Physiology of Capillaries and Veins Flashcards
What do we mean by microcirculation?
Arterioles, capillaries, venules
How many capillaries are in peripheral circulation and what is their total surface area?
~10 billion
500 - 700 m^2
What is the greatest distance a capillary can be to a cell?
20-30 microns
What is the blood flow in capillaries?
Slow
0.5-1.0 mm/sec
What is the total thickness of the capillary wall?
0.5 microns
What are the internal diameters of capillaries?
4-9 microns
What permits passage of ions and small molecules in continuous capillaries?
Intercellular clefts
What are continuous capillaries not permeable to?
Proteins
Where are continuous capillaries found?
Muscle, lungs, adipose tissue
Are fenestrated capillaries permeable to proteins?
Yes, but only small ones
Where are fenestrated capillaries found?
Kidney, endocrine gland and intestine - high-exchange rate
What are discontinuous capillaries also known as?
Sinusoidal capillaries
What do discontinuous capillaries allow to pass?
RBCs, WBCs, various serum proteins
Where are sinusoidal capillaries found?
Liver, spleen and bone marrow
How many times does a nutrient artery branch when entering an artery?
6-8 times
How many times do arterioles branch?
2-5 times
What do arterioles supply blood to?
Terminal arterioles
How do metarterioles branch from terminal arterioles?
At right angles
What are the distal sections of metarterioles devoid of smooth muscle called?
Thoroughfare or preferential channels
Where do capillaries branch from?
Terminal arterioles
Metarterioles
What are true capillaries?
Smooth muscle fibre encircles the capillary at its origin - precapillary sphincter
What is the role of the precapillary sphincter?
Regulates entry of blood into capillary
What do true capillaries do?
Delivery of nutrients to/removal of wastes from cells and deliver blood into venules
What do capillaries deliver?
Nutritional flow
Where do thoroughfare channels deliver blood to?
Directly into venules
What structures are present in some tissues such as fingertips and ears that bypass capillaries?
Arteriovenous shunts
What name is given to blood in the tissue where arteriovenous shunts are present?
Non-nutritional flow
What is the name used for capillary blood pressure?
Hydrostatic pressure
How does hydrostatic pressure increase?
Dilation of arterioles by increasing inflow
Increased venous pressure reduces outflow
What does a larger volume of blood in capillaries do?
Increase transmural pressure
What does increased transmural pressure trigger?
Contraction of terminal arterioless
What does decreased transmural pressure trigger?
Relaxation of terminal arterioles
What overrides local vasomotion?
Extrinsic neural regulation of upstream vessels
How does transcapillary exchange occur?
Diffusion
Filtration
Pinocytosis
Where is interstitial fluid in the body?
Lies outside cells and the circulatory system directly bathing the cells
What is the rate of diffusion determined by?
Fick’s Law
Define Fick’s Law.
J=PS(Co-Ci)
J = rate of diffusion
P = capillary permeability
S = capillary surface area
Co = concentration of substance outside capillary
Ci = concentration of substance inside capillary
What is a limitation of diffusion at capillaries?
Flow-limited transport
Diffusion limited transport
What is meant by flow-limited transport?
Small molecules pass rapidly through pores and accumulate at arterial end of capillary if blood flow is slow, opposite if it is fast
Larger molecules diffuse at slower rate so will diffuse further down the capillary
What would increase supply of diffusible solutes in capillaries?
Increase in blood flow
Increase in capillary density
Explain diffusion limited transport.
Molecules > 60000 kDa cannot diffuse
If capillary density is low, or during oedema, increased distance between capillaries and tissue may limit efficient exchange
Where in the capillaries, which side has a higher density of pores and increased permeability?
Venous end
What may density of pores depend on?
Tissue type
Fenestrated and discontinuous capillaries have increased permeability
What molecules may diffuse through endothelial cell walls without the presence of pores?
Lipid-soluble molecules
What determines the direction and rate of movement of lipid-soluble molecules?
Relative values of PO2 and PCO2 in blood and interstitial fluid
What is filtration at capillaries?
