SSEH2260 Final Exam

Cardiovascular system

Cardiac Cycle

• orderly flow of blood throughout the heart

• maintained by valves

• the cardiac cycle refers to what is happening in the ventricles

• diastole - passive ventricular filling during relaxation

• systole - AV valves close, semilunar valves open, ventricles eject blood

  • contraction produced by the QRS complex as seen on ECG

Cardiac Output (Q)

• amount of blood pumped by the heart (L/min)

• Q = HR x SV

• around 5L/min at rest, increases with exercise until it eventually plateaus at Qmax - differs depending on individual fitness

• in a sedentary person, Q increases to 20-22L/min during exercise. in an athlete, Q increases to 35-40L/min during exercise. This is to match the need for increased oxygen supply to the working muscles

Stroke Volume (SV)

• SV = EDV - ESV

• Stroke volume is the amount of blood ejected with each contraction of the left ventricle

• Preload determines SV

• during exercise, stroke volume can increase by up to 1.8 times at rest

  • in untrained individuals, SV may go from 70-110 mL, while in trained individuals, from 100-180mL

• athletes achieve a higher Q solely through enhanced SV

• to increase SV during exercise, you need to increase venous return (blood back to the heart). this can be done by:

  • increasing ventricular filling during diastole (frank starling law)

  • Normal ventricular filling, followed by a forceful contraction

  • training adaptations - increase blood volume

• Exercise has an effect on ventricular contractility - ventricular contraction as a given preload

  • positive inotropic effect: increase contractility

  • negative inotropic effect: decrease contractility

• Factors causing inotropic effects:

  • autonomic NS activity

  • hormones

  • changes in ion concentrations

End Diastolic Volume (EDV)

• the total amount of blood filling the left ventricle

• EDV is affected by filling time

  • as heart rate decreases, filling time increases

• Filling amount (venous return)

  • this is dependent on cardiac output, blood volume, peripheral blood distribution and skeletal muscle activity

• this is the amount of blood at the end of ventricular filling, i.e. the amount of blood that flows from the atrium into the ventricle

• preload is directly proportional to EDV, the greater the volume in the ventricle at end diastole, the greater the stretching of the ventricle

• the frank starling curve can be used to represent stroke volume (y) vs EDV/pre load (x)

  • sarcomeres lengthen and contract to pump blood

  • in athletes a larger EDV results in increased SV due to training

End systolic volume

• the amount of blood left following contraction

• this is affected by

  • ventricular preload

    • how much fills the ventricles (EDV)

  • ventricular contractility

  • ventricular afterload

    • the pressure which the ventricle has to overcome to eject blood (BP)

Fick Principle

• maximal oxygen consumption (VO2 max) reflects the bodies ability to transport and utilise oxygen. this is dependent on:

  • ventilatory responses to exercise

  • external respiration: transfer of O2 in the lungs

  • transport of O2 in the blood

    • dependent on cardiac output, peripheral resistance and O2 carrying capacity

  • peripheral utilisation of O2: extraction at the tissue level as reflected by the a-vO2 diff

• VO2 = (Q x a-vO2diff) / 100

• in an untrained individual at rest, the VO2 is around 300, and at maximal exercise 3100

• in an endurance athlete, VO2 at maximal exercise is around 5570

• during exercise VO2 increases ~10x in untrained individuals due to:

  • increase in cardiac output via increase in SV (1.5x) and HR (2.5x)

  • increase in a-vO2 diff (the utilisation of O2 increases around 3x)

Heart Rate During Exercise

• increases directly in proportion to workload

• max heart rate = 220 - age

• heart rate and oxygen uptake are linearly related but HRmax is lower in trained athletes despite increased oxygen consumption

• heart rate at submaximal exercise intensities is lower after training despite similar submaximal cardiac outputs

• due to increased sympathetic tone, the SA node is accelerated, increasing heart rate

• due to decreased parasympathetic tone and less parasympathetic innervation, heart rate decreases

  • this inhibits acetylcholine and stops brachycardia

• initially (until ~120bpm) during exercise there is a decrease in parasympathetic activity, followed by an increase in sympathetic activity

• HR response i rapid initially partly due to feed-forward anticipatory increase stimulated by central command

• as exercise continues, HR is increasingly controlled by catecholamine release

  • this is a slower, hormonal response

• the largest increase in HR occurs during anticipation for an event to occur, increasing from ~60bpm - 130bpm

• Heart transplant patients have no neural control of heart rate, so the peak HR occurs after the event has finished as hormones take a while to circulate

