Physiology 8 - Control of Blood Volume Flashcards
See diagrams at end of physiology 7 lecture
And at start of 8
Which organ is largely involved in blood volume and hence long term blood pressure control?
Kidneys
Is kidney function modifiable?
Yes
Where is angiotensin released from?
The liver
What is the enzyme to angiotensin’s substrate?
Renin
What is the action of angiotensin II?
Constricts renal arteries (vasoconstriction), decreasing blood flow through kidneys and also is involved in renal retention of salt & water
Angiotensin I?
Converted to angiotensin II by an enzyme in the lungs
Study some of the diagrams
On this lecture
In order to change long term MABP, one of what two factors must be changed?
Renal output
Salt and water intake
What is the equilibrium point on an MABP vs input/output graph?
The MABP at which intake of salt and water = renal output of salt and water
What causes decreased osmolarity?
Decreased water or increased salt intake
What does decreased osmolarity do to the oncotic pressure?
Decrease it
Decreased oncotic pressure is detected by the hypothalamus. What changes does the hypothalamus initiate in order to correct this decrease?
Stimulation of hypothalamic thirst centre
Increased antidiuretic hormone (ADH) output –> decreased water loss at kidneys
What 3 factors lead to ADH release?
Increased oncotic pressure
Hypovolaemia (>10%)
Angiotensin II
How does hypovolemia stimulate ADH release?
Atrial baroreceptors normally inhibit/restrict ADH release
Decreased BV in atria –> decreased atrial pressure –> decreased atrial baroreceptor firing –> increased release of ADH
Briefly summarise the mechanism and location of action of ADH.
It increases blood volume by increasing water permeability in the renal collecting ducts. It therefore reduces urine production.
What type of cells is renin released from?
Kidney juxtoglomerular cells
What type of receptors regulate renal activity?
Beta receptors
What conversion does renin perform?
Angiotensinogen –> Angiotensin I
Which enzyme converts angiotensin I to angiotensin II?
ACE (angiotensin converting enzyme)
Where is ACE found?
The lungs
Briefly describe the three mechanisms of action of angiotensin II.
Constricts renal arteries thereby reducing blood flow via the kidneys.
Also causes release of aldosterone from the adrenal glands, which increase Na+ and H20 reabsorption.
Stimulates release of ADH from the pituitary
What is the overall effect angiotensin has on the circulatory system?
Increase in blood volume
What is the atrial natriuretic hormone, when is it released and what effect does it have?
A 28-amino acid peptide synthesised and stored in muscle cells of the atria. It is released in response to stretch of the atria. It helps oppose the effects of the RAAS system, therefore counteracting volume overload.
What is a major cause of whole blood loss?
Haemorrhage
What is a major cause of plasma loss?
Burns
What is a major cause of sodium loss?
Vomiting
List the percentages of blood loss that enable the classification shock into 4 classes.
Class 1: 10-15% loss
Class 2: 15-30% loss
Class 3: 30-40% loss
Class 4: >40% loss
Which classes of shock will require blood transfusion?
Classes 3&4
What is the immediate response/reflex to hypovolaemia?
Baroreceptor reflex
Describe the intermediate response to hypovolaemia.
Arteriolar constriction (–> increased TPR) –> decreased capillary hydrostatic pressure –> net movement of fluid from interstitium to capillaries –> increase in BV
Why is the intermediate response only a temporary solution to decreased BV?
Because net movement of fluid is out of the interstitium, tissues become dehydrated
What is a common consequence of severe hypovolaemia?
Heart failure
What type of resuscitation fluids are used in fluid replacement therapy?
Colloid or Hartman’s
Blood
List 3 other factors affecting BP.
Cortex and emotions
Time of day
Respiration
How is long term control of BP achieved?
Control of BV
