Smaill: Cardiovascular Regulation Flashcards
What are the ways in which tissue fluids and electrolytes are gained or lost?
- Sweating
- Diuresis
- Changes in fluid intake or diet
- Blood Loss
- Diarrhoea
- Vomiting
Despite this osmolality and electrolyte concentrations on body fluid compartments vary little despite wide variation in fluid intake and diet.
What are the fluid compartments of the body?
Body is 60% fluid.
- 2/3 is intracellular fluid
- 1/3 is extracellular fluid
- Interstitial Fluid - Cells in contact directly.
- Plasma
What is special about the barrier separating the plasma and interstitial fluid?
This barrier (capillary wall) is porous and as a result, there is free movement of electrolytes and fluid between the two fluid spaces.
- Sodium moves freely so are in equilibrium.
- Plasma proteins unable to diffuse freely so greater concentration of plasma proteins in plasma compartment.
What is the key pressure involved with fluid exiting the capillaries?
This is the hydrostatic pressure which is the pressure the blood exerts on the capillary walls. This decreases throughout the vascular tree i.e. higher hydrostatic pressure in arteries than capillaries.
Other than vessel size; what other factors affect hydrostatic pressure?
The resistance through the circuit; with a decrease in resistance leading to increased hydrostatic pressure.
- Changes in vessel diameter - Vasodilation/ Vasoconstriction
What is the difference between the barrier of the extracellular fluid and intracellular fluid?
The intracellular fluid has an impermeable membrane. As a result, water movement is totally driven by osmosis. Therefore, osmosis has a huge impact on cell volume.
What occurs if hypotonic saline is added to the interstitial fluid/ECF?
- Osmolality and Osmolarity of the interstitial fluid will drop.
- Water movement from the interstitial fluid into the intracellular fluid.
- Osmotic Equilibrium is reached.
- Swelling of the cells.
What is the primary determinant of the osmolality in the interstitial fluid?
Sodium ions and as a result cell volume - and therefore cell function - is determined by sodium and water balance in the extracellular compartment.
How is plasma osmolality kept regular?
- Cells in the supraoptic and Paraventricular Nuclei of the hypothalamus sense changes in effective plasma osmolality by altering their volume.
- This modulates the synthesis and release of ADH by the posterior pituitary.
- ADH causes aquaporin (H2O channels) to bind to the wall of the collecting ducts.
- These aquaporins act to increase water reabsorption from the urine in the collecting duct.
- Decreases H2O secretion.
- Increased plasma osmolality also stimulates the thirst centres in the hypothalamus.
What is the maximum effective concentration of ADH?
~4.3pg/ml
This is the concentration where the H2O reabsorption from the collecting ducts are unable to increase further as the urine has reached full concentration.
How are Extracellular Volume and Blood Volume Related?
Extracellular Volume directly affects the volume of the Interstitial Volume as the two remain in equilibrium via osmosis (the movement of water). As a result, if hypotonic saline is added to the ECF water will flow from the ECF into the interstitial fluid. As the ISF and Plasma directly linked changes in the ISF - such as an increase in volume - there will also be changes in the volume of the plasma - such as an increase in its volume. Thus increasing the blood volume as well.
What are the effects of hypotonic saline being added to the ECF?
- Water movement into ISF from the ECF.
- Increased Plasma Volume.
- Increased Blood Volume.
- Increased cardiac filling pressure and therefore stroke volume.
- Assuming no change occurs to HR and Peripheral Resistance there will be increased CO and Mean Arterial Pressure.
What are the roles of cardiac receptors?
Both myelinated and unmyelinated cardiac receptors are sensitive to small changes in the cardiac filling.
- If there is >10% loss of BV; arterial pressure falls and arterial baroreceptor firing is reduced.
- This 10% is important as it enables us to donate blood but also allows us to maintain cardiovascular homeostasis.
What determines the input and output of water?
- Input is mainly determined by the thirst mechanism.
- The output is largely controlled by the handling of H2O by the kidney.
- There are obviously a number of other areas of loss but the kidney is the main control in the output process.
How does a reduction of blood volume effect the kidney?
Reduced Pressure - due to reduced blood volume - at the terminal end of the juxtaglomerular apparatus activates the renin-angiotensin-aldosterone cascade.
- Upregulation of the thirst mechanism.
- Reduction in water and sodium excretion in the kidney.
What is the neuro-humoral response to a reduction in ECF volume?
- Fall in firing of cardiovascular baroreceptors
- Reduced afferent input to the SNS
- Immediate increase in HR, Contractile state
- Decrease in PNS input to the heart
- Widespread vasoconstriction of precapillary and sometimes evem post capillary due to SNS activation
What happens to water transfer in the vascular system due to an increase in sympathetic tone?
A fall in blood volume results in an increase in precapillary resistance - due to vasoconstriction in response to norepinephrine. Therefore, hydrostatic pressures at the level of the capillary decrease leading to a reduction in the water transported out of the capillaries. - known as the internal transfusion
- The greater the blood volume loss the greater the reabsorption in the capillary bed.
What occurs with blood loss of less than 10%? (graph)
- No change in the firing of systemic arterial baroreceptors as there is very little change in pressure. However, there is a profound decrease in the firing of cardiac receptors.
- Levels of aldosterone and ADH both increase substantially.
- Increased heart rate and inotropic state.
Cardiac Receptors play a central role in maintaining blood volume fluctuations of less than 10%.
What occurs when blood loss increases above 10%? (graph)
- Arterial Baroreceptors firing is progressively reduced.
- Further increases in the release of aldosterone and ADH.
- Increased HR and inotropic state.
- Constriction of resistance and capacitance vessels becomes increasingly intense.
- In severe blood loss blood flow to some vascular beds may be cut completely.
What is the cardiac response to posture?
- Increased cardiac filling when supine.
- End-diastolic volume decreases substantially when standing.
- Venous pooling in the lower limbs due to increased vasodilation in the feet. NOT BECAUSE IT’S HARD TO PUSH AGAINST GRAVITY
- This venous pooling causes 400mL (10%) of blood to leave the circulation and therefore is essentially lost from blood volume. Hence, the importance of being able to accommodate for a blood loss of 10%.
- SV drops as a result of this
- No loss of perfusion pressure so cerebral perfusion remains
- Venous pooling in the lower limbs due to increased vasodilation in the feet. NOT BECAUSE IT’S HARD TO PUSH AGAINST GRAVITY
Are baroreceptors able to be reset?
Baroreceptors are able to be reset if you experience chronic increases or decreases in blood pressure. When this occurs your threshold is shifted and they will operate in the same way at higher pressures as they would at lower pressures.
- Therefore only really useful in acute response.
What did the experiment comparing intact baroreceptors with eliminated baroreceptors conclude?
When baroreceptors were eliminated the pressure was far more variant in response to changes in posture and exertion etc, however, the average pressure being maintained is about the same.
- This shows that baroreceptors buffer rapid changes in pressure but that the pressure at which they operate is being set by something else.
What happens when all of the arterial baroreceptors and cardiac receptors have been denervated?
- When you denervate all of the arterial baroreceptors and the cardiac receptors there is a clear grouping and elevation of mean arterial pressure, however, it is higher than that when the individual has not been denervated.
- This indicates that the things that control blood fluid volume - and therefore blood volume - are really important in the long-term control of mean arterial pressure.