  • during a heart transplant, nerves are cut so HR can only be adjusted during exercise via catecholamine release

The heart

• Cardiac tissue has a large blood supply, myocardium has to be fed from the coronary arteries, blood does not diffuse through

• as the heart rate increases, time to fill decreases, decreasing stroke volume

• the heart has its own internal rhythm regulated by the SA node

• heart rate is also regulated by the medulla

  • the medulla receives signals from various receptors/detectors in the body

• parasympathetic nerve endings concentrate in the atria, including the SA and AV nodes

• sympathetic fibers supply the SA and AV nodes and the muscle of the atria and ventricles

• In athletes, cardiac muscle may be up to 1.5 times thicker to allow increased cardiac output

  • not all exercise increases the left ventricular mass equally. while swimming, running, and wrestling all increase ventricular mass, wrestling shows the biggest increase

  • cardiac muscle grows due to the relationship between the pressure in the ventricles and adaptation, constant stretching causes an increase in size

  • cardiac muscle grows to match the workload imposed on the ventricle

  • a volume overload of the left ventricle leads to eccentric hypertrophy (dilation of the cavity and some thickening of the walls)

  • a pressure overload (increased afterload) of the left ventricle leads to concentric hypertrophy (thickening of the walls).

  • LV mass increases more in endurance athletes compared to resistance athletes. In fact, more resistance athletes experienced a decrease in ventricular mass

Cardiovascular Control Centres

• medulla oblongata of the brainstem contains the control centre for the cardiovascular system

• Control centre receives input (afferent signals) from receptors (e.g. baroreceptors which are sensitive to blood pressure changes)

• corrective signals are sent via the nervous system and via hormone release

• neural influences flow through the sympathetic and parasympathetic components of the autonomic nervous system

• impulses from aortic and carotid arterial mechanoreceptors, cardiac mechanoreceptors, skeletal muscle ergoreceptors (afferent) travel to the ventrolateral medulla. the medulla then sends signals to the effectors (heart, skeletal muscles, kidneys, etc)

• sympathetic nerves stimulate the SA node to induce tachycardia through noradrenaline

• parasympathetic nerves (via vagus nerves) induce brachycardia through acetylcholine

Response to prolonged exercise

• during steady state prolonged exercise, particularly in the heat, SV gradually decreases while HR gradually increases

  • this is due to blood flow being redirected to the skin to dissipate heat, as well as decreased plasma volume due to movement into interstitial fluid (sweating)

  • these changes decrease venous return, decrease EDV, and decrease SV

• Heart rate increases to compensate for decreased stroke volume, i.e. to maintain cardiac output

• the increase in HR that occurs when exercising at the same pace over time to maintain Q is known as cardiovascular drift

  • increased stroke volume increases performance

The Arterial System

• when fully dilated, the body’s blood vessels can hold ~20L of blood (4x total blood volume)

• smaller arterial branches = arterioles

• arterioles can alter their internal diameter = regulates blood flow

  • vasoconstriction and vasodilation

• the ability to redistribute blood is important during exercise where blood is diverted to the active muscles

Capillaries

• arteries form smaller vessels ~0.01mm in diameter called capillaries

• usually contain ~5% of total blood volume

• some capillaries are so narrow that only one blood cell can squeeze through

• capillary density of human skeletal muscle averages 2000-3000 capillaries per mm^2 of tissue

• around each capillary is a pre-capillary sphincter (not skeletal muscle)

  • the pre-capillary sphincter is a ring of smooth muscle that encircles the capillary and controls the diameter of the capillary. The capillary cannot constrict or dilate on its own.

  • constriction and relaxation of the sphincter provides an important local means for blood flow regulation within a specific tissue

The Venous System

• deoxygenated blood moves from the tissue into the capillary into small veins - venules

• the venules empty into the body’s largest veins - inferior vena cava and superior vena cava

• this large vessel returns mixed venous blood to the right atria

Venous Return

• since humans stand upright, ~70% of blood volume lies below heart level

• the driving force for venous return (post capillary) is 15mmHg (0-30mmHg)

• downwards hydrostatic pressure on the blood is 100mmHg. the veins lack this pressure to return blood from the extremities into the right atrium

• venous return determines preload and therefore cardiac output and BP

Structural features

• regional differences in venous structure - lower limb veins are stiffer as the compression allows more blood to be sent upwards

• muscle pump generates ~90mmHg of pressure. decreased volume increases pressure and forces the blood upwards

  • active “cool-down” prevents peripheral pooling

  • athletes who collapse at the end of a long race can be revived by lying down and elevating legs (this aids venous return)

