Guyton Chapter 16 - The Microcirculation And Lymphatic System

Question 1

Are the arterioles highly muscular?

Answer 1

Yes

Question 2

What differs the arterioles from the metarterioles regarding?

Answer 2

The arterioles are highly muscular.

The metarterioles (the terminal arterioles) do not have a continuous muscular coat but smooth muscular fibers encircle the vessel at intermittent points.

Question 3

Can the arterioles’ diameter change?

Answer 3

Yes it can change manyfold

Question 4

What is the precapillary sphincter?

Answer 4

A smooth muscle fiber usually encircle the capillary at the point where each true capillary originates from a metarteriole.

Question 5

Describe the structure of the capillary wall?

Answer 5

The wall is composed of a unicellular layer of endothelial cells and is surrounded by a thin basement membrane on the outside of the capillary.

Question 6

What is an intercellular cleft?

Answer 6

A thin slit curving channel that lies at the bottom of the figure between adjacent endothelial cells. Each cleft is interrupted periodically by short ridges of protein attachments that hold the endothelial cells together, but between these ridges fluid can percolate freely through the cleft.

Question 7

What are plasmalemmal vesicles also called caveolae?

Answer 7

They are believed to play a role in endocytosis.

They form from oligomers of proteins called caveolins that are associated with molecules of cholesterol and sphingolipids.

Question 8

Name the special types of pores that occur in the capillaries in the brain, liver, gastrointestinal capillary membrane and in the glomerular capillaries of the kidney.

Answer 8

Brain: the junctions between the capillary endothelial cells are mainly tight junctions that allow only extremely small molecules such as water, oxygen and carbon dioxide to pass into or out of the brain tissues,

Liver: the opposite is true. The clefts between the capillary endothelial cells are wide open so that almost all dissolved substances of the plasma including the plasma proteins can pass from the blood into the liver tissues.

3. The pores of the gastrointestinal capillary membrane are midway between those of the muscles and those of the liver.
4. In the glomerular capillaries of the kidney, numerous small oval windows called fenestrae penetrate all the way through the middle of the endothelial cells.

Question 9

What is vasomotion?

Answer 9

The cause that blood flows intermittently is vasomotion which means intermittent contraction of the metarterioles and precapillary sphincters (and sometimes even the very small arterioles as well)

Question 10

What is the most important factor found that affect the degree of opening and closing of the metarterioles and precapillary sphincters?

Answer 10

The concentration of oxygen in the tissues.

Question 11

Are oxygen and carbon dioxide lipid soluble?

Answer 11

Yes

Question 12

How much of the total volume of the body consists of interstitium (spaces between cells)?

Answer 12

One sixth of the total body volume

Question 13

What 2 major types of solid structures does the interstitium contain of?

Answer 13

1. Collagen fiber bundles
2. Proteoglycan filaments

Question 14

What does the proteoglycan filaments in the interstitium consist of?

Answer 14

98% hyaluronic acid and 2% protein

Question 15

How is the gel in the interstitium created and what does it contain?

Answer 15

It’s derived by filtration and diffusion from the capillaries. It contains almost the same constitutents as plasma except for much lower concentrations of proteins because proteins do not easily pass outward through the pores of the capillaries.

Question 16

What is called tissue gel?

Answer 16

The interstitial fluid is entrapped mainly in the minute spaces among the proteoglycan filaments. The combination of proteoglycan filaments and fluid entrapped within them has the characteristics of a gel and therefore is called tissue gel.

Question 17

Is it easy for fluid to diffuse through the gel?

Answer 17

No it’s not (because of the large number of proteoglycan filaments).
Instead fluid mainly diffuses through the gel - that is moves molecule by molecule from one place to another by kinetic thermal motion rather than large numbers of molecules moving together.

Question 18

What does the hydrostatic pressure in the capillaries tend to do with fluid and its dissolved substances?

Answer 18

Tend to force them through the capillary pores into the interstitial spaces.

Question 19

What does osmotic pressure tend to do with fluid movement from interstitium?

