WEEK IX (Capillaries) Flashcards
What are Capillaries?
Sites for exchange of materials between blood and tissue cells
How are capillaries suited to serve as site of exchange?
- There are no carrier-mediated transport systems across capillaries
- Materials are exchanged across capillary walls by diffusion
Which law describes diffusion across capillaries?
Fick’s Law
Describe the factors that enhance diffusion across capillaries
- Capillaries have thin walls and small diameters -> SHORT DIFFUSION DISTANCE
- Branching and capillary network -> LARGE SURFACE AREA
- Slow down the flow of blood -> MAXIMISE TIME OF EXCHANGE
What are capillaries composed of?
A single layer of flat endothelial cells which serve as the lining for other blood vessels and are surrounded by a thin basement membrane
What doesn’t have capillaries?
Smooth muscle and connective tissue
Materials entering or leaving the capillaries can freely pass through the basement membrane (TRUE/FALSE)
TRUE
What is the importance of capillaries having a smaller diameter than red blood cells?
Red blood cells have to pass through capillaries in single file, squeezing through due to their larger size -> Pushes red blood cells against capillary wall -> Short diffusion distance
Blood flows more slowly in capillaries compared to other parts of the circulatory system (TRUE/FALSE)
TRUE
What is responsible for the reduced velocity of blood flow in the capillaries?
The extensive branching of capillaries
What is the difference between Flow rate and velocity?
FLOW RATE: The volume of blood flowing through any level of the system is equal to the cardiac output (CO)
[if the heart pumps out 5L of blood, the same amount of blood must flow through the arteries, arterioles, capillaries and veins]
VELOCITY: Velocity of flow differs among different segments of the vascular tree because it is inversely proportional to the total cross-sectional area of all vessels at a given level
What is Flow rate?
FLOW RATE = the volume of blood per unit of time passing through a specific segment of the circulatory system
What is Velocity?
VELOCITY = speed or distance per unit of time with which blood moves forward in a particular segment of the circulatory system
What is the equation to calculate Velocity of blood flow?
V = Q/A
V = Velocity (cm/sec)
Q = Blood flow (mL/min)
A = Cross-sectional area (cm2)
Velocity is directly proportional to blood flow and inversely proportional to the cross-sectional area at ant level of the cardiovascular system
What happens when capillaries rejoin to form veins?
The total cross-sectional area decreases leading to an increase in blood flow velocity as it returns to the heart
Why is blood velocity slower in the capillaries than in the veins?
The same volume of water, spread over a larger cross-sectional area covers a shorter distance in a given period
Describe endothelial cells in capillaries
- Continuous/closely joined
- Narrow water-filled clefts or pores at junctions between cells
- Size of capillary pores varies from organ to organ
What are the different types of capillaries?
- CONTINUOUS CAPILLARIES
- FENESTRATED CAPILLARIES
- DISCONTINUOUS CAPILLARIES (SINUSOIDS)
Describe Fenestrated Capillaries
- Endothelial cells have small circular pores allowing for increased permeability
- Found in organs with high metabolic activity
- Found in kidneys, endocrine glands and intestines
Describe Continuous Capillaries
- Endothelial cells have tight junctions with small pores -> Restrict passage of large molecules and cells
- Found in lungs and central nervous system
Describe Discontinuous Capillaries (Sinusoids)
- Endothelial cells have irregular shapes with large gaps and wide openings -> allows passage of large molecules, cells and plasma
- Found in organs involved in filtration or blood cell production
- Found in liver, bone marrow and spleen
Describe Brain Capillaries
- Endothelial cells joined by tight junctions resulting in the absence of pores
- Tight junctions prevent the transcapillary passage of materials between these cells
What is the function of the blood-brain barrier?
It protects the brain by restricting the passage of certain substances from the bloodstream into the brain tissue -> The absence of pores in brain capillaries helps maintain the integrity and specialised functions of the brain
What is the function of fenestrations?
- Allow for the rapid movement of fluid across the capillaries in the kidneys and intestines
- Enhances the efficiency of fluid movement and nutrient absorption
What are Precapillary sphincters?
Consist of a ring of smooth muscle that surrounds the entrance of a capillary which regulates blood flow into the capillary bed
What is the function of Precapillary sphincters?
- Control blood flow
- Regulate the distribution of blood within the capillary network
_______________ have a high degree of myogenic tone and respond to local metabolic changes
Precapillary sphincters
What is the importance of Precapillary sphincters in capillaries?
Capillaries themselves lack smooth muscle and cannot actively regulate blood flow -> Increased tissue metabolic activity leads to relaxation of precapillary sphincters, increasing the number of open capillaries -> Decreased tissue activity causes contraction of pre capillary sphincters, diverting blood flow away from the capillary bed
What happens when muscle activity increases?
