Short-Term Effects of Cardiovascular Exercise
When you start exercising, the cells in your muscles start utilising energy and as a result there is an increase in waste products from this cell respiration. Therefore there is an increase in carbon dioxide in the blood. The brain detects this and sends a signal to the lungs to breathe faster and deeper therefore speeding up the gases exchange in the lungs. The brain also sends a signal to the heart to beat faster, making the heart rate go up. The working muscles squeeze on the veins and push more blood back to the heart.
This causes the blood to rush into the heart at higher pressure. Stroke volume (the amount of blood leaving the heart in one beat) increases as a result of more blood entering the heart under pressure and the heart contracting harder (therefore pushing more blood out of the heart). At rest, stroke volume is approximately 70 – 80ml per beat. In a well trained person, stroke volume at rest is 100 – 110ml per beat. During activity, stroke volume increases and in well trained people it can be as high as 200ml per beat.
The heart can therefore pump more blood around the body as a result of the increase in heart rate and stroke volume (i.e. cardiac outputincreases). This means more oxygen reaches the muscles each minute and more carbon dioxide is carried away. Arterioles in the muscles widen to facilitate this increased blood flow. Blood gets shunted to where it is most needed from where it is least needed, i.e. from your gut to your muscles. To make this happen, blood vessels widen (known as vasodilation) and constrict (known as vasoconstriction)as appropriate.
Summary of Short-Term Effects of Cardiovascular Exercise
|Increased stroke volume (better filling and emptying of the left ventricle)Increased heart rate (the heart beats faster)Increased cardiac output (a combination of increased heart rate and stroke volume)Increased systolic pressure (because the myocardium is contracting more forcefully)Maintained diastolic pressureIncreased oxygen demands by the working muscles Increased respiratory rate (more oxygen needs to enter the blood and more carbon dioxide must be expelled)Increased blood flow to the working muscles (where the oxygen is needed)Decreased blood flow to the gut (where oxygen supply can be minimal for the duration of the exercise)|
Long-Term Adaptations to Regular Cardiovascular Training
The heart needs to adapt to cater for any stresses placed upon it, and the muscle of the myocardium has a similar ability as skeletal muscle to become stronger (especially the portion around the left ventricle). The normal resting heart rate for an adult is between 60 – 80 beats per minute (bpm). However, a fit well conditioned adult can have a low resting heart rate of 45 – 60bpm. Regular exercise can cause as much as a 20 to 30bpm reduction in resting heart rate. This is due to greater parasympathetic nervous system control and increase stroke volume of the heart.
For example, an untrained adult may have a stroke volume of 70ml. If their resting heart rate is 72bpm then:
Resting Cardiac Output = 70 x 72 = 5140 ml/ min (or roughly 5.1 litres per minute of blood)
A well trained adult may have a stroke volume of 100ml. Therefore, to supply the same amount of blood at rest their heart rate would only need to be:
5140/100 = 51.4or roughly 52bpm
Fit individuals can maintain a certain cardiac output with fewer heart beats than an unfit individual. This also gives a longer duration in diastole between beats, which as we saw earlier, improves coronary blood supply.
There are many long-term adaptations to regular cardiovascular training, affecting not just the heart, but also the lungs, blood, circulation and muscles. The table on page 14 summarises those adaptations.
Summary of Long-Term Adaptations to Regular Cardiovascular Training
|Heart:Increased stroke volumeDecreased heart rate at rest and for any given exercise intensityIncreased cardiac outputBetter cardiac blood supply, hypertrophy of cardiac muscle (especially left ventricle)Lungs: Increased number of capillariesIncreased use of ‘dead space’ in the lungs (residual volume)Better oxygen and carbon dioxide exchangeBlood and circulation:Increased blood volumeIncreased red blood cell countIncreased haemoglobin content (oxygen carrying capacity)Increased ‘tone’ in smooth muscle of artery wallsMore efficient circulationMuscles:Increased number of capillariesIncreased size and number of mitochondriaIncreased aerobic/anaerobic enzymes|
The Vascular System
The vascular system is a network ofvessels which allow blood to flow from the heart to every cell of the body, and then back again to the heart. It can be conveniently divided into two main parts: the systemic circulation and the pulmonary circulation.
In the systemic system, oxygen rich blood is distributed to the body tissues. Blood rich in oxygen leaves the heart via the aorta when the left ventricle contracts forcefully. It then branches into many smaller arterieswhich run throughout the body. Finally the oxygen-rich blood passes from the smallest of the arteries (arterioles) and enters capillaries, which are very small blood vessels that deliver oxygen and nutrients to the tissues. At the same time, capillariescollect carbon dioxide and waste products from the tissue and transport it back to the heart via a network of venulesand then veins.
The pulmonary system comprises the lungs and the blood vessels between the lungs and heart. Deoxygenated blood is pumped from the heart through the pulmonary arteriesto the lungs. Oxygen is then infused into the blood and carbon dioxide leaves the blood and is exhaled by the lungs. The newly oxygen enriched blood then travels back to the heart through the pulmonary veins into the left atrium.
