Exercise and Posture (M1 13/11)

Question 1

LO: What are the effects of changes in posture on arterial blood pressure?

LO: What are the changes in the cardiovascular system during exercise?

Answer 1

These are the LO’s dodo brain.

Question 2

What happens in orthostatic hypotension (going from lying to standing)?

Answer 2

• Get redistribution of 500 ml of blood from the intrathoracic vessels (around the thorax area) into the veins of the lower limbs due to the effect of gravity (blood moves down the body). This is a process known as venous pooling, and takes about 15 seconds.
• Venous pooling causes venous return to decrease, SV decreases, so CO falls. So arterial blood pressure fall, hypotension, and so may cause dizziness.
• Then compensatory mechanisms will restore the BP: activation of the baroreceptors AND Cardiopulmonary receptors also fire less and this decreases parasympathetic supply to SAN
• This causes autonomic changes: vasoconstriction (increased total peripheral resistance), tachycardia and positive inotropy. The latter two changes increase cardiac output. Increases in cardiac output and SV lead to a partial restoration of arterial pressure.

Question 3

Describe the Baroreceptor Response to Orthostatic Hypotension

Answer 3

1. Arterial blood pressure falls upon standing
2. The carotid and aortic baroreceptors fire less, as there is less stretch
3. Less AP’s travel along the afferent nerves (the vagus nerve on the aortic arch or the glossopharyngeal on the carotid sinus).
4. The cardio-vascular control centre in the brain increase sympathetic output and decrease parasympathetic output.
5. BP rises due to tachychardia, vasoconstriction and positive ionotropy,

Question 4

What is clinical postural hypotension defined as?

Answer 4

• Decrease in systolic BP of 20 mmHg or more after two min of standing.
• Compared to supine (lying down) values.
• Can lead to syncope (fainting).

Question 5

What is the effect of excercise on blood pressure?

Answer 5

1. Cardiac output increases
2. Get redistribution of blood to skeletal muscles, for this to happen blood flow through skin, gut and renal vessels is reduced
3. Systolic BP increases
4. Peripheral resistance decrease
5. Pulse-pressure increases (represents increased SV)

• Interestingly, diastolic BP decreases, but this is just because it represents overall vasodilatation due to metabolites.

Question 6

What mechanisms are activated when excercising?

Answer 6

Overall in excercise, HR can increase 3 fold, and stroke volume increase by 50%.

1. Atrial booster pump: do to redistribution there is increased venous return (less blood in the peripheries) so the atria are stretched, and pump more blood into the ventricles.
2. Increased ventricular suction: there is an increase in sympathetic supply during exercise, so the ventricles suck more blood in during diastole.
3. There is increased myocardial contractility (the heart muscles can contract more strongly) because or adrenaline and noradrenaline. They have a positive ionotropic effect. Also there is increased contractility because filling time increases- more stretch, greater strength of contraction according to the Frank-Starling mechanism.
4. Heart rate increases due to increase in sympathetic activity:

Tachycardia occurs at the onset of exercise: this initial increase is thought to be due to vagal withdrawal (this refers to the actvity of the vagus nerve which acts on pacemakers on the heart to lower heart rate)

Increase in sympathetic supply, linked to proprioreptors (receptors for self, position and movement) in muscle preparing person for exercise. Body senses that you are moving more, so increases heart rate.

5. During excercise there is a modification of the baroreceptors that want to keep a constant arterial BP, by the CNS. This allows BP to increase during excercise.
6. Venous return is increased by three different pumps:

• Venous return rate is determined by the pressure gradient between the superior/inferior vena cava and the pressure in the right atrium. This means that if right atrium pressure is decreased, there will be a pressure gradient for blood to move into the right atrium form the inferior/ superior VC and so venous return will be increased. Breathing facilitates venous return. When a person inspires, the diaphragm flattens. This decreases the intra thoracic pressure. This is transmitted across the right atria, lowering right atrial pressure and thus facilitating venous return.
• Also the decrease in thoracic pressure causes and increase in abdominal pressure, this squeezes the inferior vena cava and blood moves toward the heart.
• The skeletal muscle contacts so blood moves from veins back up to heart.

Question 7

What is positive ionotropy?

Answer 7

Positive ionotropy: when an agent causes an increase in the strength of the heart’s muscular contraction.

Question 8

Describe the baroreceptor reflex in hypotension?

Answer 8

• The body’s homeostatic mechanism that helps to maintain blood pressure at nearly constant levels.
• When blood pressure falls, there is less stretch of the baroreceptors, so they fire less action potentials.
• Autonomic neurons within the cardio-vascular control centre in the medulla respond by increasing sympathetic outflow and decreasing parasympathetic outflow.
• These autonomic changes cause vasoconstriction (increased total peripheral resistance), tachycardia and positive inotropy.
• The latter two changes increase cardiac output. Increases in cardiac output and SVR lead to a partial restoration of arterial pressure.

Question 9

Possible causes/treatment of postural hypotension?

Answer 9

• Hypovolaemia – caused by excessive use of diuretics or vasodilatators, dehydration, prolonged bed rest, anaemias
• Addison’s disease, atherosclerosis, diabetes and some neurological disorders
• Cardiac problems: electrical and mechanical

Treatment:

Depends on cause but includes anti-diuretics and a1-adrenergic receptor agonists (e.g. Midodrine)

Question 10

What is the Effect of Heart Rate on Phase Duration and Filling Time?

Answer 10

Resting heart rate is 60 BPM:

• Cycle Length: 1s
• Diastole: 0.66s
• Systole: 0.34s

When excercising heart rate is around 180 BPM:

• Cycle Length: 0.33s
• Diastole: 0.13s
• Systole: 0.20s

During excercise there is much less time for filling/diastole.

If HR increases above 200BPM, stroke volume actually decreases ast there is not enough time for filling, so the ventricles are contracting without having any blood in them. This means DIASTOLIC INTERVAL is CHIEF FACTOR LIMITING MAXIMUM USEFUL HEART RATE

Question 11

What effect does training have on the heart?

Answer 11

• Skeletal and heart muscle mass hypertrophies: increases in size
• Muscle develop more capillaries, thus rate of diffusion increases, so increase in O2 delivery to working tissues
• End diastolic volume (the amount the ventricles fill during systole) increase by up to 200ml
• Stroke volume increases by up to 100ml
• Have a lower resting BP, which can now increase 4 fold during excercise. They have a lower HR because the effect of the parasypathetic and sympathetic systems.
• The max HR does not change.

Question 12

Generally what are the effects of excercise on the body?

Answer 12

• Lowers blood pressure
• Improves circulation.
• Improves endothelial function
• Improved lipid metabolismIncreases ‘good’ HDL cholesterol and reduces bad ‘LDL’ cholesterol levels. LDL can form fatty deposits in the arteries and contribute to heart disease –atherosclerosis
• Increases fat loss & helps weight-loss.This reduces the incidence and severity of obesity and the consequent risk of type 2 diabetes
• Improves glucose tolerance.
• Exercise also reduces stress by releasing endorphins