Quest 5 (Cardiovascular System)

does the heart hypertrophy with exercise?

Yes. With training it hypertrophies from 1% BW to 1.1% BW and is a rapid response to training

describe the blood flow through the heart

deoxygenated blood → superior and inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary artery → lungs → oxygenated blood → pulmonary veins → left atrium → mitral valve → left ventricle →aortic semilunar valve → aorta → systemic circulation

describe the left vs right side of the heart

Systemic:

• left side of heart

• blood pumping to the body

Pulmonary:

• right side of heart

• blood pumping to the lungs to be oxygenated

describe the spleen in horses

• Accessory organ

• holds onto RBCs

  • only releases RBCs into body during extreme endurance or intense speeds

  • works to blood dope and increase hematocrit/O2 in the blood

why is the left ventricular wall of the heart thick?

pumps blood into the aorta and then to muscles and other tissues

describe blood pressure

• Pressure in arteries depends on the rate of blood flow (Q) and the amount of resistance to flow

  • resistance is the degree of constriction or dilation of arterioles which regulates the downstream capillaries

• venous blood return uses muscle contractions to bring the blood back to the heart

describe the cardiac cycle

• Systole → contraction

• Diastole → relaxation

What are the functions of the cardiac cycle?

• water

• oxygen → superior because of spleen

• carbon dioxide

• fuels for energy production

• electrolytes

• hormones

• metabolic products

what is stroke volume?

amount of blood ejected from the heart in one pump

• Resting: over 1 liter in trained horse

• Exercise: max = 300 l/m

how do we estimate the size of the heart?

1. Heart score on ECG

   1. Measure the time it takes for electrical activity to go through the heart (QRS wave)

   2. Theory → bigger heart has a larger/longer QRS complex

   3. Not used anymore due to inaccurate findings

1. Echocardiography/ultrasound

   1. Bigger heart does not always mean more athletic

   2. Abnormally large can lead to diseases, etc

Describe the cardiac muscles

• Primarily mononuclear

• centrally located nuclei

• striated

• branched fibers

• rich in glycogen

• lots of mitochondria → works to convert ATP

what cells help the heart contract?

Pacemaker cells

• set the tone for contraction of the heart

• These cells rarely fail in horses

Intercalated discs

• connect cells mechanically and electrically so they all contract at once

Purkinje fibers

• Tells the heart to contract

what controls heart rate?

Medulla Oblongota which impacts the vagal nerve:

• Balances nerve signals:

  • Parasympathetic

    • Should be dominant

    • Powered by the vagal nerve and cholinergic receptors

    • Keeps heart rate low

  • Sympathetic

    • Can overpower the vagal nerve tone

    • adronergic cell receptors

    • increases heart rate

Describe how inhaling and exhaling impacts heart rate

Inhaling:

• Thorax pressure decreases

• Venous return increases (pushes blood into heart)

Exhaling:

• Thorax pressure increases

• Venous return decreases

Breathing depth also impacts cardiovascular return

what is the Frank-Starling Mechanism?

• The ability of the heart to change its force of contraction and stroke volume in response to changes in venous return

  • Basically, the more force and stretch of the heart, the more force used to push blood out (like a rubber band → the further back you pull, the harder it hits)

• Protective mechanism and helps with exercise

  • More blood enters, more blood pushed out, more O2 delivery

• Respiration impacts blood pressure:

  • Inhale → blood into heart

  • Exhale → blood out of heart

• Unique to the cardiac muscle

what do the baroreceptors of the heart do?

• Negative feedback loop

• Maintains blood pressure:

  • act to decrease heart rate with increased pressure → elevated blood pressure reflexively causes decrease in pressure

  • decreased pressure depresses baroreceptors → increases blood pressure

how do the baroreceptors impact volume reflex?

• if the blood volume is too high the atrium stretches, so the heart reduces the load

  • Inhibits sympathetic nervous system

  • vasodilation in the kidneys

  • increased urinary output

• Volume reflex via ANF on the collecting tubules of the kidney

what does the bainbridge reflex do?

