Acute Responses to Exercise

Acute Cardiovascular Responses to Exercise

As the body begins exercise there is a greater demand for oxygen, to accommodate for this increase in oxygen demand the body makes a number of changes to meet the new oxygen requirements. The changes that occur to the cardiovascular system which involes the heart and blood vessels happens automatically in response to exercise. It is important to remember that these changes are only temporary and only last for the duration of exercise. 

Increased Heart Rate

Heart rate is controlled by the autonomic part of the central nervous system, it has two aspects the parasympathetic bracnh which is responsible for decreasing heart rate returning it back to resting levels after exercise. The second branch is the sympathetic system which increases heart rate and is responsible for maintaining a high heart beat during exercise.

Increased Endocrine system action

The endocrine system produces the hormones adrenaline and noradrenaline during exercise as both hormones play a role in stimulating the heart, increasing heart rate and force of contraction. Adrenaline is also responsible for the slight anticipatory rise in heart rate just prior to commencing exercise as the body anticipates that exercise is about to occur.

Increased Stroke Volume

Frank Starling Mechanism: Increased amounts of blood returning to the heart (increased venus return) results in an increased force of contraction due to stretching of the muscle wall. There is increased neural stimulation the autonomic nervous system stimulates the heart to push more blood around the body thus increasing stroke volume.

As exercise continues blood vessels dilate to facilitate greater blood flow this also results in less resistance against the blood and greater venus return.
However stroke volume can only be increased to a point as the heart can only pump out so much blood at a time. This limit is usually met a sub-maximal activity at around 50% of VO2 max.

Blood Pressure

Is the pressure exerted by the blood on the arterial walls as it is ejected from the heart. Normal resting blood pressure being around 120/80 mmHg.

  • Systolic (contraction phase of the heart)- increased during exercise
  • Diastolic( relaxed phase of the heart) – decreases during exercise, due to decreased resistance of blood flow from vessel dilation.

Increased Redistribution of blood flow

Blood is redistributed from organs and areas that do not require a lot of blood immediately, therefore during exercise some of that blood is redirtcted to the working muscles. Blood is predominately redirected from areas such as the stomach, liver and kidneys to the muscles.

Blood plasma volume decreases

Blood plasma levels decreases due to sweating, evaporation and excretion of metabolic by-products, this can lead to dehydration.

Acute respiratory responses to exercise

The respiratory system is needed to get in oxygen and expel carbon dioxide in order to increase oxygen supply to the muscles and to remove carbon dioxide the respiratory system sees increases in the respiratory rate, tidal volume and minute ventilation. The aim of these mechanisms is to increase the amount of oxygen getting into and distributed around the body and also to increase the efficiency in the removal of carbon dioxide.

Increased Respiration Rate (RR)

  • Is the number of breaths per minute.
  • Is mainly regulated by CO2 levels within the blood.
  • Levels can increase from rest of around 12 breaths/min to 35-50 breaths/min.

Increased Tidal Volume (TV)

  • Is the amount of air breathed in and out per breath.
  • Resting levels are usually around 0.5L whilst exercising can be as high as 3.5L.

Increased Minute Ventilation (VE)

  • Is the amount of air inspired or expired per minute by the lungs. It is calculated by respiration rate x tidal volume.
  • At rest remains at around 6L/min and at exercise around 75-190L/min.

Increased Oxygen Uptake (VO2)

  • Is the volume of oxygen transported to,taken up and utilised by the body for energy production.
  • Can be measured in METs = metabolic equivalent.

Increased Diffusion

  • Diffusion is the movement of particles across a membrane in this case it is the movement of gases from blood vessels to muscle cells.
  • This is due to the increase in the amount of oxygen being used by the muscles increasing the difference in oxygen concentrations and carbon dioxide levels.

 

Acute muscular adaptations to Exercise

In response to exercise the muscles themselves must become more efficient in their use of fuels in particular oxygen and be able to provide the muscular contractions needed for movement in exercise.

Increased motor unit and muscle fibre recruitment

During exercise the body will require a greater number of muscle contractions therefore more motor units and in turn more muscle fibres need to be activated. This is especially true for powerful strength activities that require a lot of force as to provide maximum force a high number of muscle fibres will need to be activated.

Increased blood flow to muscles

During exercise blood is redirected from organs that are not needed for the immediate activity and is redirected to the working muscles.

Increased muscle enzyme activity

Enzyme activity is increased this improves the efficiency of the mitochondria and therefore energy can be produced faster.

Increased muscle temperature

This is because heat is a by-product of the energy systems. Heat may be helpful to enzyme activity to some extend but if too high may be detrimental and hyperthermia or dehydration could set in.

Increased oxygen supply and use

The muscles cells use more oxygen whilst exercising than at rest as they are engaging in more aerobic respiration whilst exercising. This increased use and uptake of oxygen from the blood results in a greater AVO2(arteriovenus) difference.