Exchange and the Lymphatic System

Question 1

What does the structure of capillaries aid?

Answer 1

Exchange of substances between them and tissues

Question 2

In what ways are capillaries specialised for exchange?

Answer 2

Large surface area : volume ratio
Small diameter
Lots of them
Thin walled - one endothelial cell thick

Question 3

What are the different types of capillaries?

Answer 3

Continuous e.g. brain
Fenestrated e.g. intestines
Discontinuous e.g. liver

Question 4

What does the structure of capillaries relate to?

Answer 4

Function

Question 5

What kind of substances are exchanged from capillaries via diffusion?

Answer 5

Self-regulating, non-saturable, non-polar substances move across membrane
Polar substances - move through clefts/channels

Question 6

What kind of substances are exchanged from capillaries via carrier-mediated transport?

Answer 6

Certain polar substances e.g. glucose

Question 7

What are the different kinds of exchange occurring at capillaries?

Answer 7

Diffusion
Carrier-mediated transport
Bulk flow

Question 8

Why is the blood-brain barrier necessary?

Answer 8

Composition of the extracellular fluid in the brain must be kept extremely constant in order to allow stable neuronal function

Question 9

What is the blood-brain barrier responsible for?

Answer 9

Tightly controlling movement of ions and solutes across the walls of the continuous capillaries within the brain and the choroid plexus

Question 10

How is any significant movement of hydrophilic solutes prevented in the blood brain barrier?

Answer 10

The junctions between the endothelial cells of the cerebral capillaries are extremely tight

Question 11

What allows the controlled movement of inorganic ions, glucose, amino acids and some other substances across the capillary wall of the blood-brain barrier?

Answer 11

Specialised membrane transporters in cerebral endothelial cells

Question 12

In the brain, the relatively uncontrolled diffusion of solutes present in other vascular beds is replaced by what?

Answer 12

A number of specific transport processes

Question 13

Why can the blood-brain barrier present a therapeutic problem?

Answer 13

Many drugs are excluded from the brain

Question 14

What interrupts the blood-brain barrier?

Answer 14

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

Question 15

What might cause the blood-brain barrier to break down?

Answer 15

Large elevations of BP, osmolarity or PCO2 and infected areas of the brain

Question 16

What is involved in clotting?

Answer 16

Formation of a platelet plug and fibrin clot

Question 17

What happens when the endothelial wall is damaged?

Answer 17

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

Question 18

What are the functions of the anti-clotting mechanisms of the endothelium?

Answer 18

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

Question 19

What is the most important consequence of Starling’s Law?

Answer 19

Stroke volume of left and right ventricles must be matches

Question 20

When do small transient differences in output of the ventricles occur?

Answer 20

All the time - breathing, rising from sitting position

Question 21

What would happen if right ventricular pressure was greater than that of the left for a significant period?

Answer 21

Pulmonary blood volume and pressure would rise and fluid would be forced into the lung causing pulmonary oedema

Question 22

What pressure pushes water out of the capillaries?

Answer 22

Hydrostatic pressure

Question 23

What pressure draws more water into the capillaries?

Answer 23

Osmotic pressure

Question 24

What fluid volume is lost and regained every day?

Answer 24

20 litres lost, 17 litres regained

Remaining 3 litres is pushed out of the capillaries into lymph capillaries

Question 25

Where does fluid draining into the lymph system go?

Answer 25

Drains into lymph vessels and back towards the low pressure of the CVS

Question 26

Why is fluid in the lymph vessels pushed back?

Answer 26

Vessels pass through muscle, contracting muscle squeezes the lymph vessels so fluid is pushed back

Question 27

What is oedema?

Answer 27

Accumulation of excess fluid

Question 28

What are the causes of oedema?

Answer 28

Lymphatic obstruction e.g. filariasis, surgery
Raised CVP e.g. ventricular failure
Hypoproteinaemia e.g. nephrosis, liver failure
Increased capillary permeability e.g. inflammation