Cardiovascular System

Structure of the Heart

The heart is a muscular double pump that sits at the centre of the cardiovascular system, working alongside the blood vessels to circulate blood around the body. A muscular wall called the septum divides the heart into a left and right side, and each side is made up of two chambers: an upper atrium and a lower ventricle.

• The atria only need to push blood a short distance down into the ventricles below, so their walls are relatively thin
• The ventricles have to force blood out of the heart entirely, so their walls are much thicker and more muscular
• The left ventricle has the thickest wall of all four chambers, because it has to generate enough pressure to send blood all the way around the whole body, whereas the right ventricle only has to push blood a short distance to the nearby lungs

Several major blood vessels connect to the heart:

• Vena cava – brings deoxygenated blood back into the right atrium
• Pulmonary vein – delivers newly oxygenated blood into the left atrium
• Pulmonary artery – carries deoxygenated blood out of the right ventricle towards the lungs
• Aorta – carries oxygenated blood out of the left ventricle towards the rest of the body

Blood flow through the heart is kept moving in one direction by four valves:

• Tricuspid valve – between the right atrium and right ventricle
• Bicuspid (mitral) valve – between the left atrium and left ventricle
• Semi-lunar valves (pair) – guard the entrances to the pulmonary artery and the aorta

Each valve opens to let blood pass through and then closes to stop it flowing backwards.

The Cardiac Conduction System

Heart muscle is described as myogenic, meaning it can generate its own electrical impulses to trigger contraction rather than needing a signal from the nervous system.

1. The sinoatrial node (SAN), in the wall of the right atrium, acts as the heart's natural pacemaker and fires an electrical signal that spreads across both atria as a 'wave of excitation', causing them to contract together and push blood down into the ventricles – this is atrial systole
2. The impulse passes through the atrioventricular node (AVN), found in the wall between the atria and ventricles, which briefly delays the signal by about 0.1 seconds – giving the atria enough time to fully empty before the ventricles begin to contract
3. From the AVN, the impulse travels down the bundle of His, which runs through the septum and splits into left and right bundle branches
4. These branches feed into a network of Purkinje fibres that spread the signal throughout the walls of both ventricles, triggering ventricular systole – the powerful contraction that ejects blood from the heart

Control of Heart Rate

Although the heart can generate its own beat, the speed of that beat is adjusted moment to moment by a cardiac control centre located in the medulla oblongata of the brain, using both nervous and hormonal pathways.

• The sympathetic nervous system sends impulses to the SAN to speed the heart up
• The parasympathetic nervous system sends impulses that slow it back down towards its resting rate

Three types of receptor feed information to the cardiac control centre:

• Chemoreceptors, in the carotid arteries and aortic arch, detect rising levels of carbon dioxide in the blood during exercise, triggering greater sympathetic stimulation and a faster heart rate
• Baroreceptors, in the walls of major arteries, detect stretching caused by changes in blood pressure: a rise in blood pressure causes a fall in heart rate, and a fall in blood pressure causes a rise in heart rate. At the very start of exercise the baroreceptors' usual 'set point' is temporarily raised, which stops heart rate being reduced when it is needed most
• Proprioceptors, in the muscles, tendons and joints, detect increased movement as exercise begins and pass this information to the medulla, again increasing sympathetic stimulation and heart rate

Heart rate is also controlled hormonally: during exercise, the sympathetic and cardiac nerves trigger the release of adrenaline, a stress hormone which acts directly on the SAN to increase both the speed and the force of each contraction, boosting cardiac output.

Stroke Volume, Heart Rate and Cardiac Output

Stroke volume is the volume of blood ejected by a ventricle with each single beat, averaging around 70ml at rest. It is influenced by three main factors:

• Venous return – the more blood that flows back into the heart, the more can be pumped back out. This is described by Starling's Law: an increase in venous return leads to greater filling and stretching of the heart during diastole, producing a stronger, more forceful contraction and a greater proportion of blood ejected
• The natural elasticity of the cardiac muscle fibres
• The overall contractility of the heart muscle itself

The proportion of blood ejected is the ejection fraction – the percentage of the blood in the ventricle at the end of filling that is pumped out on the next beat. At rest this is typically around 60%, but with regular training it can rise to as much as 85%.