• increases heart rate due to increase blood volume

• if the blood volume is high, the Bainbridge reflex dominates

  • stretch receptors → sympathetic dominates

  • increased heart rate decreases pressure in the right atrium where receptors located

  • Stretch also decreases vasopressin (anti-diuretic hormone)

    • increases urine volume, decreasing pressure

what are some hormones that impact the cardiovascular system?

1. Atrial Natriuretic Factor (ANF) hormone

2. Vasopressin (ADH)

what does the atrial natriuretic factor do?

• controls blood volume

• acts on kidney when blood pressure increases

  • No Na+ (sodium) uptake, less H2O uptake, more H2O loss, so less blood volume

what does vasopressin do?

• causes mild vasoconstriction when blood pressure is low

• retains water at kidneys if pressure is low

Basically works to keep blood pressure high

explain the kidney’s role in blood volume regulation

• juxtaglomerular apparatus in kidney

• Pressure factors → pressure diuresis (increases fluid intake in kidney = more urine)

• Natriuretic factors → if pressure is too high, inhibits Na reuptake, and high urinary volume

• Ionic factors → aldosterone axis (adrenal) and angiotensin axis - Na+ reabsorption and water retention

  • aldosterone stimulates vasopressin release

how much does a horses heart rate increase from rest to maximal?

6x = 40 → 240

how does heart rate change when exercising?

• horses resting heart rate does not change with increased fitness like humans due to already having a low resting rate

• Horses stroke volume does not change much due to already having a strong heart (also different from humans)

• Difficult to assess horses resting heart rate due to having a highly developed sympathetic nervous system (they are prey animals)

when looking at heart rate during exercise, is it better to have a lower or high HR max?

Lower heart rate max while going a fast speed is the fittest animal

is heart rate a good indicator of fitness in horses?

no, but it is okay if it is one of several measurements

• during the onset of exercise, the sympathetic nervous system releases catecholamines to increase heart rate

what is cardiovascular drift?

• prolonged strenuous submax exercise at a constant work rate

  • increases heart rate → adjustment of HR due to adjustment of exercise length and increases to help with evaporative cooling

  • decrease in stroke volume and arterial pressure

  • effort to cool body

what happens to the heart during initial exercise and recovery?

• heart rate increases → overshoots in the beginning, but settles down

• cool down gradually → when done exercising, HR spikes again to start recovery and cool down

what can the recovery heart rate tell you?

• sharp decline in heart rate around 1 min

• total recovery time is normally around 10 minutes. This is the length of time to return to normal resting heart rate

• If return to normal resting rate is abnormally long, this could be an indicator that the horse is developing problems

  • Ex: Lameness = pain, which increases heart rate

how can the cardiovascular training response be measured?

• Measured to improve pump function

• HR max/V-HR max/V140/V200

  • least important measure of the cardiovascular system

• V140 and V200 work to find the submax, which can help to tell the velocity at which the heart rate reaches 140 (endurance) or 200 (racing)

  • Want the velocity to increase with a decrease in HR as fitness increases

how does plasma/blood volume change in exercise?

• Intense exercise causes plasma volume to shrink (both humans and horses) due to water going elsewhere like evaporative cooling or splenic dumping (horses), which increases RBCs and decreases plasma in ratio

  • Plasma volume decreases 5-20%

• Total blood volume:

  • decreases in humans

  • unchanged or increases in horses

how does sweating and blood volume change in exercise?

• blood volume larges goes to sweat

  • Sweat 5-10 L/Hr during exercise

  • Blood volume = 50L, so 5-10 hours to deplete

• Fatigue or exhaustion

  • significant dehydration

  • offer H2O whenever available (supplement with electrolytes)

describe oxygen consumption and uptake with exercise

• Oxygen consumption is tied to VO2 max

  • VO2 = Q x D (a-v)O2

• More O2 is extracted during exercise

  • The more O2 taken up/time of exercise

    • the more ATP is generated

    • more fuel to do work

describe the onset of blood lactic acid accumulation (OBLA)

• when we shift from aerobic to anaerobic, the cori cycle in the liver can get backed up with converting lactic acid back to glucose, so lactic acid remains in the blood stream or in the muscle cells

• velocity at which lactate in plasma = 4 mmol/L

how does cardiac output (Q) change with exercise?

