During exercise the heart needs to be increased in order to ensure that the working muscles receive adequate of nutrients and oxygen, and that waste products are removed. Before you even start to exercise, there is an increase in your heart rate. This is called the anticipatory rise, which occurs because the sympathetic nervous system releases adrenaline. Once the exercise has started, there is an increase in carbon dioxide and lactic acid in the body, which is detected by chemoreceptors. The chemoreceptors trigger the sympathetic nervous system to increase the release of adrenaline, which further increases heart rate. As exercise continues, the body becomes warmer, which will also help to increase the heart rate because it increases the speed of the conduction of nerve impulses across the heart.
An ECG is a test that measures the electrical activity of the heart. The information obtained from an ECG can be used to discover different types of heart disease. In the cases of athletes, it can also demonstrate thickening of the heart muscle. Each section is given a letter to identify it, which corresponds to the electrical activity of the heart:
- P wave – this occurs just before the atria contract
- QRS complex – this occurs just before the ventricles contract
- T wave – this occurs just before the ventricles relax
Arteries carry blood filled with nutrients away from the heart to all parts of the body. The blood is sometimes compared to the river of life, but the arteries are more like a river in reverse. Arteries are thick walled tubes with a circular covering of yellow elastic fibres, which contain a filling of muscle that absorbs the tremendous pressure wave of a heartbeat and slows the blood down.
The heart contracts about 70 to 80 times each minute throughout life, though the rate varies with age, emotion, exercise and other influences. Each beat is a cycle of events, which lasts about 0.8 seconds. Blood pours into the right and left atria from the great veins; they then contract simultaneously, emptying their contents into the ventricles. Atrial contraction lasts about 0.1 seconds. The ventricles then begin to contract and the atrio – ventricular valves are closed by the rising pressure; the closure of these valves causes the first heart sound, which can be heard through a stethoscope placed over the apex of the heart.
Ventricular contraction continues lasting about 0.3 seconds in all. When the pressure in the ventricles is greater than that in the arteries the pulmonary and aortic valves are forced open, and the blood flows into the aorta and pulmonary trunk. As the ventricles relax the pressure in them decreases, the pressure in the great vessels forces the aortic and pulmonary valves to close, causing the second heart sound. This can be heard over the near end of the second right rib. During ventricular contraction the atria are relaxed. Following ventricular contraction the whole heart is relaxed for approximately 0.4 seconds. During this time blood is flowing into both atria and through the open atrio – ventricular valves into the ventricles. The phase of the cardiac cycle in which contraction occurs is known as systole, and the phase of relaxation is known as diastole.
The blood pressure is the force, which, the blood exerts on the walls of the blood vessels. It caries in the different blood vessels and also with the heart beat. The pressure is greatest in the large arteries leaving the heart, and gradually falls in the arterioles until, when it reaches the capillaries, it is so slight that the least pressure from without will obliterate these vessels and drive the blood out of them.
Two values are given when a person has their blood pressure taken. The average male is 120/80. The two values correspond to the systolic value and the diastolic value. The higher value is the systolic value and the lower is the diastolic. Blood pressure is measured in milligrams of mercury, mmHg.
The value for a person’s blood pressure is determined by the cardiac output (Q), which is a product of stroke volume and heart rate, and the resistance the blood encounters as it flows. This can be put into an equation:
Blood pressure – Q * R
Where Q - cardiac output ( stroke volume * heart rate )
R - resistance to flow
A reduction on blood pressure is detected by baroreceptors in the aorta and the carotid artery. This detection is passed to the central nervous system, which then sends a nervous impulse signal to the arterioles to constrict. This increases the pressure of the blood and also has the effect of increasing the heart rate.
When blood pressure is increased, the baroreceptors detect this and signal the CNS, which makes the arterioles dilate, and reduces blood pressure.
During exercise there is an increase in heart rate, which will result in an increased cardiac output. As previously stated, cardiac output = heart rate * stroke volume, therefore, as heart rate increases, cardiac output will also increase.
Dilation of the blood vessels feeding the working muscle acts to reduce blood pressure, but this is counteracted by the increase in blood pressure caused by increased cardiac output.
Exercise raises systolic pressure, but there is only a slight change in diastolic pressure. Immediately after exercise there is a falling systolic pressure as the skeletal muscular pump is no longer pumping blood from the muscles to the heart. This can lead to blood pooling in the muscles and cause the athlete to faint as not enough blood is being pumped to the brain.
Oxygen
Only 1.5% of oxygen is carried in the blood plasma. The majority of oxygen is transported in blood by haemoglobin. Oxygen reacts with haemoglobin to make oxyhaemoglobin. The reaction of oxygen with haemoglobin is temporary and completely reversible. The binding of oxygen to haemoglobin is dependant on the parcel pressure of oxygen. Oxygen combines with haemoglobin in oxygen – rich situations, such as in the lungs. Oxygen is released by haemoglobin in places where there is little oxygen, such as in exercising muscle. Myoglobin is haemoglobin like pigment found in muscle fibres, which binds only less oxygen to it compared to haemoglobin. It takes up oxygen from the haemoglobin in the blood and stores oxygen within the muscle itself.
Oxygen dissociation curve
This is an S shaped curve that represents the ease with which haemoglobin will release oxygen when it is exposed to tissues of different concentrations of oxygen. The curve starts with a steep rise because haemoglobin has a high affinity for oxygen. This means that when there is a small rise in the partial pressure of oxygen, haemoglobin will pick up and bind oxygen to it easily. Thus, in the lungs the blood is rapidly saturated with oxygen. However, only a small drop in the % saturation of haemoglobin. Therefore in exercising muscles, where there is a low partial pressure of oxygen, the haemoglobin will readily unload the oxygen for use by the tissues.
Changes in blood carbon dioxide level and hydrogen ion concentration causes shifts in the oxygen dissociation curve. These shifts enhance oxygen release in tissues and increase oxygen uptake in the lungs. This is known as the Bohr effect.
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