Answer 19

To cause fluid movement by osmosis from the interstitial spaces into the blood.

Question 20

Name the 4 Starling forces that determine whether fluid will move out of the blood into the interstitial fluid or in the opposite direction

Answer 20

1. The capillary pressure which tends to force fluid outward through the capillary membrane
2. The interstitial fluid pressure [Pif] which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative
3. The capillary plasma colloid osmotic pressure which tends to cause osmosis of fluid inward through the capillary membrane
4. The interstitial fluid colloid osmotic pressure which tends to cause osmosis of fluid outward through the capillary membrane.

Question 21

What happens if the sum of Starlings forces are positive/negative?

Answer 21

If it’s positive, there will be a net fluid filtration across the capillaries.

If it’s negative - there will be a net fluid absorption from the interstitial spaces into the capillaries.

Question 22

How are starlings forces in normal conditions?

Answer 22

Slightly positive

Question 23

What is the capillary filtration coefficient?

Answer 23

The rate of fluid filtration in a tissue is also determined by the number and size of the pores in each capillary as well as the number of capillaries in which blood is flowing. These factors are usually expressed together as the capillary filtration coefficient.

Question 24

Describe the equation for the rate of capillary fluid filtration.

Answer 24

Filtration = capillary filtration koefficient x net filtration pressure

Question 25

Name 3 methods for measuring interstitial fluid hydrostatic pressure

Answer 25

1. Direct cannulation of the tissues with a micropipette
2. Measurement of the pressure from implanted perforated capsules
3. Measurement of the pressure from a cotton wick inserted into the tissue.

Question 26

What is the normal interstitial fluid pressure?

Answer 26

Usually several millimeters of mercury negative with respect to the pressure that surrounds each tissue.

Question 27

What is responsible for the osmotic pressures on the two sides of the capillary membrane?

Answer 27

The proteins of the plasma and interstitial fluid

Question 28

What are the plasma proteins a mixture of and what are there molecular weight?

Answer 28

Albumin: 69 000
globulins: 140 000
fibrinogen: 400 000

Question 29

What is osmotic pressure determined by?

Answer 29

The number of molecules dissolved in a fluid rather than by the mass of molecules

Question 30

How much of the total colloid osmotic pressure consists of albumin, globulins and fibrinogen?

Answer 30

About 80% results from albumin fraction
20% globulins
Almost none from the fibrinogen

Question 31

What is the average protein concentration of the interstitial fluid compared to plasma?

Answer 31

Usually only 40% of that in plasma or about 3 g/dl

Question 32

What is the average capillary pressure at the arterial ends of the capillaries compared to the venous ends and what is the difference in what happens with the fluid?

Answer 32

Average capillary pressure at the arterial ends of the capillaries is 15-25 mmHg greater than at the venous ends.

Because of this difference, fluid filters out of the capillaries at their arterial ends but at their venous ends, fluid is reabsorbed back into the capillaries.

Question 33

What is the net filtration after summation of forces at the arterial ends of the capillary?

Answer 33

The net filtration pressure is 13 mmHg (tending to move fluid outward through the capillary pores).

Question 34

With the 13 mmHg filtration pressure - How much of the plasma in the flowing blood filter out of the arterial ends of the capillaries into the interstitial spaces

Answer 34

About 1/200 of the plasma

Question 35

What is net reabsorption pressure at the venous ends of the capillaries?

Answer 35

7 mmHg

Question 36

How much of the fluid that has been filtered out of the arterial ends of the capillaries is reabsorbed at the venous ends? What happens to the rest of the fluid?

Answer 36

About nine tenths of the fluid that has filtered out of the arterial ends of the capillaries is reabsorbed at the venous ends.

The remaining one tenth flows into the lymph vessels and returns to the circulating blood.

Question 37

For the total capillary circulation, we find a near equilibrium between the total outward forces (28.3 mmHg) and and the total inward force (28 mmHg). What does this slight imbalance of forces cause?

Answer 37

It causes slightly more filtration of fluid into the interstitial spaces than reabsorption.