• Heart rate – the number of times the heart beats each minute, averaging around 72bpm at rest
• Cardiac output – the total volume of blood pumped by the heart in one minute

As exercise intensity increases, heart rate rises in direct proportion, up to a ceiling known as maximum heart rate, estimated as 220 − age. Trained performers tend to have a lower resting heart rate alongside a higher maximum heart rate, giving them a much wider heart rate range than untrained individuals – elite endurance athletes have been recorded with resting heart rates in the low thirties, far below the average of ~72bpm in an untrained adult.

• During short, maximal efforts, heart rate shows an anticipatory rise just before exercise begins (driven by adrenaline), a sharp increase once anaerobic exercise starts, then falls away quickly during recovery
• During longer, sub-maximal exercise, heart rate rises to a steady state, where oxygen supply matches oxygen demand, before declining more slowly during recovery as the body clears waste products such as lactic acid

Regular aerobic training causes the heart muscle to enlarge and strengthen – cardiac hypertrophy. A bigger, stronger heart can eject more blood with every beat, so it doesn't need to beat as often at rest, producing bradycardia (a resting heart rate below 60bpm), which makes the heart more efficient and reduces its own oxygen requirements.

• Cardiac output during exercise rises with intensity due to increases in both heart rate and stroke volume, levelling off once maximum intensity is reached
• Trained performers can reach a substantially higher maximum cardiac output than untrained performers, even though resting cardiac output is similar between the two groups
• During exercise, the proportion of blood sent to the working muscles rises dramatically compared with rest, while flow to organs such as the intestines falls, and flow to the brain and kidneys stays broadly constant

Stroke volume only continues to rise with exercise intensity up to around 40–60% of maximum effort, after which it plateaus – as heart rate climbs closer to its maximum, the time available for the ventricles to fill during diastole becomes shorter, limiting how much blood can be pumped out on each beat.

Cardiovascular Drift

During prolonged exercise (typically once ten minutes or more has passed) in a warm environment, heart rate does not stay level during steady state as might be expected – instead it gradually climbs even though intensity hasn't changed. This is cardiovascular drift, accompanied by a gradual fall in both stroke volume and arterial blood pressure alongside the rising heart rate.

Likely explanation: sweating causes a loss of fluid from the blood plasma, reducing plasma volume; this reduces venous return and, by Starling's Law, reduces stroke volume. Heart rate then rises to compensate, helping to maintain cardiac output and supply extra energy for cooling the body. Drinking enough fluid before and during exercise helps to minimise this effect.

Cardiovascular Disease and the Benefits of Exercise

Coronary heart disease (CHD) is the leading cause of death both in the UK and worldwide. It develops through atherosclerosis – the coronary arteries supplying the heart muscle with oxygenated blood gradually become narrowed and blocked by fatty deposits known as atheroma. High blood pressure, high cholesterol, physical inactivity and smoking are all risk factors.

• As the arteries narrow, the heart muscle receives less oxygen, producing the chest pain and discomfort known as angina
• If a piece of atheroma breaks away it can form a blood clot that blocks a coronary artery entirely, cutting off oxygen to part of the heart muscle and causing a heart attack

Because the heart is itself a muscle, regular exercise keeps it healthy: it becomes bigger, stronger and more efficient, increasing stroke volume and helping to maintain the flexibility of blood vessels, normal blood pressure and healthy cholesterol levels. Guidance from the American Heart Association recommends at least 150 minutes of moderate exercise (e.g. brisk walking) each week.

High blood pressure places extra strain on the heart and arteries, and if left untreated raises the risk of heart attack, heart failure, kidney disease, stroke and dementia. Regular aerobic exercise can lower both systolic and diastolic blood pressure by around 5–10mmHg, cutting the risk of a heart attack by up to 20%.

