hb) is the protein in red blood cells that carries oxygen, carbon dioxide and carbon monoxide (CO) in the blood. The oxygen binds to the haemoglobin in the red blood cells and, at the same time, carbon dioxide dissociates from the haemoglobin and diffuses from the blood into the alveoli in the lungs to be removed from the body.

The red blood cells are transported via the circulatory system.

  • Oxygen from the lungs is carried by the red blood cells to the heart where it is pumped from the heart to the body.
  • Carbon dioxide from the cells is carried by the red blood cells to the heart and into the lungs.

The lifecycle of the cardiovascular and respiratory systems

Early years

As with all of the body’s systems, the cardiovascular and respiratory systems experience rapid growth in early years as we move away from the comfort of the womb and adapt to our new environment.

At birth, the mass (weigh) of the right ventricle is equal to that of the left ventricle. However, with time and exposure to high volume and pressure of blood flow, the left ventricle grows in size at a significantly higher rate; by early childhood it is double the size of the right ventricle (Klabunde, 2005).

At birth, about 15% of a person’s total quota of alveoli have formed (i.e. 45 million out of the 300 million that are acquired by adulthood). A child will develop a lot of air sacs very quickly within the first six months of their life (Strang, 1977).

A baby’s breathing rate is higher (30-60 breaths per minute) than that of an adult (12-20 breaths per minute).

By the time a child reaches the age of three, their lungs and heart look like a mini version of that of an adult. From this stage onwards, the lungs get bigger as the person grows; when they stop growing, so does the size of their major cardiorespiratory organs.

Later life

Important changes occur in the cardiovascular and respiratory systems with advancing age, even in apparently healthy individuals. Some examples of the deterioration that occurs within these systems, in older age, are:

  • Thickening and stiffening of the large arteries, which cause systolic blood pressure to rise with age, while diastolic pressure generally declines after the sixth decade.
  • A decline in aerobic exercise capacity of approximately 10% per decade from the age of 40 onwards.
  • Changes in the shape of the ribcage, as bones become thinner and reshape, meaning it cannot expand and contract as well during breathing.
  • Weakening of the diaphragm, which can prevent a person from breathing enough air in or out.
  • A loss of function in the part of the brain that controls breathing. When this happens, the lungs are not able to get enough oxygen or expel enough carbon dioxide from the lungs, making breathing more difficult.

Short-term effects and long-term benefits of exercise on the cardiovascular and respiratory systems