Capillaries 1

Question 1

Why do we need to transport solutes and fluids

Answer 1

Metabolism creates a need to transport solutes and fluids. This is in order to supply the body with
oxygen, nutrients, maintain electrolyte balance, and allow hormones to access different cells
amongst other requirements.

Question 2

Where are the metabolites in the body found

Answer 2

These metabolites is found in a water solution that can be intracellular
(majority) or extracellular fluid.

Question 3

What can the extracellular fluid be

Answer 3

Extracellular fluid can be interstitial fluid or the fluid in the circulatory
system (plasma) that both have similar compositions. Movement is partly facilitated by concentration
gradients that are generated by metabolism, as well as the membrane barriers to this movement.

Question 4

What molecules can diffuse through the membrane

Answer 4

Smaller uncharged molecules like oxygen and carbon

dioxide can diffuse across the membrane directly.

Question 5

Give a mechanism that the cell membrane is involved in

Answer 5

The cell membrane provides support and
protection to the cell and is a way cells recognise each other (e.g. for immunity). It controls what
enters and what leaves the cell thus regulating its function.
As mentioned above, cell membranes selectively facilitate movement through them. This transport
can be active or passive.

Question 6

What is passive transport

Answer 6

Passive transport is movement of molecules down a gradient that can be
concentration (diffusion), pressure (convection), osmotic (osmosis) or electrical (electrochemical flux)
dependant. This means this process does not require energy. The passive diffusion can be simple
(generally smaller molecules like water and carbon dioxide) or facilitated (generally larger molecules
like ions and glucose).

Question 7

What is active transport and give examples

Answer 7

Active transport is generally the movement of molecules against a
concentration gradient and is generally used to create a concentration gradient for different
purposes (e.g. ATPase pumps, endocytosis and exocytosis). This process requires energy that is
usually found in the form of ATP.

Question 8

Where does solute and fluid exchange happen in the body

Answer 8

Solute and fluid exchange in the body occurs at capillaries that are blood vessels of the finest
diameter.

Question 9

Why are capillaries important

Answer 9

They connect terminal arterioles to venules with every cell of the body reasonably close to
one (especially highly metabolically active cells like brain cells).

Question 10

Give examples of solute exchange - what exchange is this

Answer 10

Solute exchange occurs across the
capillaries with the molecules involved including oxygen, glucose, amino acids, hormones and drugs.
This movement is facilitated by simple diffusion.

Question 11

How does the fluid itself move with the solutes and what facilitates this

Answer 11

The fluid itself can move along with the solutes it
contains in bulk movement. This movement is facilitated by pressure gradients that can move
interstitial fluid into plasma or vice versa.

Question 12

Why is the movement of fluid essential

Answer 12

This movement of fluid is essential to maintaining blood

(maintains blood pressure) and tissue volumes.

Question 13

What determines the rate of solute transport 3 points

Answer 13

The rate of solute transport is determined by the properties of passive diffusion.

This is dependent
on concentration gradients, distance and rate of movement.

The physical properties of the solutes
involved also determine the rate of solute transport (e.g. Fick’s law) as well as

the properties of the
capillaries across which the movement takes place.

Question 14

What is passive diffusion based on - when is this suitable

Answer 14

Passive diffusion does not require energy but is
based on the random movement of molecules. The general trend of this movement is that molecules
will move from an area of high to low concentration. This movement is suitable over short distances,
a reason why capillaries cannot be too far from any cells of the body.

Question 15

Why is passive diffusion suitable across short distances

Answer 15

The reason why this movement
is suitable to short distances only is because it is proportional to the square of time (i.e. time will
increase exponentially with distance). This is why small animals can rely on diffusion whilst larger
animals require a specialised circulatory system.

Question 16

How is solute transport affected

Answer 16

Solute transport is also affected by properties of the
solute. The concentration gradient will affect the movement (as mentioned before) as well as the size
and lipid solubility (lipophilic or lipophobic) of the solute. Properties of the membrane itself also
affect the movement. Its thickness and composition will affect the movement (e.g. presence of
aqueous pores or channels).

Question 17

What affects carrier transport

Answer 17

The availability of carriers will also affect active or carrier-mediated
transport.

Question 18

What does ficks law consider

Answer 18

Fick’s law considers the properties of solutes and membranes affecting transport to
calculate a predicted amount of substance that is transported across a membrane per unit of time mass/unit time or m/t in Js).

Question 19

What 4 factors affects the amount of substance transported calculated by ficks law

Answer 19

According to Fick’s law, this is determined by 4 factors;

the ease of
movement through the solvent (D),

the surface area for diffusion (A),

the concentration gradient (ΔC)

and the distance (x) (Js = - D A ΔC/x).
In terms of capillaries, this movement across membranes is from inside the capillary through the
walls of the capillary into the interstitial fluid.