Cholesterol is carried around the body in two main forms:

• LDL (low-density lipoprotein) – carries cholesterol to the body's tissues, considered 'bad' cholesterol since higher levels are linked to greater risk of heart disease
• HDL (high-density lipoprotein) – carries excess cholesterol back to the liver to be broken down, considered 'good' cholesterol because it lowers heart disease risk

Regular physical activity lowers LDL levels while raising HDL levels.

A stroke occurs when the blood supply to part of the brain is cut off, causing brain cells to die and potentially leading to brain injury, disability or death.

• Ischaemic stroke (more common) – happens when a blood clot blocks the supply
• Haemorrhagic stroke – happens when a weakened blood vessel in the brain bursts

Regular exercise helps lower blood pressure and maintain a healthy body weight, which together can reduce stroke risk by as much as 27%.

The Vascular System and Blood Pressure

The vascular system is the network of blood vessels that carries blood around the body, delivering oxygen and nutrients to the tissues and removing waste products such as carbon dioxide. There are two distinct circuits:

• Pulmonary circulation – carries deoxygenated blood from the heart to the lungs and oxygenated blood back again
• Systemic circulation – carries oxygenated blood from the heart to the rest of the body and deoxygenated blood back

Blood travels through five main types of vessel in sequence: arteries → arterioles → capillaries → venules → veins, before returning to the heart. Each vessel type is structurally suited to its role:

• Arteries – carry blood at the highest pressure, so have a thick elastic outer layer to absorb pressure fluctuations, a narrow lumen and a smooth inner lining
• Veins – carry blood at much lower pressure, so their muscular and elastic walls are comparatively thin, but they have a wide lumen and contain valves to prevent backflow
• Capillaries – narrow enough to allow only a single red blood cell through at a time, slowing blood flow right down; combined with walls only one cell thick, this creates ideal conditions for diffusion of gases and nutrients into and out of the surrounding tissue

Blood pressure is the force that blood exerts against the walls of the blood vessels (blood flow × resistance).

• Systolic pressure – the higher pressure produced when the ventricles contract
• Diastolic pressure – the lower pressure that remains while the ventricles are relaxed and refilling

Blood pressure is conventionally measured at the brachial artery in the upper arm, with a typical resting reading of around 120/80mmHg, and it falls progressively the further a vessel is from the heart – highest in the aorta, lowest in the veins. During exercise, systolic pressure tends to rise (increased force of ventricular contraction and stroke volume), while diastolic pressure tends to fall (vasodilation in the working muscles).

Venous Return

Venous return is the flow of blood back into the right side of the heart through the vena cava. At rest, as much as 70% of the body's total blood volume is held within the veins, giving a large reserve that can be called upon when needed. During exercise, venous return increases, and by Starling's Law this drives a corresponding increase in stroke volume.

Because pressure in the large veins is very low and their wide lumen offers little natural resistance, the body relies on several active mechanisms to keep venous return moving, particularly during exercise:

• Skeletal muscle pump – contracting and relaxing muscles change shape and press against nearby veins, squeezing blood along towards the heart
• Respiratory pump – breathing in and out changes the pressure within the chest and abdominal cavities, compressing nearby veins and helping push blood onward
• Pocket (one-way) valves within the veins prevent blood flowing backwards once it has passed through
• Additional support from the thin layer of smooth muscle in vein walls, gravity assisting return of blood from the upper body, and a suction effect created by the heart itself

At rest, the valves and smooth muscle in the vein walls are generally sufficient to maintain venous return, but during exercise the increased demand for oxygen means the skeletal muscle pump and respiratory pump become essential. Continuing to move through an active cool-down after exercise keeps these pumps working and helps prevent blood pooling in the veins.

Venous return is also affected directly by blood pressure: a rise in systolic pressure increases venous return, while a fall reduces it. More precisely, venous return is driven by a pressure gradient equal to venous pressure minus right atrial pressure, divided by venous vascular resistance – because both venous pressure and right atrial pressure are normally very low, even small changes (e.g. pressure changes in the chest during inspiration) can produce a comparatively large change in venous return.

