21-09-22 - Microcirculation, Venous Blood Flow, and Venous Return Flashcards
Learning outcomes
- To identify the major routes across capillary membranes of fluids, solutes and larger molecules/proteins.
- To explain how Starling’s forces contribute to fluid homeostasis and the net transcapillary movement of water across capillary beds, including the importance of the lymphatic system.
- To describe the factors which affect venous return and consequently determine cardiac output and blood pressure.
What type of capillaries are used in capillary beds?
What molecules can pass through continuous capillaries?
What does the interstitium consist of?
Where is interstitial fluid found?
Where does interstitial fluid come from?
Where is most of the interstitial fluid held?
How does diffusion rate in the tissue gel compare to that in the free fluid?
- Continuous capillaries are used in capillary beds
- Only small molecules, such as water, glucose, hormones, and gas can move through continuous capillaries
- The interstitium consists of collagen and proteoglycan filaments between cells
- Interstitial fluid comes from substances that leak out of blood capillaries
- Interstitial fluid is found trapped amongst the filaments of the interstitium
- A majority of the interstitial fluid is held in the gel, with 1% being free water in the form of free vesicles
- Diffusion occurs in the tissue gel 95-99% as rapidly as in the free fluid
What is diffusion?
What substances move across blood capillaries by diffusion?
What is bulk flow?
What substances can move across blood capillaries by bulk flow?
What is filtration?
What is reabsorption?
What substances can not move through the capillary wall?
What is the pressure and velocity of flow like in capillaries?
Why is it like this?
- Diffusion is the gradual movement of concentration within a body due to a concentration gradient
- Nutrients, oxygen, and metabolic end products (e.g CO2) can move across blood capillaries by diffusion
- Bulk flow is movement of an entire body due to a pressure gradient
- Crystalloids (e.g Na+, Cl-, K+) are small water-soluble molecules that can move across blood capillaries via bulk flow
- When moving from the blood to the interstitial space, bulk flow is termed filtration
- When moving from the interstitial space to the blood, bulk flow is termed reabsorption
- Colloids, such as plasma proteins, are filtered out and remain in the capillary due to permeability being low for proteins because of their size and shape
- Capillaries have low pressure and low flow velocity in order to accommodate these exchanges
What is oncotic pressure (colloid osmotic pressure)?
What is oncotic pressure based on?
What substances form oncotic pressure?
How are areas of high and low oncotic pressure formed?
What are the pressure values for these areas of high and low oncotic pressure?
How does oncotic pressure affect movement of water?
- Oncotic pressure (aka colloid osmotic pressure) is a form of osmotic pressure
- It is based on the charge of protein groups that attract water
- Oncotic pressure is formed by plasma proteins – predominantly albumin, and to a lesser extent globulins
- Formation of oncotic pressure:
1) The permeability for albumin is 1/1000th that of water, meaning albumin is filtered out and not able to move out of the capillary into the interstitial space
2) When these plasma proteins are present in high concentration in the capillary, this will form an area with high oncotic pressure within the capillary, compared to an area of low oncotic pressure in the interstitial space where albumin concentration is low - Oncotic pressure in the capillaries is 28mmHg
- Oncotic pressure in the interstitial space is 5-8mmHg
- Water will flow from areas of low oncotic pressure to areas of high oncotic pressure, meaning fluid will move from the interstitial space to the capillary
What is the role of capillary hydrostatic pressure?
How does hydrostatic pressure affect fluid flow?
What is the hydrostatic pressure like in the capillaries and interstitium?
What is capillary hydrostatic pressure at the arteriole and venule end of capillaries?
Why does this change?
What does this change cause to happen?
- Hydrostatic pressure forces fluid out of the capillaries into the interstitium
- Hydrostatic pressure will cause fluid to flow from areas of high hydrostatic pressure to areas of low hydrostatic pressure (pushes away fluid)
- This means the hydrostatic in the capillaries is high, while the hydrostatic pressure in the interstitium is low (essentially negligible in most cases)
- The hydrostatic pressure at the arteriole end of capillaries is 30-40mmHg
- The hydrostatic pressure at the venule end of capillaries is 10-15mmHg
- As hydrostatic pressure pushes fluid out of the capillaries, this causes the pressure by the blood on the vessel wall to be lower, which causes a decrease in pressure
- This decrease in pressure as we get to the venule end of the capillary leads to more reabsorption occurring via oncotic pressure then filtration by hydrostatic pressure
What do Starling forces look at?
Describes the 6 steps in the process of how starling forces change as we move through the capillary
- Starling forces looks at how hydrostatic and oncotic pressures in the interstitial and capillary environment balance and determine movement of fluid
- How starling forces change as we move through the capillary:
1) Oncotic pressures stay relatively the same throughout the capillary
2) There is a small amount of oncotic pressure pulling fluid into the interstitial environment, but this is counteracted by the fact we have a lot of albumin in the capillary that pulls fluid in. Albumin concentration isn’t ever diluted as new blood with more albumin is constantly supplying albumin
3) Hydrostatic pressures are high at the arteriole end of the capillary and negligible in the interstitial space throughout the whole capillary
4) Hydrostatic pressures start pushing fluid out of the capillary, but are weak at the venule end, leading to a decreased amount of fluid being pushed out
5) Net fluid movement at the arteriole end is into the interstitial space via filtration, while net fluid movement at the venule end is into the capillary via reabsorption
6) Overall, we lose a slight bit of fluid, meaning there is a growing amount of fluid in the interstitial space. This fluid gets drained into the lymphatics and returned to the CVS, with 2-3 litres of fluid being lost and drained everyday
How much fluid does the lymphatic system drain a day?
