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Cardiovascular Response to Exercise This article will endeavor to outline the physiological effects of exercise on 1) Cardiac function 2) Blood Pressure 3) Neural Control 4) The Respiratory System 5) Heat Production

With respect to 'exercise', a lot of research in this field is focused on pure exercise which develops physical fitness. However, a wider definition that includes the term 'activity' is more useful in health promotion. The difference between the two you ask? Well, activity in general is deemed to include everyday tasks. For example, running on a treadmill is exercise. Cycling to the shop is an activity.

1) Cardiac Function There are three primary functions of the Cardiovascular (CVR) System; transport, protection and regulation. TRANSPORT: This refers to the transport of nutrients, oxygen and hormones to the cells of the body. It also refers to the transport of wastes produced by bodily functions, e.g. carbon dioxide, nitrogen and heat emitted by the functions of the body. PROTECTION: This is achieved by supplying white blood cells (WBCs) and antibodies, and also by coagulating the blood when an injury has occurred. REGULATION: By maintaining a stable body temperature, balancing the pH of the blood and providing water to the cells and tissues of the body.

Cardiac Output (CO) is determined by Heart Rate (HR) and Stroke Volume (SV). CO = HR X SV

Average man CO at rest = 5l/min HR at rest = 70bpm SV at rest = 70ml Trained athlete CO at rest = 5l'm HR at rest = 50bpm SV at rest = 100ml

During maximum exercise: CO = 50-35l/m HR = 200bpm SV = 175ml

HEART RATE The heart has an intrinsic pacemaker which kicks in at around 100bpm. Increasing the HR from 70-100 is achieved either by: - reducing parasympathetic activity OR - increasing sympathetic activity To increase the HR above 100, increase sympathetic activity only. HRmax = 220 - [age in years]

STROKE VOLUME In upright humans, up to 70% of the circulating blood volume is below the heart, with the vast majority of this blood being stored in the venous capacitance vessels. Limb movement (muscle pump) during exercise enhances venous return to the heart, which causes an increase in stroke volume. Frank-Starling mechanism: a) Increased venous return increases the ventricular filling (end-diastolic volume) and therefore increases preload, which is the initial stretching of the cardiac myocytes prior to contraction. b) Myocyte stretching increases sarcomere length, which causes an increase in force generation. This mechanism enables the heart to eject the additional venous return, thereby increasing stroke volume. c) Stroke volume plateaus at 40-60% maximal capacity. Vo2 max = CO X (a-v)O2 difference

Cardiac output (CO) limits aerobic exercise capacity -frequency of pumping -efficiency of ventricular filling -efficiency of ventricular emptying

What determines BP? MAP = CO X TPR

What determines TPR? Arteriole (vessel) diameter.

During dynamic exercise... 1 - BP rises with exercise intensity. Sypathetic activity; adrenaline release; CO increases. 2 - TPR adjusts. Reduced blood flow to all major regional beds except for brain; increased BF to skeletal (10-20%)/cardaic (400%) muscle.

3) NEURAL CONTROL Baroreflex in exercise In exercise, BP is kept higher than during rest. This increases nutritional perfusion when increased vasoconstriction. This is to prevent a sudden fall in BP if exercise stops. Exercise stops = fall in venous return and CO. Need to reset the baroreflex - upwards shift.

BP in dynamic exercise with large muscle groups: - heart pumps large volumes of blood into low pressure system - vasodilation of arterioles supplying skeletal muscles gives rise to decrease in TPR - Systolic BP rises, diastolic BP remains constant. Static exercise with large muscle groups: - no skeletal/respiratory muscle pump facilitating venous return - high intramuscular pressures with high force muscle contractions, occluding BVs - Cardiac output increases but considerable ventricular afterload - Heart pumps lower volumes into high pressure system - systolic BP can increase alarmingly to > 300 mg Hg - Diastolic BP can increase to >150 mg Hg

EXERCISE AND THE RESPIRATORY SYSTEM Resp Rate (RR) and Tidal Volume (TV) increase Alveolar ventilation and diffusion of gases increase Aerobic exercise: lungs are able to supply muscles with O2 Anaerobic exercise: lungs can't supply enough O2 e.g. weight lifting

PULMONARY CIRCULATION within the thorax pulmonary gas exchange is not impaired during exercise. Arterial O2 saturation remains unchanged from rest. At rest, the equilibrium for gas exchange is reached about 1/3 of the way along the capillary. 3 x CO will still give full O2 saturation. At max work load, efficiency of perfusion and ventilation.

HEAT PRODUCTION Muscle activity produces metabolic heat which elevates core temperature. CO to exercising skeletal muscle is compromised. This reduces amount of blood available for muscle perfusion. Withdrawal of sympathetic vasoconstrictor from cutaneous arterioles. Sweat production (sympathetic system)

Cardiac drift is the tendency for heart rate to rise gradually during exercise due to dehydration and rising internal temperature.

EXERCISE ADAPTATIONS Aerobic.... - increase SV - increase number of mitochondria - increase in number of capillaries leading to increased endurance and less fatigue Anaerobic.... - increase strength and bulk of muscle - training of types IIa and IIb muscle fibres - formation of lactic acid