4.1 Capillaries, Oedema and Lymph Flashcards
What are some properties of a capillary
-capillaries have a 5-10 μm diameter
-have a one cell thick layer wall
-function as part of a capillary bed
How do arterioles link to capillaries
Arterioles branch of into smaller metarterioles. Precapillary sphincters then connect the metarterioles to the capillaries. The sphincters can be constricted or relaxed to control the blood flow through the capillary bed
How are different types of capillaries classified
Based on how easy it is for substances to move into and out of them
Describe continuous capillaries
They are the most common type of capillary and the least leaky. Plasma membranes of endothelial cells form continuous tubes with intracellular clefts in between.
Describe fenestrated capillaries
The plasma membranes have fenestrations (windows) allowing small substances to get in and out. They are found in regions where there is a high level of filtration/absorption needed (e.g renal corpsucle).
Describe sinusoid capillaries
They are very leaky due to having little to no basement membrane, large fenestrations and larger intracellular clefts. They have a wider and more winding shape
How do different substances cross capillaries
Lipid soluble molecules diffuse through the plasma membranes but water soluble substances diffuse through intercellular spaces and fenestrations.
Describe hydrostatic pressure
Fluid must move down its pressure gradient, from an area of high pressure to low pressure, this is the hydrostatic pressure and is measured in millimetres of mercury (mmHg). Hydrostatic pressure is higher at the arteriole end of the capillary than the venous end.
What is osmotic pressure
Known as plasma colloid osmotic pressure (aka oncotic pressure), it is mostly determined by the proteins in the plasma (as other contributors such as ions, sugars and amino acids can leak out of the capillaries and reach an equilibrium, but proteins cant). Plasma colloid osmotic pressure stays more constant at either end of capillary compared to the hydrostatic.
How is net pressure calculated and hence how is net filtration pressure calculated
Net pressure = hydrostatic pressure - osmotic pressure
Net filtration pressure = = net hydrostatic pressure - net osmotic pressure
it is written as NFP = (Pc - Pi) - (πc - πi)
where Pc is capillary hydrostatic pressure, Pi is interstitial hydrostatic pressure, πc is capillary osmotic pressure and πi is interstitial osmotic pressure.
What is the main determining force in net hydrostatic pressure
Pc, the capillary hydrostatic. This depends on the arterial and venous pressure + pre capillary (arteriolar) and post capillary (venular) resistance. An increase in arterial or venous pressure would increase Pc. An increase in precapillary resistance would decrease Pc but an increases in postcapillary resistance would increase Pc.
What is the Starling equation and what does it describe
The Starling equation describes fluid movement across capillary membranes
Net fluid movement (either filtration or reabsorption) = the filtration coefficient X NFP
What is the filtration coefficient
It is also known as the hydraulic conductance. It measures the water permeability of the capillary wall which determines the magnitude of fluid movement for the given pressure differences. This coefficient varies between tissues.
Describe the lymphatic system.
The lymphatic system picks up excess fluid and other waste substances from the interstitual fluid. Lymphatic capillaries merge into larger lymph vessels. The lymph flows in only one direction (due to flap valves and the pressure). The lymph nodes are sites along the lymphatic vessels (distributed in series) where the lymph is filtered, at these nodes there is a congregation and proliferation of lymphocytes. Cleaned lymph then returns from the nodes to the lymph vessels to the lymph trunks to the right lymphatic duct or the thoracic duct.
What occurs at the right lymphatic duct.
It is where lymph from the upper right of body accumulates, it then drains into venous blood at the junction of the right internal jugular and the subclavian veins.
What occurs at the thoracic duct
It is where lymph from everywhere else in the body accumulates, it drains into venous blood at the junctions of the left internal jugular and the subclavian veins
How is lymph moved through the lymphatic system
By contraction of the smooth muscles of the lymphatic vessels, contraction of the surrounding skeletal muscles and by thoracic pressure changes. The lymph moves only in one direction due to the valves and pressure differences.
What are some properties of lymph capillaries.
-lymphatic capillaries are a closed end network close to blood capillaries
-they have no basement membrane and no tight junctions
-they are anchored to connective tissue via fine filaments (no attachments to the actual cells)
-lymphatic capillaries have a one way flow (as the pressures don’t encourage it + there are valves)
Describe oedema
Oedema is a phenomenon that occurs when fluid does accumulate in interstitial tissue. It can be a generalised condition of organ specific. It increases the diffusion distance between capillaries and tissues which is very dangerous, it also limits the removal of waste. It can occur due to alterations in the elements of starlings equation (↑ Pc, ↓ πc, ↑ Kf) or due to impaired lymphatic tissue. Loose tissue (e.g subcutaneous tissue around eyes or scrotum) is more prone to oedema than firm tissue (e.g muscles) or encapsulated structures (e.g kidney).
Give a common example of oedema
Inflammation. Where when the capillary permeability is increased, it causes increased fluid loss from capillaries and movement of plasma proteins.
What is lymphoedema
Lymphoedema is oedema due to impaired lymphatic drainage (often due to surgical removal or irradiation of certain lymph nodes)
