Autonomic Reflexes Flashcards
What is an alternative equation for cardiac output
CO= HR x force of contraction
Summarise the autonomic control of heart rate
No autonomic control: intrinsic rate 100-110bpm
Vagus nerve: lowers the HR to normal 72bpm
Describe the baroreceptor reflex at rest
Parasympathetic nerve stimulated to decrease HR
Inhibitory nerve leading to the sympathetic nerve inhibited to reduce sympathetic response (some baseline activity)
PSNS > SNS
What happens when BP increases
Massive increase in firing of baroreceptors
Significant inhibition of the sympathetic system, reducing the attempt to increase HR
Significant stimulation of the parasympathetic nerves leading to significant decrease on HR
Attempt to reduce CO to normalise BP
What happens when BP decreases
Decreased baroreceptor firing
Decreased parasympathetic inhibition of HR
Decreased inhibition of the sympathetic nervous system, leading to increased HR and vasoconstriction
Increases CO to normalise BP reflexively
What is the effect of sympathetic stimulation on the heart
The sympathetic stimulation of the heart causes:
§ An ionotropic effect – increased SV due to increased contractility.
§ A chronotropic effect – increased HR due to the effect on the nodal tissue, speeding up rate of contractions. This means cardiac output increases.
Describe the problem that a change in posture presents
Problem - Movement from supine to standing position is a severe challenge to human circulation
Vertical position – Usual pressure resulting from cardiac contraction b) Effect of gravity on column of blood
e.g. In a foot capillary;
= 25mmHg
= 80mmHg
Total = 105mmHg
Pressures in legs increase, heart stays the same, head decreases
Describe the effect of gravity and use this to explain orthostatic hypotension
Gravity: causes unequal distribution of blood volume when standing; blood pressure above heart is reduced and that below is increased because blood pools in veins (distension occurs)
Orthostatic hypotension: increased formation of tissue fluid and venous pooling leads to decreased return, decreasing CO due to the Frank-Starling relationship
What is the consequence of increased hydrostatic pressure in the legs
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
Bulk Flow
What is the effect of increased venous distension and capillary fluid loss
reduced ventriclular filling during diastole (end-diastolic volume) - reduced volume of ejected blood during the resulting systolic contraction (stroke volume)
How do we normally compensate for postural hypotension
BP Fall Reflex Arc: Decreases the rate of baroreceptor firing, leading to…
Decreased parasympathetic discharge to heart - increases HR (/\ CO)
Increased sympathetic discharge to heart - increases contractility (/\ SV) and HR (/\ CO)
Increased sympathetic discharge to veins - increases tone, pressure and hence venous return to increase EDV; Frank-Starling allows for increased SV and hence CO
Increased sympathetic discharge to arterioles - causes vasoconstriction to increase TPR
BP = TPR x CO - therefore increasing both increases BP
Sympathetic Discharge: baseline tonic activity that is normally inhibited baroreceptor
Also increased release of adrenaline
Describe transient hypotension
When you stand up from sitting down, you feel the effect of gravity pulling down the blood in your body more. In the normal reaction of your body, this:
§ Decreases venous return so contractility of the heart decreases so CO decreases.
§ This stimulates the BRs less so SNS is stimulated more which compensates the BP.
Describe clinical postural hypotension
When you stand up from sitting down, you feel the effect of gravity pulling down the blood in your body more. In the abnormal reaction of your body, this:
§ Decreases venous return so contractility of the heart decreases so CO decreases.
§ The SNS reflex, however, is impaired meaning the SNS cannot compensate which may result in fainting as blood cannot be pumped at a high enough pressure to reach the head.
§ By fainting, your body lies flat so blood can get to the head.
Cerebaral hypo-perfusion as arterial pressure is not maintained
Summarise autonomic control of the pupils
The parasympathetic nerve that innervates the eye is the oculomotor nerve (CN III) – this nerve goes to the ciliary ganglion.
§ The PNS stimulates pupil constriction.
§ The SNS stimulates pupil dilation.
Describe pilocarpine
§ Pilocarpine is a drug (an ACh drug) that activates the PNS and can act to constrict the pupils.
§ This process is known as miosis – the muscarinic ACh receptors are activated in the iris.