What does the lymphatic system consist of?
What does the lymphatic system drain?
What does it pass through?
What 4 things is the lymphatic system important controlling?
- The lymphatic system drains about 2-3 litres of fluid lost from the capillaries a day
- The lymphatic system consists of large, fenestrated walls of capillaries
- The lymphatic system drains via lymphatic vessels and passes through lymph nodes
- The lymphatic system is important in controlling:
1) Concentration of proteins in interstitial fluid (as proteins can pass through fenestrated capillaries
2) Volume of interstitial fluid
3) Interstitial fluid pressure
4) Parts of immune response
What is resistance like in systemic venous circulation?
What effect does this have on blood pressure?
What volume is present in this system?
What is venous return to the heart a major determinant of?
How is this done?
- Systemic venous blood resistance is low, as we are past where major resistance will be (small arteries/arterioles)
- This low resistance results in the systemic venous circulation being low in pressure (between 3-18mmHg)
- The systemic venous circulation holds about 60% of total blood volume
- Venous return to the heart is a major determinant of cardiac output
- This is done via the Frank-Starling mechanism, which matches venous return to stroke volume, with greater degrees of stretch leading to greater contractility
What are 4 factors that aid/facilitate venous return?
- 4 factors that aid/facilitate venous return:
1) Sympathetic Innervation
* Sympathetics in arterioles is about maintenance of resistance
* Sympathetics in veins squeeze the veins (vasoconstriction) , decreasing the venous compliance and capacity, which increases blood pressure and opens the valves
* The blood wants to flow from an area of high pressure to low pressure, so the blood will move through the vein faster, with the venous valves in place to ensure the blood travels towards the heart
* This increase in venous return leads to an increase in cardiac output, which is important during cases of blood loss or exercise
2) Skeletal muscle pumps
* Collection of skeletal muscles (mainly in legs) that’s contraction aids the venous return to the heart by squeezing the veins and decreasing capacity and compliance, which increases the pressure and open the valves
* The blood wants to flow from an area of high pressure to low pressure, so the blood will move through the vein faster, with the venous valves in place to ensure the blood travels towards the heart
3) Inspiratory movement
* During inspiration:
* The diaphragm descends – this increases abdominal pressure, which is transmitted passively to intraabdominal veins (venous blood is pushed through)
* The pressure in the thorax decreases – this decreases the pressure in the intrathoracic veins and right atrium (venous blood is pulled into the right atrium)
* Both of these changes lead to a greater pressure difference between the peripheral veins and heart, which will draw more blood into the right atrium
* This results in greater venous return.
4) Blood volume
* The greater the volume in the veins, the greater the venous pressure and blood flow
* The heart can accommodate this increase in blood volume because of the Frank-Starling mechanism
* Blood loss (e.g from haemorrhage) will predominantly be from the venous system.
* This decrease in blood volume will lead to decreased pressure, which impedes venous flow
Vein valves and muscle pumps diagrams
Postural effects of standing completely still.
What is mean arterial blood pressure (MABP)?
What is the MABP when standing still?
How does Pressure change as we move up and down the body?
What will this be at the feet?
Why is this?
What stops working as we stand still?
Why does this change when we are recumbent?
- Postural effects of standing completely still
- Mean arterial blood pressure is the average blood pressure in an individual during a single cardiac cycle
- These pressures values on the diagram are where the pressure values normalise at with gravity being the only thing affecting fluid in the body
- The MABP standing still is about 100mmHg and located at the level of the heart
- Pressure increases by 1mmHg for each 13.6mm below the surface
- This is +90mmHg by the feet, meaning the blood pressure is 190mmHg at the feet
- We have venous return flowing to the level of the right atrium, so the heart level would at 0mmHg
- The gravity draining the venous blood from the head means there’s negative pressure in veins above the level of the heart
- There is a +90mmHg venous pressure at the feet, and a -39mmHg venous pressure at the head.
- This greater difference in pressure means we have to have a lot of pressure in order to overcome gravity, as we are also fighting against a negative pressure
- After standing still for a while, blood begins to pool in the legs due to gravity (oedema) with 10-20% of blood volume gathering in the legs in 15-30 minutes
- The venous valves and muscle pumps needed to pump blood to the heart and head stop working when standing still, which will lead to inadequate venous return, a reduction in blood pressure, and eventually fainting
- When we are recumbent, the whole body is roughly on the same level, meaning the effects of gravity isn’t causing blood to pool and negative pressures aren’t being generated, which will make it easier to pump blood back to the heart
- There is a normalised effect of gravity across the body
- The arterial pressure and venous pressure will be low, and roughly be the same throughout the whole body, leading to adequate circulation
