regulation of fluid balance in the microcirculation Flashcards
What is the main characteristic of continuous capillaries?
Continuous capillaries are the least permeable type, found in most tissues. The endothelium forms a monolayer joined by tight junctions.
Where are continuous capillaries commonly found?
Continuous capillaries are found in most tissues.
What do fenestarted capillaries contain and what is unique about this?
Fenestrated capillaries contain pores (fenestrae) in their endothelial cells, making them 10x more permeable to small hydrophilic molecules compared to continuous capillaries.
In which organs are fenestrated capillaries typically located?
Fenestrated capillaries are found in the kidneys, joints, and intestinal mucosa.
What is the defining feature of sinusoidal (discontinuous) capillaries?
Sinusoidal capillaries have large spaces between endothelial cells, allowing large molecules like proteins to diffuse across the capillary wall.
Where can sinusoidal capillaries be found?
Sinusoidal capillaries are present in the liver, bone marrow, and spleen.
What is the glycocalyx, and what role does it play in diffusion across the capillary wall?
The glycocalyx is a glycoprotein layer covering the luminal surface of the endothelium, creating a barrier to diffusion that depends on the size of the permeating species.
How does water primarily flow across the endothelial wall?
Water flows across the endothelial wall via intercellular clefts, also known as the paracellular route.
Which substances can pass through the endothelial cells of capillaries, and how?
Lipid-soluble substances, such as O₂ and CO₂, pass through endothelial cells by diffusion.
How do small water-soluble substances like Na⁺, K⁺, glucose, and amino acids cross the capillary wall?
These substances pass through small pores in the endothelial cells.
What happens to plasma proteins during diffusion across the capillary wall?
Plasma proteins generally cannot cross the capillary wall and are retained in the plasma.
How are exchangeable proteins transported across the capillary wall?
Exchangeable proteins are moved across the wall via vesicular transport mechanisms.
What is Fick’s First Law of Diffusion?
Relative permeability of capillaries to different substances
How much plasma is pumped through the capillaries each day?
Approximately 4000 liters of plasma are pumped through the capillaries each day.
How much fluid flows across the capillary walls in both directions daily?
Around 80,000 liters of fluid flows across the capillary walls daily.
What is the net volume of fluid filtered daily by the microcirculation?
Only about 2–4 liters are net filtered daily by the entire microcirculation.
How is filtered fluid returned to the bloodstream?
It is returned via the lymphatic system.
What happens if net filtration increases locally or systemically?
Fluid accumulates in tissues, resulting in a condition called oedema.
What balance maintains the fluid flow across the capillary walls?
Filtration and absorption are almost perfectly balanced, ensuring minimal net fluid loss.
What is the typical hydrostatic pressure at the arteriolar and venous ends of an open capillary?
~40 mmHg at the arteriolar end and ~15 mmHg at the venous end.
What is the hydrostatic pressure in the tissue spaces (interstitium)?
~0 mmHg (atmospheric pressure).
What does the hydrostatic pressure gradient (Pcap - Pint) do?
It drives water out of the capillaries.
How does gravity affect hydrostatic pressure in the microcirculation?
Gravity increases hydrostatic pressure below the heart and decreases it above the heart.
Why is the effect of gravity often neglected in typical discussions of capillary hydrostatic pressure?
Because standard explanations focus on capillary dynamics without considering positional effects.
What exerts osmotic pressure in solution?
Substances dissolved in solution.
How is the osmotic pressure gradient calculated?
It is the difference in osmotic pressures between two compartments multiplied by the reflection coefficient (σ).
Why do electrolytes and glucose not affect the distribution of water across capillaries?
They cross the capillary wall very easily and have a reflection coefficient of 0.
Thus, the crystalloid osmotic pressures do not affect the distribution of water.
What is the reflection coefficient of proteins, and why is it significant?
Proteins have a high reflection coefficient (~0.9) because their permeability across the capillary wall is very low.
Therefore, the difference in the oncotic (or colloid osmotic) pressures across the capillary wall concentrations has a powerful effect on the movement of water across the capillary wall
What are the typical osmotic pressures for plasma (πp) and interstitial fluid (πint)?
Plasma: ~25-30 mmHg; Interstitial fluid: 5-10 mmHg.
What is the effect of the osmotic pressure gradient (πp - πint) across the capillary wall?
It tends to draw water into the capillaries.
What does the hydrostatic pressure gradient do in the capillary system?
Drives water out of the capillaries into the interstitium.
What does the osmotic pressure gradient do in the capillary system?
Draws water into the capillaries from the interstitium due to the presence of proteins.
What is the typical protein concentration in the capillaries versus the interstitial fluid?
Capillaries: 60-80 g/L; Interstitial fluid: 20-30 g/L.
What determines the net movement of fluid across an exchange vessel
It is driven by the overall pressure gradient acting on the fluid, as described by the Starling equation:
What is the reflection coefficient (σ) in the Starling equation?
It determines the permeability of the capillary wall to proteins and other solutes.
What is the average daily net whole-body filtration in the microcirculation?
2-4 liters per day.
How does standing affect fluid filtration in the lower extremities?
Standing increases 𝑃cap, transiently increasing filtration in the lower extremities.
How does the body respond to increased
𝑃cap in the lower extremities while standing?
Reflexes cause arteriolar constriction, reducing 𝑃cap and partially compensating for the rise in filtration.
What does the Net Filtration Pressure (NFP) equation represent
Net Filtration Pressure (Jv) = (Pc – Pif) – (Πp – Πif)
Pc : Capillary pressure
𝑃if: Interstitial fluid pressure
Πp: Plasma colloid osmotic pressure
Πif : Interstitial fluid colloid osmotic pressure
What is the significance of a positive NFP?