Bulk movement of fluid through the pores in capillaries into interstitial area
Why does a tissue gel form in the interstitial area?
Due to the proteoglycan content of extracellular environment
What determines direction and magnitude of movement of fluid between capillaries and interstitium?
Hydrostatic forces
Oncotic forces
What raises hydrostatic pressure?
Vasodilation of arterioles
Constriction of venules
What is capillary blood pressure dependent on?
Arterial and venous pressures
What is the average capillary hydrostatic pressure at both arterial and venous end of capillaries?
arterial - 32mmHg
venous - 15 mmHg
What is the negligible force that opposes filtration?
Interstitial hydrostatic force
What is the capillary wall semi-permeable to?
Proteins
What determines the rate of water movement?
Osmotic pressure in mmHg
Is the protein concertation of tissue fluid or plasma greater?
Plasma
What is the name of the osmotic pressure that promotes movement of water out of tissue fluid into capillaries?
Colloid osmotic pressure or oncotic pressure
What is the colloid osmotic pressure of plasma?
25 - 28 mmHg
What does oncotic pressure do?
Oppose hydrostatic pressure
What are opposing hydrostatic and oncotic pressures collectively known as?
Starling forces
What is the direction of fluid movement determined by?
Starling equation
Where does filtration and absorption occur at an idealised capillary and why?
Arterial end - filtration
Venous end - absorption
Due to drop in hydrostatic pressure along capillary
Why do hydrostatic and oncotic pressures usually equilibrate quickly in most capillaries?
Due to rapid movement of water
In the entire capillary circulation does net filtration or net reabsorption occur?
Net filtration
In what form is net filtration returned to vasculature?
Lymph fluid
Name a tissue where only filtration occurs.
Renal glomerulus
Name a tissue where only reabsorption occurs.
Intestinal
What is the determining factor for Starling forces?
Magnitude of Pc (capillary pressure)
What is pinocytosis?
Movement of fluid across endothelial cells by vesicles
What is pinocytosis mostly responsible for?
Exchange of large molecules
Where do lymph ducts drain?
Left and right subclavian veins
How does interstitial fluid and suspended particles enter the lymphatic system?
One-way valves created by adjoining endothelial cells
What is the rate of total lymph production?
120ml/hr
Where is two thirds of all lymph derived from?
Liver and intestinal capillaries - leaky sinusoidal capillaries
What is oedema?
Excess fluid accumulation in extracellular spaces
What underlying causes give rise to oedema?
Excessive capillary filtration and/or poor lymphatic drainage
How does oedema occur in the Starling equation?
Increased capillary filtration coefficient
Increased capillary hydrostatic pressure - NB
Decreased plasma oncotic pressure
What clinical causes lead to oedema?
Increased blood pressure or venous obstruction
Increased tissue protein concentration
Decreased plasma protein concentration
Obstruction of lymphatic vessels
What is a common form of oedema?
Pulmonary oedema
When is pulmonary oedema usually caused?
Left-ventricular failure/congenitive heart failure
How does congenitive heart failure lead to pulmonary oedema?
Build-up of blood in pulmonary circulation as left ventricle backs up blood on the left side of the ehart
Is pulmonary oedema serious?
Yes, as excess fluid interferes with gas exchange
When would blood be transferred from veins to arterial circulation?
Exercise
Haemorrhage
What is central venous pressure?
Pressure of blood in right atrium
What does a reduction in CVP do?
Increase venous return
What increases overall venous pressure?
Increase in blood volume
Reduction in compliance
What is the drop in arterial pressure that causes a sense of dizziness when going from a lying down to a standing position?
Orthostatic hypotension
What immediately corrects orthostatic hypotension?
Baroreceptor reflex
What causes the venous pump?
Muscular activity in legs that compresses veins (reducing compliance)
What happens in individuals with prolonged increases in venous pressure stretch and damage venous valves?
Varicose veins
What happens on standing and even following muscular activity of individuals with varicose veins?
Downward movement of blood into ankles
Venous and capillary pressures become very high
Leakage of fluid from capillaries causes constant oedema and swelling in the feet