Types of Blood Vessel
Blood vessels provide the means through which blood is transported and they vary in construction and shape depending upon their function. There are five main types:
- Arteries:These carry blood away from the heart under high pressure. Arteries have thicker walls, more elastic fibres and more smooth muscle than veins and capillaries in order to maintain this pressure and give a rebound ‘boost’ to the pumping of the heart.
- Arterioles:Arteries subdivide into smaller vessels called arterioles. They still contain smooth muscles and elastic fibre, but not in the quantities found in arteries. The main function of arterioles is to regulate blood flow to the capillary beds.
- Capillaries:These have very thin walls, sometimes made of just a single layer of cells. They have very few elastic and muscles fibres. These thin walls allow the exchange of gases and nutrients from the body’s tissues. Capillaries within the lungs and muscles increase in number when an individual trains regularly. This adaptation results in more efficient gas and nutrient exchange.
- Venules:Once the capillaries have passed through the tissue they contain carbon dioxide and waste products and start joining up to form venules which then flow into veins.
- Veins:Veins all take blood back towards the heart, eventually emptying their deoxygenated blood into the vena cava and thence the right atrium. Pressure in the veins is low in comparison to the arteries, and so they have relatively thin walls and non-return valves to prevent pooling of blood in the legs due to gravity.
The diagram shows the differences between arteries and veins respectively:
The Effects of Disease on Blood Vessels
As a person gets older the natural ageing process causes the artery walls to become less supple and elastic, builds up scar tissue and starts to thicken. This is referred to as arteriosclerosis. If the person smokes, is inactive, suffers high levels of psychosocial stress, has a poor diet, and so on, then this process of hardening the arteries is accelerated. This is undesirable because a hardened artery will not ‘give’ sufficiently when blood flows through it, leading to increased blood pressure (hypertension). Also, a hardened artery is much more likely to suffer damage to its smooth inner lining of endothelial cells. The body reacts to this by repairing damage with scar tissue, involving more thickening of the artery wall and an accumulation of fatty deposits or plaques. This build up of fatty deposits inside the artery is known as atherosclerosis. If this process continues then it can lead to eventual blockage of the artery. A blockage in a coronary artery may cause a heart attack. This is shown in the diagram that follows. If the blockage affects an artery to the brain, then a stroke may result.
Heart attacks and strokes are the most common causes of death in developed countries with ‘circulatory diseases’ listed as the cause for 33% of adult deaths in the UK (statistics.gov.uk, 2009). Consequently there has been a huge amount of research into the why and how arteriosclerosis/atherosclerosis develops. This has resulted in a number of recognised ‘risk factors’ that relate to CHD. Some of these you can do nothing about (such as your age) but many of them are factors that can be controlled with lifestyle changes or medication. Thetable on the following page shows the major risk factors for CHD.
|Risk factor for CHD||Possible Mechanism||Can it be modified?|
|Family history||If close relatives suffered from CHD then this statistically increases your own risk, probably through genetic factors||No|
|Gender||Men have a higher risk of CHD than women. This may be due to patterns of fat storage, hormones, stress response, etc. After menopause the risk level tends to even out.||No|
|Lifestyle||Diverse factors such as alcohol and drug use, quality of sleep, dental health and social class all affect CHD||Yes|
|Smoking||Smoking causes many changes in the body leading to accelerated damage to the smooth internal lining of arteries, and consequent build up of atherosclerotic plaques||Yes|
|Environment||Environment includes things like pollution, weather, sunlight, the political situation where you live, sanitation, etc. These can all affect general health and stress levels.||Yes|
|Age||CHD is a progressive condition, and the narrowing of arteries gradually develops as you age.||No|
|Physical inactivity||A sedentary lifestyle leads to a de-conditioned heart muscle, with a poorer coronary blood supply. It is also associated with many other risk factors in this table, such as type 2 diabetes, hypertension, obesity, and high blood cholesterol||Yes|
|Diet||A huge subject, complicated by much misleading information in the media. A diet with adequate micronutrients from fruit and vegetables, whole grains, etc. seems to be protective, whereas a diet too high in fat/saturated fat/trans fats and refined calories generally seems to contribute to CHD , whether directly, or indirectly via obesity, type 2 diabetes, etc. it is hard to say||Yes|
|Hypertension||High blood pressure places stress on the heart and also accelerates damage to the artery walls.||Yes|
|Type 2 diabetes||Diabetes (type 1 or 2) has wide ranging effects on the vascular system, greatly increasing risk of CHD||Yes|
|Obesity/Overweight||Linked to poor diet and inactivity. Obesity increases the load on the heart simply by having greater body mass to supply with oxygenated blood and to move around. Central obesity – storing fat around the internal organs – is recognised as a particular risk||Yes|
|High blood cholesterol||Blood cholesterol refers to lipoproteins carrying cholesterol and fat in the blood. The main culprits here are thought to be low density lipoproteins (LDL). High density lipoproteins (HDL) are seen as cardio protective||Yes|
|Stress||Chronic stress leads to hypertension and a number of hormone changes that increase risk of CHD. Stress can be caused by many factors, such as socioeconomic status, illness, environment, fear, anger, enforced exercise, etc.||Yes|
|Hormone replacement therapy (HRT)||HRT for post-menopausal women is effective for preventing osteoporosis, but at the same it increases risk of CHD.||Yes|