53.1 Changes in Posture Flashcards
What is orthostasis?
Physiological response to counteracting a fall in blood pressure when standing up from a supine or seated position.
What is the orthostatic challenge?
The sudden re-introduction of the effects of gravity to the fluids of the body, causes around 0.5L of blood to pool towards the veins of the lower limbs unless corrected.
What is the first physiological thing that happens when we stand up?
Right atrial mean pressure decreases (blood from body to the heart)
How does the circulation change when moving from lying to standing?
Blood pools to the lower limbs into dependent vessels from intrathoracic compartments
Reduces venous return to the heart leading to a fall in cardiac output
What are the consequences of orthostasis which need to be overcome?
-Stretch and high pressure in dependent capillaries
-Sub atmospheric pressure in subdural venous sinuses of the skull
What is the consequence of stretch and high pressure in dependent capillaries?
*Increased filtration, can result in oedema
*Impaired exchange of gases, nutrients, and waste. Could lead to hypoxia/ impaired tissue function.
What are the consequences of sub atmospheric pressure in subdural venous sinuses of the skull?
*Sub-atmospheric pressure in the venous sinuses helps facilitate the return of blood from the brain to the heart. If this pressure is too low, it can lead to impaired venous return, potentially causing increased intracranial pressure
*The pressure in the venous sinuses influences the absorption of CSF. Sub-atmospheric pressure aids in the drainage of CSF into the venous system. Disruption in this pressure can lead to conditions like hydrocephalus, where excess CSF accumulates in the brain
Which receptors detect the fall in blood pressure? Where do they signal to?
*Arterial blood pressure decreases is detected first by low-pressure cardiopulmonary stretch receptors within the arterial walls.
*The pooling resulting from reduced venous return quickly decreases afferent signalling, which triggers the baroreflex through the solitary nucleus.
*if hypotension reaches arterial and carotid baroreceptors, these signal through CNX and IX (resp.) to produce similar response to baroreflex.
What reflex is triggered during orthostasis?
Baroreflex
What happens as a result of baroreceptor activation?
Solitary nucleus signals to paraventricular nuclei to send sympathetic efferent fibres to:
-Increase heart rate
-Vasocontrict arterioles (specifically in splanchnic circulation) to raise peripheral pressure
Other than the baroreceptor reflex, what else is important in maintaining blood pressure during orthostasis?
Muscle pumps:
Muscles of legs and abdomen tighten when a person stands
Why is the dip in arterial pressure during orthostasis smaller in a cool environment than a warm one?
Arterioles at lower extremities are already constricted in a cool environment
In a warm environment the arterioles are more dilated so orthostasis leads to a faster transfer of blood from the thoracic pool to the legs
What is the cardiovascular reflex response to change in posture?
- The action of standing leads to a rapid pooling of venous blood in the legs due to gravity, leaving less blood in the large central veins for return to the heart. More than 0.5L of blood is redistributed in this way upon standing.
- The reduction in preload leads to a reduction in stroke volume (Frank–Starling law; Fig. 6.35, graphic p.445) and, consequently, cardiac output. Arterial pressure falls momentarily. Baroreceptors in the large veins and atria detect the fall in central venous pressure.
- Signals from the low-pressure baroreceptors are processed in the cardiovascular centres of the medulla and the sympathetic nervous system is stimulated (Fig. 6.43).
- Heart rate increases, peripheral vascular resistance increases, central veins contract—all of which returns arterial and venous pressure to near-normal levels within a few seconds.
