Exchange and the Lymphatic System Flashcards
What does the structure of capillaries aid?
Exchange of substances between them and tissues
In what ways are capillaries specialised for exchange?
Large surface area : volume ratio
Small diameter
Lots of them
Thin walled - one endothelial cell thick
What are the different types of capillaries?
Continuous e.g. brain
Fenestrated e.g. intestines
Discontinuous e.g. liver
What does the structure of capillaries relate to?
Function
What kind of substances are exchanged from capillaries via diffusion?
Self-regulating, non-saturable, non-polar substances move across membrane
Polar substances - move through clefts/channels
What kind of substances are exchanged from capillaries via carrier-mediated transport?
Certain polar substances e.g. glucose
What are the different kinds of exchange occurring at capillaries?
Diffusion
Carrier-mediated transport
Bulk flow
Why is the blood-brain barrier necessary?
Composition of the extracellular fluid in the brain must be kept extremely constant in order to allow stable neuronal function
What is the blood-brain barrier responsible for?
Tightly controlling movement of ions and solutes across the walls of the continuous capillaries within the brain and the choroid plexus
How is any significant movement of hydrophilic solutes prevented in the blood brain barrier?
The junctions between the endothelial cells of the cerebral capillaries are extremely tight
What allows the controlled movement of inorganic ions, glucose, amino acids and some other substances across the capillary wall of the blood-brain barrier?
Specialised membrane transporters in cerebral endothelial cells
In the brain, the relatively uncontrolled diffusion of solutes present in other vascular beds is replaced by what?
A number of specific transport processes
Why can the blood-brain barrier present a therapeutic problem?
Many drugs are excluded from the brain
What interrupts the blood-brain barrier?
Circumventricular organs i.e. areas of the brain which need to be influenced by blood borne factors or need to release substances into the blood e.g. pituitary and pineal glands
What might cause the blood-brain barrier to break down?
Large elevations of BP, osmolarity or PCO2 and infected areas of the brain
What is involved in clotting?
Formation of a platelet plug and fibrin clot
What happens when the endothelial wall is damaged?
Platelets get out and stick to the collagen to form a platelet plug, fibrinogen is converted to fibrin by thrombin when a signal from the platelets turns thrombin on
What are the functions of the anti-clotting mechanisms of the endothelium?
Stop blood contacting collagen to prevent platelet aggregation
Produce prostacyclin and nitrous oxide which both inhibit platelet aggregation
Produce tissue factor pathway inhibitor which stops thrombin production
Express thrombomodulin which binds and inactivates thrombin
Express heparin which inactivates thrombin
Secrete tissue plasminogen activator which converts plasminogen into plasmin and digests clots
What is the most important consequence of Starling’s Law?
Stroke volume of left and right ventricles must be matches
When do small transient differences in output of the ventricles occur?
All the time - breathing, rising from sitting position
What would happen if right ventricular pressure was greater than that of the left for a significant period?
Pulmonary blood volume and pressure would rise and fluid would be forced into the lung causing pulmonary oedema
What pressure pushes water out of the capillaries?
Hydrostatic pressure
What pressure draws more water into the capillaries?
Osmotic pressure
What fluid volume is lost and regained every day?
20 litres lost, 17 litres regained
Remaining 3 litres is pushed out of the capillaries into lymph capillaries
Where does fluid draining into the lymph system go?
Drains into lymph vessels and back towards the low pressure of the CVS
Why is fluid in the lymph vessels pushed back?
Vessels pass through muscle, contracting muscle squeezes the lymph vessels so fluid is pushed back
What is oedema?
Accumulation of excess fluid
What are the causes of oedema?
Lymphatic obstruction e.g. filariasis, surgery
Raised CVP e.g. ventricular failure
Hypoproteinaemia e.g. nephrosis, liver failure
Increased capillary permeability e.g. inflammation