• valves prevent the backflow of blood and break the continuous column into smaller supported sections. varicose veins cause circulatory failure

The Countercurrent Flow Mechanism

• increased heart rate increases pulsatility, increasing the contribution of the counter flow mechanism

The Respiratory Pump

• during inhalation, blood is forced from the abdomen to the heart

  • pressure in the inferior vena cava increases → blood back to the heart

  • increase in venous return, augmenting stroke volume and cardiac output

• during exhalation, semilunar vales prevent blood backflow. Blood is forced from the abdomen to the heart

  • pressure in the inferior vena cava decreases

Blood Pressure

• normal systolic blood pressure in an adult is 110-120mmHg

• normal diastolic blood pressure in an adult is 70-80mmHg

• Hypotension is classified as a SBP of less than 100 and DBP of less than 60

• blood flow and pressure vary considerably in systemic circulation

• during the cardiac cycle, BP varies from 120-80mmHg in the aorta/large vessels

• pressure decreases in proportion to the resistance of the vascular circuit

  • 30mmHg in arterioles, ~0mmHg at the right atrium

Blood Pressure Regulation

• Blood pressure is regulated by the medulla oblongata and responds to afferent messages from baroreceptors located in the aortic and carotid bodies

• Blood pressure is also regulated via hormones

  • adrenaline, atrial natriuretic peptide, anti-diuretic hormone, angiotensin II

• with a drop in blood pressure, a pressure error is registered by the control centres and autonomic nervous system activity is modified

  • pressure = (HRxSV) x TPR

  • Parasympathetic nervous system inhibition → HR and SV decreases

  • Sympathetic nervous system activation increases HR and SV and thereby Q, increasing TPR

Mean Arterial Pressure (MAP)

• average force exerted by the blood against the arterial walls during the entire cardiac cycle

  • the heart remains in diastole longer than systole

• MAP = DBP + (1/3 x (SBP-DBP)

  • SBP-DBP = pulse pressure

Hypertension

• an elevated systolic or diastolic blood pressure is referred to as hypertension

  • hypertension is classified as a SBP of 140mmHg or higher and DSP of 90mmHg or higher

• systolic BP at rest can exceed 200mmHg in individuals with atherosclerosis (plaque) or thickening of the connective tissue of the artery

  • plaque can cause blood clots → stroke

• diastolic blood pressure can also exceed 100mmHg under these conditions

• hypertension causes chronic strain on the cardiovascular system

• untreated chronic hypertension can lead to:

  • heart disease

  • stroke

  • kidney failure

  • eye problems

• Hypertension can be prevented by:

  • regular physical activity

  • weight loss

  • stress management

  • smoking cessation

  • reduced sodium and alcohol consumption

  • adequate potassium, calcium and magnesium intake

  • medications such as beta blockers, alpha blockers, Ca channel blockers, diuretics, ACE inhibitors, ARBs

Blood Pressure and Exercise

Response

• Regular aerobic exercise reduces blood pressure by ~6-10mmHB

  • reduced sympathetic nervous system (decreased peripheral resistance)

  • increased renal function (removal of sodium = decreased fluid)

• Generally, blood pressure during exercise is affected by:

  • Cardiac output (Q)

    • Increased during exercise → increased blood pressure

  • Blood Viscosity

    • increases during exercise → increased blood pressure

  • Resistance to flow

    • as a result of changes in the vascular system

• BP = Q x total peripheral resistance

During Exercise

• during the initial stage of exercise, there is an increase in systolic blood pressure during the first few mins of steady state exercise

  • SBP usually levels off at around 140-160mmHg

• DBP remains relatively unchanged

• for every 3ml/Kg you would expect SBP to go up by 10mmHg

  • this is most likely due to increased cardiac output due to increased venous return

• During maximal graded exercise there is a linear increase in SBP proportional to workload

  • BP can be 200mmHg or higher during max exercise, most likely due to large cardiac output (increased venous return = increased cardiac output = increased blood flow)

  • DBP remains relatively stable during exercise

• During steady state exercise, there is rhythmic muscle activity → vasodilation to active muscles → decreased total peripheral resistance → increased blood flow

  • venous valves prevent backflow

  • muscle pump increases pressure

Rate Pressure Product (RPP)

• indirect index of myocardial oxygen consumption (VO2 of the myocardium)

