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What is the primary function of capillaries in the cardiovascular system?
The diameter of capillaries is larger than that of red blood cells.
False
What is the primary mechanism through which lipid-soluble substances are exchanged in capillaries?
Diffusion
Capillaries contain only about ___% of the blood volume.
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Match the following components with their roles in the capillary system:
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What is the primary function of fenestrated capillaries?
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Hydrostatic pressure pushes fluid into the blood vessel.
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What term describes the mass movement of water and dissolved substances across capillary walls?
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The pressure generated by proteins that remains in the capillary and counterbalances hydrostatic pressure is known as __________ pressure.
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Match the type of pressure with its function:
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What is the primary mechanism through which excess fluid in interstitial tissues can lead to edema?
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High protein levels in blood plasma decrease osmotic pressure in interstitial fluid.
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What term describes the drainage of interstitial fluid back to the venous system?
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Edema can occur due to an increase in ________ pressure in blood vessels.
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Match the following causes of edema with their descriptions:
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Which of the following substances are known to easily diffuse across a cell membrane in capillaries?
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Capillaries prevent the exchange of waste products with interstitial fluid.
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What regulates the net movement of fluid across capillary walls?
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Capillaries receive blood from the smallest __________.
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Match the types of capillary exchange mechanisms with their descriptions:
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What primarily drives the filtration of fluid out of capillaries?
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Osmotic pressure is responsible for pushing fluid out of the capillaries.
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What is the main effect of increased hydrostatic pressure at the beginning of a capillary?
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The movement of fluid back into the capillaries is primarily driven by __________ pressure.
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Match each type of pressure with its corresponding function:
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Which of the following can lead to edema formation?
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Edema is caused only by high venous pressure.
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What is the primary function of the lymphatic system in relation to interstitial fluid?
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A decrease in __________ pressure due to the loss of proteins can cause fluid to move out of blood vessels into the interstitial space.
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Match the following causes of edema with their descriptions:
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Study Notes
Capillaries
- Only contain 5% of the total blood volume
- Key site for exchange of water, nutrients, waste products and gasses between blood and interstitial fluid
- 7-8µm diameter, 0.5mm in length
- Red blood cells are roughly 7.5µm in diameter
- Single layer of endothelial cells with dense networks
- Receive blood from the smallest arterioles and metarterioles, which connect arterioles to the capillary network
- The density of capillaries varies depending on the metabolic activity of the tissue
- Metarterioles have rings of smooth muscle that can open and close on demand (e.g., in skeletal muscle, all open during exercise)
- The small diameter of capillaries results in slow blood flow, however, the extensive parallel ramifications reduce resistance
Transcapillary Exchange
- Diffusion depends on the type of capillary and the properties of the substance. It follows a gradient between blood and interstitial fluid.
- Lipid soluble substances exchange freely across cell membranes (e.g., oxygen and carbon dioxide)
- Most capillaries possess pores or clefts that allow transfer of water and lipid-insoluble molecules (e.g., sodium, chloride, glucose, amino acids). The size and number of pores vary depending on the tissue.
- Some capillaries are fenestrated (e.g., intestines, liver, kidneys). These capillaries have vesicles that fuse to form large gaps across endothelial cell membranes, which allow water and water-soluble macromolecules to pass through.
Bulk Flow = Fluid Exchange
- Refers to mass movement of water and dissolved substances
- Occurs across capillary walls.
- Towards interstitial fluid = filtration
- Towards the intravascular fluid = absorption.
- Maintains fluid balance between intravascular and interstitial fluid.
- Depends on:
- Pressure gradients (hydrostatic and osmotic)
- Permeability of the vessel (porosity)
- Size of diffusion surface
- Blood flow
Pressure Gradients
- Hydrostatic pressure:
- Filtration pressure
- Pressure from the heart, arterioles and capillaries
- Pressure in interstitial fluid is close to 0
- Pushes fluid out of the vessel
- Colloid osmotic pressure (oncotic):
- Due to large proteins that remain in the vessel
- Counterbalances hydrostatic pressure
Starling Forces
- The net movement of fluid depends on the balance between filtration (hydrostatic) and osmotic (oncotic) pressures.