Question 20

What are the three types of capillary

Answer 20

There are broadly 3 types of capillaries in terms of

structure; continuous (upper right), fenestrated (middle right) and discontinuous (lower right).

Question 21

What is the most and least permeable type of capillary

Answer 21

Discontinuous capillaries are the most permeable type whilst continuous capillaries are the least
permeable type.

Question 22

What are most capillaries

Answer 22

Most capillaries are continuous. These have moderate permeability dependant on
tight gaps between neighbouring cells and a constant basement membrane.

Question 23

What is movement across the capillary

Answer 23

Movement across this
capillary can be transcellular or paracellular. However, due to tight junctions between the cells,
paracellular movement is restricted.

Question 24

Where are junctions between cells and movement restricted

Answer 24

The tightest gaps are found in the brain and this is the basis
behind the blood brain barrier (tightly regulates movement of plasma and interstitial fluid of the
brain).

Question 25

Where are continuous capillaries also found

Answer 25

Continuous capillaries are also found in many other types of tissues including muscle, skin, fat
and connective tissue

Question 26

Compare and describe the permeability of fenestrated started capillaries

Answer 26

Fenestrated capillaries have a higher permeability than continuous capillaries.
This allows for more water movement. They have a structure that is continuous but has some gaps in
it that allow for a high water turnover.

Question 27

Where are fenestrated capillaries found

Answer 27

This type of capillary is found in tissues with a high water
turnover like salivary glands, the kidneys, synovial joints, anterior eye and choroid plexus
(cerebrospinal fluid).

Question 28

Compare discontinues capillaries to fenestrated ones - where are they found

Answer 28

Discontinuous capillaries have even larger fenestrations that give them a
disrupted membrane. These are generally found in very specific locations as the holes are so large
that some RBCs are able to squeeze through them. These specific tissues are where cell movement is
required like in the liver, spleen and bone marrow.

Question 29

What are the three structural features of capillary walls that influence solute transfer

Answer 29

There are three structural features of capillary walls that influence solute
transfer.
• The intracellular clefts (gaps) can vary in width between 10 and 20 nm allowing or blocking the movement of solutes.
• The system of caveolae (invaginations in the cell membrane) and vesicles influence the movement of larger solutes.

Question 30

Describe the system of caveolae

Answer 30

In this system, the solute enters the cell by endocytosis and exits by exocytosis.

Question 31

Describe the glycocalyx

Answer 31

The
glycocalyx represents a negatively charged basement membrane that surrounds the endothelial cells
on the lumen side (endothelial basement membrane found on the interstitial side).

Question 32

How is the glycocalyx regulated

Answer 32

The glycocalyx
can be highly regulated in that it can be formed and broken down rapidly as required (still being
researched).

Question 33

When does Ficks law need to be adapted

Answer 33

Fick’s law is very good at describing movement through a membrane that is slightly permeable, has
the same permeability throughout it and has the same solute either side of the barrier. However,
when discussing permeability of molecules that can only travel through pores and channels to
different solutes, this law needs to be adapted. A porous membrane reduces the surface area for
diffusion, increases length of movement through the membrane and affects diffusion due to
hydrostatic issues.

Question 34

How can the most dominant way of solute movement be calculated

Answer 34

The most dominant way
that solutes move to cells can be calculated through the example of glucose. The glucose
concentration in plasma is roughly 1 g/litre. There are around 8 litres of plasma that flow into tissues
per day meaning a maximum of 8 g of glucose can be filtered per day. However, glucose consumption
in a human adult is around 400 g/day. This means that filtration only accounts for around 2% of
glucose transport with the remaining glucose moved into tissues by diffusion (GLUT transporters).

Question 35

What factors control diffusion 3 ish thingies

Answer 35

Diffusion rate can be controlled by a few factors.

Blood flow controls diffusion rate in that the more
blood that flows through the capillaries, the more solutes available for diffusion.

Blood flow can be
controlled by arterioles that make up a large part of peripheral resistance.

Interstitial concentration
gradients of different ions also affect diffusion rates (e.g. interstitial oxygen concentration falls during
exercise and therefore diffusion rates increase).

Finally, there is a concept known as recruitment of
capillaries.

Question 36

Why are capillaries not perfused at the same time

Answer 36

Capillaries are not all perfused at the same time (catastrophic loss of blood pressure if
they were).

Question 37

How is perfusion in capillaries controlled

Answer 37

Their perfusion is controlled by vasoconstriction or vasodilation of pre-capillary
arterioles. Increasing the number of capillaries available increases surface area for diffusion.