A positive NFP results in steady filtration of fluid from the plasma into the interstitial fluid (IF).
What force drives fluid out of the capillaries into the interstitial space?
Capillary pressure (𝑃𝑐) drives fluid out of the capillaries.
Which force opposes the movement of fluid out of the capillaries?
Interstitial fluid pressure (𝑃𝑖𝑓) opposes fluid movement out of the capillaries.
What happens to capillary filtration when arterioles are open?
When arterioles are open (relaxed), capillaries are primarily filtering fluid due to higher capillary hydrostatic pressure (𝑃cap)
How does vasoconstriction affect capillary hydrostatic pressure?
Vasoconstriction reduces capillary hydrostatic pressure (𝑃cap), making absorption more dominant than filtration.
What are the typical pressures when arterioles are relaxed?
What are the typical pressures when arterioles are contracted?
How do vasodilators contribute to oedema?
Vasodilators reduce peripheral resistance, increase capillary hydrostatic pressure, and enhance filtration, potentially leading to oedema.
Why do organs specialized for water reabsorption, like the kidneys, favor reabsorption?
Proteins are continually washed out of the interstitium, lowering their concentration and creating a large osmotic pressure gradient (ΔΠ) that favors reabsorption.
How does fluid reabsorption in the nephron aid in water absorption in the kidneys?
Fluid reabsorption dilutes proteins in the interstitium, reducing interstitial concentration and helping water move back into the blood.
Which other organ exhibits a similar reabsorption mechanism to the kidneys?
The intestinal mucosa.
How does low interstitial protein concentration affect reabsorption?
It increases the oncotic pressure gradient, which favors water reabsorption into the capillaries.
What is the typical pressure distribution in the kidney’s capillaries during reabsorption?
P cap: ~40 mmHg
Πcap: ~27 mmHg
𝑃int: ~0 mmHg
Πint : ~2 mmHg
What happens to plasma volume in the lower extremities during an orthostatic challenge?
Plasma volume is lost due to increased capillary hydrostatic pressure, causing fluid to move out of the capillaries into the interstitial space.
What is the capillary hydrostatic pressure (𝑃cap) during an orthostatic challenge?
approximately 70 mmHg
What is the typical pressure distribution in the kidney’s capillaries in lower extremities in response to orthostatic challenge
What is oedema?
Oedema occurs when fluid filtration exceeds absorption and lymphatic flow, causing fluid to accumulate in the interstitial compartment
What is the primary cause of oedema?
An imbalance where filtration is greater than the sum of absorption and lymphatic flow.
What are the four main factors that promote oedema?
1) Increased capillary hydrostatic pressure
2) Increased capillary or venular permeability
3) Decreased plasma oncotic pressure
4) Lymphatic obstruction.
How does increased capillary hydrostatic pressure cause oedema?
It forces more fluid out of the capillaries into the interstitial space. Examples include heart failure and venous obstruction (e.g., thromboembolism).
What is the role of venular permeability in oedema?
Increased permeability, often due to inflammation, allows proteins to leak into the interstitium, increasing osmotic pressure and fluid retention.
How does decreased plasma oncotic pressure lead to oedema?
Conditions like malnutrition, burns, or liver dysfunction reduce plasma protein levels, decreasing oncotic pressure and leading to fluid retention in the interstitial space.
What can cause lymphatic obstruction leading to oedema?
Lymphatic obstruction can result from lymph node removal, lymphatic inflammation, or invasion by parasites.
What role does the lymphatic system play in preventing oedema?
The lymphatic system helps return excess interstitial fluid to the bloodstream, preventing accumulation in tissues.
What is the pressure gradient through capillaries?
The pressure at the arterial end is approximately 30 mmHg, while at the venous end it is about 10 mmHg.
What is the Net Filtration Pressure (NFP) at the arterial and venous ends of capillaries?
NFP is positive at the arterial end (promotes filtration) and negative at the venous end (promotes reabsorption).
What happens to fluid at the arterial end of capillaries?
Some fluid leaves the capillaries due to positive NFP
What happens to fluid at the venous end of capillaries?
Fluid returns to the capillaries due to negative NFP.
What percentage of lost fluid returns to venous capillaries?
About 90% of the lost fluid returns to venous capillaries.
How is the remaining 10% of fluid handled?
The remaining 10% of lost fluid is removed by the lymphatic system.
What is the main function of the lymphatic system?
It preserves fluid balance, transfers fat absorbed in the small intestine to the circulatory system, and transports foreign materials to lymph nodes for immunosurveillance.
What structures in lymphatic capillaries allow one-way fluid entry?
Intercellular clefts allow the one-way entry of fluid, containing water, salts, proteins, and bacteria, driven by tissue compression.
What is a lymphangion, and what is its role?
A lymphangion is a segment of a lymphatic vessel between valves, containing smooth muscle that contracts rhythmically to pump fluid forward.
How do lymphangions contribute to lymphatic circulation?
They act like a miniature heart with pacemaker cells, creating rhythmic contractions to pump fluid forward.
What pressure is required for the lymphatic system to effectively pump fluid?
Compression of 40-50 mmHg is needed to ensure proper lymphatic pumping.
Why is the rhythmic contraction of lymphatic trunks important?
It prevents stagnation and is crucial for processes like stopping envenomation.
What occurs at the lymph node during lymphatic drainage?
Reabsorption of some lymph fluid occurs, and lymphocytes can become activated by antigens entering via the afferent lymphatics.
What is the function of afferent lymphatic vessels?
They carry lymph fluid into the lymph nodes
How do lymphocytes cycle through the lymphatic system?
Lymphocytes move from blood to lymph glands to blood and can become activated in lymph glands by antigens.