Transport of Oxygen and the Oxyhaemoglobin Dissociation Curve

Oxygen that diffuses into the capillaries at the working muscles is transported around the body in two ways:

• A small proportion, around 3%, dissolves directly into the blood plasma
• The remaining 97% binds to haemoglobin in the red blood cells to form oxyhaemoglobin

When fully saturated, each haemoglobin molecule can carry four molecules of oxygen, which happens where the partial pressure of oxygen is high (e.g. in the capillaries of the lungs). At the tissues, where the partial pressure of oxygen is lower, oxygen is released from oxyhaemoglobin in a process called oxyhaemoglobin dissociation. Within the muscle itself, oxygen is stored by myoglobin, which has an even higher affinity for oxygen than haemoglobin and holds it ready for use by the mitochondria, where aerobic respiration takes place.

The relationship between the percentage saturation of haemoglobin and the partial pressure of oxygen is represented by the S-shaped oxyhaemoglobin dissociation curve:

• In the lungs, where oxygen partial pressure is high, haemoglobin becomes almost fully saturated
• At the tissues, where oxygen partial pressure is lower, haemoglobin releases a portion of its oxygen load

During exercise, this curve shifts to the right, meaning haemoglobin gives up its oxygen more readily at any given partial pressure – the Bohr shift. This rightward shift is caused by three linked factors that all increase during exercise: a rise in blood and muscle temperature, a rise in the partial pressure of carbon dioxide in the blood, and a fall in blood pH caused by that additional carbon dioxide. Together, these changes make more oxygen available to the working muscles exactly when it is needed most.

Redistribution of Blood (The Vascular Shunt Mechanism)

The distribution of blood around the body changes considerably between rest and exercise, as more blood is redirected towards the working muscles where it is needed most. This process is known as shunting, or the vascular shunt mechanism, and it is coordinated by the vasomotor centre in the medulla oblongata.

During exercise, chemoreceptors detect rising levels of carbon dioxide and lactic acid in the blood and relay this information to the vasomotor centre, which then adjusts blood flow through:

• Vasodilation – the widening of blood vessels to increase flow (arterioles supplying the working muscles)
• Vasoconstriction – the narrowing of blood vessels to reduce flow (arterioles supplying less essential organs, such as the intestines and liver)

This is controlled through varying levels of stimulation from the sympathetic nerves within the vessel walls, alongside pre-capillary sphincters – small rings of muscle at the entrance to capillaries which relax to allow more blood through to active tissue, and contract to restrict flow elsewhere.

• At rest, only around 15–20% of cardiac output reaches the skeletal muscles, with much larger shares going to organs such as the intestines and kidneys
• During vigorous exercise, as much as 80–85% of a greatly increased cardiac output can be directed to the working muscles, while the share reaching the intestines falls sharply
• Blood flow to the brain remains broadly constant throughout, protecting brain function
• Flow to the heart muscle and skin both increase – the heart because it needs extra oxygen to beat faster, the skin because blood is needed there to help dissipate heat and cool the body down

This redistribution has a practical consequence: eating shortly before competing is best avoided, since a full stomach draws blood towards digestion rather than towards the working muscles, which can harm performance. More broadly, redistribution during exercise serves several purposes: increasing oxygen supply to working muscles, removing waste products such as carbon dioxide and lactic acid, directing extra blood to the skin to help regulate body temperature, and supplying the heart muscle itself with the additional oxygen it needs to keep beating faster.

Arterio-venous Oxygen Difference

The arterio-venous oxygen difference (A-VO2 diff) is the difference between the oxygen content of the blood arriving at the muscles through the arteries and the oxygen content of the blood leaving the muscles through the veins.

• At rest, this difference is small, since the muscles are not demanding much oxygen from the blood passing through them
• During exercise, the muscles extract much more oxygen from the blood supplied to them, so the arterio-venous oxygen difference becomes much greater
• This increased extraction is matched by greater gaseous exchange at the alveoli, with more oxygen taken into the blood and more carbon dioxide removed
• Training increases the arterio-venous oxygen difference further still, as trained performers become more efficient at extracting oxygen from the blood reaching their muscles