- Hydrostatic pressure difference pushes fluid out.
- On the other end, osmotic pressure difference pushes fluid in, and remains constant along capillary length.
- Along the length of the capillary:
- In the first portion, hydrostatic pressure is stronger resulting in more fluid exiting the vessel.
- Toward the end, hydrostatic pressure decreases, resulting in less fluid exiting the vessel. If hydrostatic pressure falls below osmotic pressure = fluid moves into the vessel.
Where Does The Fluid Go?
- The net effect is generally accumulation of fluid into interstitial tissue.
- The lymphatic system picks up the fluid and drains it to the large veins (lymphatic drainage).
- Fluid that remains in the blood collects into venules and returns to the heart.
- If the lymphatic system cannot keep up with drainage, edema formation occurs.
Physiological Significance
- Diffusion: Enables nutrient and gas exchange.
- Bulk flow: Stabilizes blood volume by using interstitial fluid as a buffer.
- Blood loss = decreases pressure = decreases hydrostatic pressure = fluid moves in
- Excess fluid intake = increases pressure = increases hydrostatic pressure = fluid accumulates interstitially
- Low protein in plasma = decreases osmotic pressure = fluid out
- High protein in plasma = increases osmotic pressure = fluid in
Edema Formation
- Refers to the abnormal accumulation of interstitial fluid.
- Four main mechanisms that contribute to its development:
- Increased hydrostatic pressure in blood vessels:
- Increased arterial pressure (hypertension)
- Increased venous pressure (right sided heart failure)
- Increased interstitial protein concentration:
- Inflammation of capillaries (become leaky)
- Decreased oncotic pressure:
- Loss of proteins (gastrointestinal disease, liver disease, poor diet)
- Obstruction of lymphatic vessels
- Increased hydrostatic pressure in blood vessels:
Edema: Clinical Example
- In cases of an 8 year old horse with dependent edema
- Swelling of legs and other "dependent" areas
- This may be due to right-sided heart failure caused by tricuspid valve insufficiency
- Venous congestion
- Increased filtration pressure
Microcirculation & Fluid Exchange
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Capillaries:
- Make up only about 5% of total blood volume.
- Important site for exchange of water, nutrients, waste products, and gases between the blood and interstitial fluid.
- Diameter of 7-8 micrometers, length of 0.5 millimeters.
- Red blood cells are about 7.5 micrometers in diameter, making them barely able to squeeze through.
- Composed of a single layer of endothelial cells.
- Form dense networks throughout tissues, ensuring that each cell is within 100 micrometers of a capillary.
- Receive blood from the smallest arterioles and metarterioles.
- Metarterioles are connections between arterioles and capillary networks.
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Capillary Density:
- Varies depending on the metabolic activity of the tissue.
- More active tissues, like muscles during exercise, require a higher density of capillaries.
- Metarterioles contain rings of smooth muscle that can open and close on demand.
- This regulates blood flow through capillaries in response to tissue needs.
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Blood Flow in Capillaries:
- Slow flow due to small diameter of capillaries.
- Resistance is reduced by the extensive parallel branching of the capillary network.
Transcapillary Exchange
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Diffusion:
- Movement of substances across the capillary wall based on their properties and concentration gradients.
- Lipid-soluble substances like oxygen (O2) and carbon dioxide (CO2) can freely diffuse across the cell membrane.
- Most capillaries have pores or clefts that permit the transfer of water and lipid-insoluble molecules like sodium (Na+), chloride (Cl-), glucose, and amino acids.
- The size and number of pores vary depending on the tissue.
- Some capillaries, like those in the intestines, liver, and kidneys, are fenestrated. These have larger gaps across the endothelial cell membrane, created by fused vesicles, allowing the passage of water and water-soluble macromolecules.
Bulk Flow
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Mass Movement of Fluid:
- The movement of water and dissolved substances across the capillary walls is called "bulk flow."
