Respiratory Response To Exercises

Body temperature increases [Ref: Ganong 22/e p632; Guyton I I/e p1062; http://www.cvphysiology.com/Blood%20Flow/BF015.htm;

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Heat is produced whenever muscles contract.

This is due to 2 reasons

1. The maximal efficiency for conversion of nutrient energy into muscle work, even under best conditions, is only 20 to 25 per cent.
2. Almost all the energy used to contract the muscle is converted into heat d/t
3. overcoming viscous resistance to the movement of the muscle and joints.
4. overcoming the friction of blood flowing through the blood vessels.
5. other similar effects.

Changes in Muscle blood flow during exercise:

• The blood flow of resting skeletal muscle is low (2-4 ml./100 g/inin). The muscle blood flow can increase a maximum of about 25 fold during the most strenuous exercise.
• Almost one half of this increase in flow is due to intramuscular vasodilatation caused by the direct effects of increased muscle metabolism.
• The remaining increase results from multiple factors, the most important of which is probably the moderate increase in arterial blood pressure that occurs in exercise, usually about a 30 per cent increase. The increase in pressure not only forces more blood through the blood vessels but also stretches the walls of the arterioles and further reduces the vascular resistance.
• The local mechanisms maintaining a high blood flow in exercising muscle include a fall in tissue P0₂, a rise in tissue PCO₂, and accumulation of K+, and other vasodilator metabolites. The rise in temperature further dilates the vessels.
• The increase in blood flow is instantaneous (and not half a minute later).

• When the contractions first begin, blood flow briefly decreases because of compressive forces exerted by the contracting muscles on the vasculature within the muscle. Each time the muscles contract arterial inflow decreases due to extravascular compression, and then arterial inflow increases as the muscles relax. This is repeated each time the muscles contract and relax. If flow were measured in the outflow vein, the venous outflow would increase during contraction and decrease during relaxation – the opposite of what occurs on the arterial side of the circulation. After just a couple of seconds, mean and peak flows begin to increase. After 15-20 seconds the increased flow will reach a steady state that is determined by the force and frequency of contraction, and the metabolic demands of the tissue. When contractions cease, blood flow may transiently increase because of the loss of compressive forces, and then over the next minute or so the flow will return to control.
• Blood flow can sometime increase at or even before the start of exercise. This shows that the initial rise in muscle blood flow in probably a neurally mediated response. Once exercise has started the high blood flow is maintained by the local mechanisms as described above.
• Blood flow to brain remains unchanged during exercise. Below given table shows that exercise causes increased blood flow to the exercising muscle with reduction of flow to liver, kidney, GI tract and inactive muscles. Blood flow to the brain remains unchanged.

Lymphatic Flow:

• The lymphatic system. unlike the circulatory system, has no pump to move lymph fluid. Any form of exercise that incorporates major muscle groups and deep breathing will encourage lymph flow. Muscle movement squeezes lymph vessels.

A i.e. Venoconstriction with decreased blood flow

During vigrous exercise, constriction of vessels in splanchnic organ, and decreased blood ‘ storage’ in liver & other portion of splanchnic bed occursQ, which may increase the volume of actively circulating blood, perfusion the muscles by as much as 30%.

Contraction of the capacitance vessels in the viscera can pump a litre of blood into the arterial circulation, in less than a minute.

C i.e. Blood flow remains unchanged

A i.e. Left shift of Hb-O₂ dissociation Curve

A right shift of Hb – 0₂ dissociation curveQ is seen in excercising muscle. Right shift occurs because of accumulation of CO2 (..1pH)2 and increased temperature of contracting muscleQ

During exercise

– The contractile muscles need more 0₂Q for energy production.

–    This need is meet by increasing the blood supply to the muscles. Cardiac activity increases to increase the stroke volumeQ. Capillary bed of the contracting muscle dilate and many previously closed capillaries open up.

–    In contracting muscle the amount of O₂ extracted from blood increases (3 fold increase in O₂ extraction from each unit of blood)Q. This is because of right shift of Hb – O₂ dissociation curve

A i.e. Hyperventilation in the beginning; B i.e. Hyperkalemia

With the onset of exercise, there is an abrupt increase in ventilationQ; followed after a brief pause by further more gradual increase. The increased stimulation is presumably due to psychic stimuli and afferent impulses from proprioceptorsQ in muscles, tendons and joints.

During muscular exercise, increase in 0₂ consumption is proportionate to the energy expended, and all the energy needs are met by aerobic process. With more muscular exertion, aerobic resynthesis of energy stores cannot keep pace with their utilization and lactate is produced (d/t anerobic breakdown of glucose):

– With vigeourous exercise buffering of increased amount of lactic acid, liberates CO₂ and this further increases ventilation. With further accumulation of lactic acid, the Tin ventilation outstrip CO₂ production and alveolar 1³092 falls as does arterial PCO2^Q. The decline in Pco2 provides respiratory compensation for metabolic acidosis

– Arterial pH and HCO-₃ decreaseQ with severe exercise due to lactic acidosis

– Strenuous exercise can cause HyperkalemiaQ by releasing K⁺ from skeletal muscle.