• associated significantly with both morbidity and mortality

• RPP = SBP x HR

• in healthy individuals, RPP will be higher than 20,000mmHg per minute

• Anything below 16,000mmHg per minute is considered insufficient

  • extra strain on the heart or kidneys

• RPP increases faster in arm exercises than leg exercises

Distribution of Blood Flow

• blood flow to tissues varies depending on the immediate needs of specific tissue (oxygen supply = blood flow x arterial oxygen content)

  • Liver and kidneys ~50%

  • Skeletal muscle ~15-20%

• Volume of flow in any vessel is directly proportional to the pressure gradient between the 2 ends of the vessel (not absolute pressure in the vessel)

  • inversely related to the resistance against the flow (caused by friction between the vessel wall and red blood cells)

  • poiseuille’s law relates resistance and radius of the vessel. Radius of the vessel and resistance to flow are the most important factors

Arterioles

• arterioles control blood flow by either constricting or dilating

• if blood vessels were all fully dilated, they could hole ~20L of blood (4x more than normal volume)

• this is done through smooth muscle contraction and relaxation

• blood flow in the arterioles is governed by two factors:

  Autoregulation

  • local metabolites act on arterial wall (detected by arterial wall → dilate, constrict, relax)

  • refers to the arterioles ability to self regulate their own blood flow

  • sensitive to local tissue environment

  • local control of blood distribution

  • in response to changes in the tissue’s local chemical (metabolites) and gas environment

    • decreased PO2, increased PCO2, increased temperature, decrease in pH, increase in nitric oxide (potent vasodilator), increased adenosine, ATP, K+

    • the above occur during exercise and result in vasodilation and increased blood flow to the local area

  Extrinsic neural control

  • stiffen veins

  • Sympathetic nervous system stimulation = release of hormones (adrenaline and noradrenaline) that cause a generalised vasoconstriction (except coronary arteries and skeletal muscle)

  • increased stimulation of nerves by the medulla causes the arterioles to constrict thus reducing blood flow

  • reduced stimulation allows arterioles to stay dilated - more blood flow to the area

  • in theory, in areas that need extra blood, sympathetic stimulation decreases and arterioles stay dilated, allowing blood flow to the area

• Functional sympatholysis

  • sympathetic nervous system activity also increases to skeletal muscles, but the action of it is modulated/inhibited by the local metabolites to blunt the constriction

Blood flow + distribution during exercise

• during exercise, blood is redirected to the areas where it is needed

• blood flow is restricted to non-active areas (kidney, liver, pancreas) and redirected to working muscles

• during heavy exercise, muscles receive 80-85% of blood flow

• redirection of blood flow during exercise is also accompanied by an increase in Q

  • cardiac output increases from 5L/min to 25-30L/min

  • this increase delivers more blood (oxygen) to the working muscles

• During exercise, sympathetic output to the non active tissues increases, resulting in increased vascular constriction and reducing blood flow to those areas

Regulation of vessel diameter

• resistance to flow is dependent upon the radius of the blood vessel. this alters resistance to flow (total peripheral resistance)

  • doubling the radius increases volume and flow

  • halving the diameter increases resistance by 16x

Thermoregulation

Body Temperature

• humans maintain a constant core body temperature in the range 36.1 - 37.8˚C

• homeotherms - maintain constant temperature

• the role of the thermoregulatory system is to maintain stable core temp

• Brain death begins at 41˚C, at 45˚C death is nearly certain

• during exercise the body gains heat faster than it can lose heat

  • Internal temperature can exceed 40˚C - adversely affecting the nervous system

  • muscle temperature can exceed 42˚C

• The body attempts to regulate its internal temperature vi

Heat Balance and Transfer

• heat is lost through

  • conduction

    • the rate of conductive heat loss depends on two factors:

      • the temperature gradient between the skin and surrounding surfaces

      • thermal qualities of surrounding surfaces - water absorbs heat faster than air. the body loses heat ~25 times faster in water than it does in air of the same temperature

    • Conduction alone cannot provide enough cooling

  • convection

    • the transfer of heat from one place to another by the motion of a gas or liquid across a heated surface - wind chill factor

  • Combined, the heat loss through convection and conduction mechanisms only accounts for about 10-20% of heat loss to the surrounding air (provided the air is cooler than the skin)

  • radiation

    • transfer of hear in the form of infra-red rays (electromagnetic)

    • primary heat loss mechanism at room temperature without sweating (~60% of heat loss at rest)

  • evaporation

    • heat loss during the “phase-shift” of a liquid to a gas

    • accounts for 20% of heat loss at rest (insensible perspiration)

    • primary avenue of heat loss during exercise (~80%)

    • very dependent on the environment

    • sweat must evaporate to allow for heat transfer

    • 1L of sweat evaporation = 580kcal heat loss

• heat is gained by the body through:

  • metabolism - basal metabolic rate

    • Can increase 20 times above resting levels during intense exercise

    • theoretically can increase core temp by 1˚C every 5-7 mins

  • environment

    • solar radiation from objects in the environment that are warmer than the body

  • muscular activity

  • hormones

  • thermic effect of food

  • postural changes

Body Temperature Control

• thermoregulatory centre of the body is in the brain (hypothalamus/thermostat)

• Thermal receptors in the skin and changes in blood temperature activate the hypothalamus

  • Increased blood and internal temperature

  • impulses go go the hypothalamus

  • vasodilation occurs in skin blood vessels so more heat is lost across the skin

  • sweat glands become more active, increasing evaporative heat loss (2-4million sweat glands)

  • body temperature decreases

Principles of heat loss + exchange

• heat flows down a temperature gradient to cooler areas

• if the body is hotter than the environment, then:

  • heat in the body moves (via the blood) from the core to the skin and cools

  • evaporation of sweat also cools the skin which then cools the blood and then the core

  • cooled blood returns to the core

• during exercise, metabolism can increase 15 fold - heat production increases 15x from rest + heat gained from the environment

• heat loss is critical during exercise

• heat flows down temperature gradient from muscle to core and then to skin

• some heat is directly conducted to the skin from the muscle underlying it

• heat transfer from the core to the skin is dependent upon skin blood flow

• the transfer of heat from the skin to the environment depends upon:

  • the temperature gradient

  • the rate of evaporation

• If ambient temperature exceeds core temperature, conduction, convection, and radiation cause heat gain

• therefore at high ambient temps, evaporation is the only effective means of heat loss

• the rate of heat loss by evaporation depends on:

  • surface area exposed to air - higher surface area = increased convection = increased evaporation

  • temperature of ambient air - higher temp = increased sweating

  • convective air movement around the body - convection improves evaporation

  • relative humidity of air - high RH impedes evaporation (most important)

    • the amount of water in ambient air compared to the total quantity of moisture that air can hold

    • for evaporation to occur effectively, RH must be lower than that of moist skin - 40mmHg → 40%

    • sweat beads represent useless water loss and can produce dehydration and overheating

    • individuals can tolerate reasonably high temps provided that the relative humidity is low

Exercise in Hot and Humid conditions

• during exercise in the heat, there is competition for cardiac output between the working muscles and the skin

• muscles require delivery of blood (oxygen) to sustain energy metabolism

• as core temperature rises, blood diverts to the skin for cooling

• This means less blood to the working muscles, resulting in:

  • less fuel (O2)

  • greater dependence on anaerobic metabolism

  • earlier accumulation of H+

  • greater use of glycogen stores

  • reduced ventilation

  • impaired exercise performance

  • premature fatigue

• the amount of blood that muscles can receive is ~30L/min

• peak cardiac output of the heart in an untrained person is ~25L/min

  • the body’s need for blood surpasses the supply

• during heat stress, 15-25% of cardiac output passes through the skin → blood vessels near the skin vasodilate → flushed appearance → increase in cutaneous blood flow

• sweating leads to plasma loss → increased viscosity → lower stroke volume and higher heart rate (cardiovascular drift)

  • this decreases venous return, and end diastolic volume

  • heart rate increases to compensate for decreased stroke volume, i.e. to maintain cardiac output → increased strain on the thermoregulatory and cardiovascular system

  • the increase in heart rate that occurs when exercising at the same pace over time to maintain cardiac output is known as cardiovascular drift

Minimising the effects of heat

• bench rotations

• cool rooms during breaks

• spray players during breaks

• ice slushies

• ice jackets

• play games during cooler times of the day

• do not rely on thirst. the thirst centre is easily switched off by sensory receptors during exercise, this can lead to voluntary dehydration during exercise

Fluid Replacement

• pre-exercise: 400-600mL water in the 60 mins prior

• 150mL every 15-20mins during

• 1.5x sweat loss post exercise, consume beyond thirst, drink salty drinks

• sports drinks replace electrolytes lost during exercise

  • during long events, excess water intake can cause hyponatraemia

  • some CHO ingestion post exercise is critical for glycogen repletion

Heat acclimatisation

• several days of activity in a hot environment will lead to improved heat tolerance and performance

• adaptations such as:

  • increase in plasma volume

  • decrease in resting core temp

  • increase in sweat rate

  • NaCl content of sweat decreases

  • reduced heart rate (compared to unacclimatised)

• once acclimatisation occurs, effects last several weeks