Relaxation of more PRECAPILLARY SPHINCTERS -> Opening of additional capillary beds -> Arteriolar vasodilation -> Increases overall blood flow to the organ
What happens during Bulk flow?
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid and is later reabsorbed
What makes Bulk flow different from Diffusion?
DIFFUSION = involves the individual movement of solutes down concentration gradients
BULK FLOW = involves movement of fluid and all its solutes as a unit
What is Ultrafiltration?
When the pressure inside the capillary exceeds the pressure outside, fluid is pushed out through the pores
What is Reabsorption?
When inward-driving pressures across the capillary wall exceed outward pressures, there is a net inward movement of fluid from the interstitial fluid back into the capillaries through the pores
What is The Starling Equation?
Jv = Kt [(Pc - Pi) - (πc - πi)]
Jv = Fluid movement (ml/min)
Kt = Filtration coefficient (water permeability of capillary wall)
Pc = Capillary hydrostatic pressure (mm Hg)
Pi = Interstitial hydrostatic pressure (mm Hg)
πc = Capillary oncotic pressure (mm Hg)
πi = Interstitial Oncotic pressure (mm Hg)
Describe fluid flow in the Starling equation
When Jv is positive = net fluid movement OUT of capillary (filtration)
When Jv is negative = net fluid movement INTO capillary (absorption)
Describe Pc (Capillary hydrostatic pressure) in the Starling equation
- An increase in Pc favours FILTRATION out of the capillary
- An increase in either arterial or venous pressure produces an increase in Pc
- Pc is higher at the arteriolar end of the capillary than at the venous end
Describe Pi (Interstitial fluid hydrostatic pressure)
- An increase in Pi OPPOSES FILTRATION out of the capillary
- Normally close to 0 or slightly -ve
Describe πc (Capillary Oncotic/Colloidosmotic pressure)
- An increase in πc OPPOSES FILTRATION out of capillary
- πc is increased by increased protein concentration in the blood (same with decrease)
- Small solutes do not contribute with πc
Describe πi (Interstitial fluid oncotic pressure)
- An increase in πi FAVOURS FILTRATION out of the capillary
- πi is dependent on the protein concentration of the interstitial fluid (normally quite low because very little protein is filtered)
What are the factors that increase filtration?
- Increase in Capillary hydrostatic pressure
(increased arterial pressure -> increased venous pressure -> arteriolar dilation -> venous constriction) - Decrease in Interstitial fluid hydrostatic pressure
- Decrease in Capillary colloid pressure
- Increase in Capillary colloid pressure (caused by inadequate lymphatic function)
What penetrates almost every tissue of the body?
Small, blind-ended terminal lymph vessels called initial lymphatics
Describe the structure of Lymph vessels
ENDOTHELIAL CELLS forming initial lymphatic walls overlap slightly, creating VALVELIKE OPENINGS in the vessel wall -> Fluid pressure on the outside of the vessel OPENS the valve -> Allow interstitial fluid to enter and become lymph -> Fluid pressure inside of vessel CLOSES the valve to prevent lymph from escaping
Why can large particles like plasma proteins and bacteria enter initial lymphatics but not blood capillaries?
Lymphatic valve like openings are larger than pores in blood capillaries
Where do lymphatic vessels empty into?
The venous system near the right atrium
What are the mechanisms that propel lymph towards the venous system?
- SMOOTH MUSCLE surrounding lymph vessels contracts rhythmically, pushing lymph through the vessels & SKELETAL MUSCLE squeeze lymph out of vessels
- ONE-WAY VALVES within the lymph vessels direct the flow of lymph towards its venous outlet in the chest
What are the functions of the lymphatic system?
- Defense against disease
- Transport of absorbed fat
- Return of filtered protein
What is the function of One-way flap valves?
Permit interstitial fluid to enter but not leave the lymph vessels
What is Oedema?
Swelling that occurs when the volume of interstitial fluid exceeds the capacity of the lymphatics to return it to the circulation
What can oedema be caused by?
- Excess filtration
- Blocked lymphatics
What can Histamine cause?
Arteriolar dilation and venous constriction which produce a large increase in hydrostatic capillary pressure and local oedema
What are some causes of Increase in Hydrostatic capillary pressure?
- Arteriolar dilation
- Venous constriction
- Heart failure
- Standing
What are some causes of Decrease in Capillary colloid pressure?
- Decreased plasma protein concentration
- Severe liver disease
- Protein malnutrition
- Nephrotic syndrome
What are some causes of Increase in Filtration coefficient (Water permeability of capillary cell)?
- Burn
- Inflammation
What is the difference between Normal Oedema and Pitting Oedema?
Pitting Oedema leaves an indentation that remains after pressure is applied and the excess fluid that builds up is composed mainly of water. In Normal Oedema this doesn’t happen and the fluid is composed of proteins, salts and water.
What is Pitting Oedema caused by?
Albumins Leakage