- Filtration: Movement of fluid from the blood vessels to the interstitial space.
- Absorption: Movement of fluid from the interstitial space to the blood vessels.
-
Essential for Maintaining Fluid Balance:
- Bulk flow helps maintain the proper fluid balance between the intravascular (blood) and interstitial compartments.
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Factors Affecting Bulk Flow:
- Pressure gradients: Hydrostatic pressure and colloid osmotic (oncotic) pressure.
- Permeability of the vessel wall: Porosity.
- Size of the diffusion surface area.
- Blood flow rate.
Pressure Gradients
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Hydrostatic Pressure:
- The pressure exerted by the blood as it flows through the capillaries.
- Filtration pressure: Pushing fluid out of the vessel.
- Pressure in the interstitial fluid is generally close to zero.
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Colloid Osmotic (Oncotic) Pressure:
- Pressure caused by the presence of proteins in the blood, primarily albumin.
- Proteins are too large to pass through the capillary wall, so they exert a pull that draws fluid back into the vessel.
- Counter-balances hydrostatic pressure.
Starling Forces
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Opposing Forces:
- The net movement of fluid across the capillary wall depends on the balance between the hydrostatic and osmotic pressure gradients.
- Hydrostatic pressure difference: Pushes fluid out.
- Osmotic pressure difference: Pulls fluid in.
- This pressure gradient remains constant along the length of the capillary.
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Along the Capillary Length:
- The first portion of the capillary is dominated by hydrostatic pressure, resulting in net fluid filtration out of the vessel.
- As blood flows towards the end of the capillary, hydrostatic pressure decreases.
- If hydrostatic pressure becomes less than osmotic pressure, fluid will move back into the vessel.
Where Does the Extracellular Fluid Go?
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Interstitial Fluid:
- The net effect of filtration and absorption is the accumulation of fluid in the interstitial space.
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Lymphatic System:
- Pick up excess interstitial fluid and drain it back into the large veins.
- This is called lymphatic drainage.
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Venules:
- Fluid remaining in the blood collects in venules and returns to the heart.
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Edema Formation:
- When the lymphatic system cannot keep up with the fluid accumulation, interstitial fluid builds up, resulting in edema.
Physiological Importance
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Diffusion:
- Essential for the exchange of nutrients, gases, and waste products between the blood and tissues.
-
Bulk Flow:
- Helps to stabilize blood volume by using the interstitial fluid as a buffer.
- This helps compensate for periods of blood loss.
- Helps regulate fluid distribution in response to changes in fluid intake or loss.
- Excess fluid intake increases blood volume and hydrostatic pressure, leading to more fluid accumulation in the interstitial space.
- Reduced plasma protein levels, as seen with liver disease, malnutrition, or protein loss from the gut, decrease osmotic pressure and promote fluid movement into the interstitial space.
- Helps to stabilize blood volume by using the interstitial fluid as a buffer.
Edema Formation
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Abnormal Accumulation of Interstitial Fluid:
- Caused by an imbalance in the Starling forces or an obstruction in lymphatic drainage.
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Four Major Causes:
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Increased Hydrostatic Pressure:
- Hypertension: Increased arterial pressure.
- Right Sided Heart Failure: Increased venous pressure.
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Increased Interstitial Protein Concentration:
- Inflammation: Capillaries become more permeable, allowing large proteins to leak out into the interstitial space.
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Decreased Oncotic Pressure:
- Hypoproteinemia: Loss of proteins from the blood due to various causes like intestinal disease, liver disease, or malnutrition.
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Obstruction of Lymphatic Vessels:
- Blockage of the lymphatic system prevents proper drainage of interstitial fluid.
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Increased Hydrostatic Pressure:
Edema: Clinical Example
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Dependent Edema in a Horse:
- 8-year-old horse with swelling in the legs and other dependent areas.
- This was due to right-sided heart failure caused by tricuspid valve insufficiency.
- The right-sided heart failure leads to venous congestion, increasing hydrostatic pressure and promoting filtration, leading to the accumulation of fluid in the interstitial space.
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