• Q = HR x SV (stroke volume)

• This increases with heart rate/exercise

• Stroke volume does not increase largely with exercise in horses

how does blood pressure and vascular resistance change with exercise?

• blood pressure increases with exercise

• systemic circulation doesn’t double (changes very little)

• pulmonic circulation increases by 2.5 with increase in arterial pressure

how do venous pressures change with exercise?

• Pulmonary = right atrium → ~20/0 rest; 30/5 exercise

• Systemic = left atrium → ~30/5 rest; 80/70 exercise

  • venous pressure from the lungs, so diastole won’t go down because of thick inelastic wall

how does EIPH change with exercise?

• induced pulmonary hemorrhage (bleeding) in racehorses

• physiological consequence of high intensity workout

• scar tissue of the lungs will occur

• Lasix (diuretic) is used

  • decreases blood volume and pressure in the pulmonary capillaries

why is the cardiovascular system important?

• Important for transport (water, glucose, O2, CO2, lactic acid, heat, etc)

  • O2, CO2, and energy (Glucose → ATP)

  • Heat

    • don’t want the brain to get hot

    • blood transports to skin for evaporative cooling

how to determine blood volume parts during exercise and how does it change?

• put blood in tube and spin it down

  • top = plasma/serum → water fraction

  • bottom = RBC/PCV → cell fraction

• Horses:

  • spleen contraction → PCV increases

  • Plasma volume decreases post-exercise

• Humans:

  • PCV increase, but not as much due to lack of splenic dumping

how does the muscle cell swell during exercise?

• muscle increases in size during acute exercise

  • opens capillaries and increases blood flow, which allows for more diffusion

• Increase in number of ATP molecules and H2O particles

how does the GI tract and BV change with exercise?

• H2O moves out of the GI tract and into the BV

• Horses:

  • at the end of the endurance bout they have a tucked up appearance in the abdominal cavity, which allows for the GI tract to act like a sponge and squeeze out water into the BV

• Plasma Volume can change during the day based on your fed state

  • Don’t feed large meal before a race because water will move to GI tract instead of to the muscles

• This is also another reason to feed hay → low hay and high pellets decreases water by 35-40%

where does BV go?

• Q = distribution of cardiac output

• At rest → brain, intestinal tract, kidneys

• During exercise → send more blood to skeletal muscle, skin (evaporative cooling), and kidney (blood flow decreases as %)

• Autonomic nervous system:

  • constricts blood blow into the kidneys and sends more into the muscle

how does heart size change with increased fitness?

• small hypertrophy

• small increase in stroke volume

• more efficient heart rate = Better Q (cardiac output)

  • more blood pumped in 1 beat = less beats

how does HR change with increased fitness?

• HR during submax exercise is lower after training

• changes may not be significant, so lower HR during submax exercise is an unreliable index of fitness

• HR may increase during actual testing due to manipulating or being near them

• Max HR does not alter with training, but the velocity at which they are running when reaching max HR does

how does plasma volume change with increased fitness?

• Increase in plasma volume happens early during training

• better thermoregulation → not sweating as much

• improved stroke volume

how does RBC change with increased fitness?

• Increase in blood volume

  • HB content (total RBC #) increases with training

• O2 extraction at muscle increased

  • increased capillarization

  • H+ increased extraction

how does capillarization change with increased fitness?

• increase in capillary #s in the lung and muscle

• slows blood flow around the lungs and muscle, which allows for better diffusion rates

  • Better O2 delivery

  • Better CO2, H+, and lactate removal

  • Slower blood flow delays OBLA

how does oxygen pulse change with increased fitness?

• VO2/HR - oxygen consumption:HR

• O2 ejected from ventricles/heartbeat

• measure of “aerobic capacity”

  • allows animals to be compared to each other

• Horses have a superior oxygen pulse compared to humans due to having more RBCs for O2 capacity

• Allows you to tell if a horse is improving over time and to compare horses to each other

• high pulse = better athlete

  • faster you go, the more O2 you use, the more ATP produced → higher max value = better athlete