Question 38

What is the net filtration? What is the normal rate of net filtration in the entire body not including the kidneys?

Answer 38

It’s the slight excess of filtration and it is the fluid that must be returned to the circulation through the lymphatics.

The normal rate of net filtration is only about 2 ml/ min

Question 39

Almost all tissues have special lymph channels that drain excess fluid directly from the interstitial spaces. There are some exceptions - which are these?

Answer 39

The superficial portions of the skin, the CNS, the endomysium of muscles, and the bones.

But even these tissues have minute interstitial channels called prelymphatics through which interstitial fluid can flow. This fluid eventually empties either into lymphatic vessels or in the case of the brain into the cerebrospinal fluid and then directly back into the blood.

Question 40

Where do essentially all lymph vessels from the lower part of the body eventually empty?

Answer 40

Into the thoracic duct which in turn empties into the blood venous system at the juncture of the left internal jugular vein and left subclavian vein.

Question 41

Most of the fluid filtering from the arterial ends of blood capillaries flows among the cells and finally is reabsorbed back into the venous ends of the blood capillaries. How much of the fluid instead enters the lymphatic capillaries and returns to the blood through the lymphatic system rather than through the venous capillaries?

Answer 41

About one tenth of the fluid.

Question 42

What is the special structure of the lymphatic capillaries that permits passage of substances of high molecular weight into the lymph?

Answer 42

The endothelial cells of the lymphatic capillary is attached by anchoring filaments to the surrounding connective tissue. At the junctions of adjacent endothelial cells, the edge of one endothelial cell overlaps the edge of the adjacent cell in such a way that the overlapping edge is free to flap inward thus forming a minute valve that opens to the interior of the lymphatic capillary. Interstitial fluid, along with its suspended particles, can push the valve open and flow directly into the lymphatic capillary.

Question 43

How is lymph derived?

Answer 43

It’s derived from interstitial fluid that flows into the lymphatics. Therefore lymph as it first enters the terminal lymphatics has almost the same composition as interstitial fluid.

Question 44

What is the protein concentration in the interstitial fluid in average?

Answer 44

It’s in average 2 g/dl

Question 45

What is the protein concentration of lymph formed in the liver and the intestines?

Answer 45

Liver: 6 g/dl
Intestine: 3-4 g/dl

Question 46

What is the protein concentration of the thoracic duct lymph?

Answer 46

3-5 g/dl

Question 47

Any factor that increases interstitial fluid pressure also increases lymph flow if the lymph vessels are functioning normally. Name 4 such factors:

Answer 47

1. Elevated capillary hydrostatic pressure
2. Decreased plasma colloid osmotic pressure
3. Increased interstitial fluid colloid osmotic pressure
4. Increased permeability of the capillaries

Question 48

When the interstitial fluid becomes 1 or 2 mmHg greater than athmospheric pressure (more than 0 mmHg), lymph flow fails to rise any further at still higher pressures. What does this result from?

Answer 48

This results from the fact that the increasing tissue pressure not only increases entry of fluid into the lymphatic capillary but also compresses the outside surfaces of the larger lymphatics, thus impeding lymph flow.

Question 49

Lymphatic pumps increases lymph flow. In addition the the pumping caused by intrinsic intermittent contraction of the lymph vessel walls, any external factor that intermittently compresses the lymph vessels also can cause pumping. Name factors that can do this in order of their importance.

Answer 49

Contraction of surrounding skeletal muscles

Movement of the parts of the body

Pulsations of arteries adjacent to the lymphatics

Compression of the tissues by objects outside the body.

Question 50

What happens with the lymphatic capillary pump each time excess fluid enters the tissue and causes the tissue to swell?

Answer 50

The anchoring filaments pull on the wall of the lymphatic capillary and fluid flows into the terminal lymphatic capillary through the junctions between the endothelial cells.
Then when the tissue is compressed, the pressure inside the capillary increases and causes the overlapping edges of the endothelial cells to close like valves. Therefore the pressure pushes the lymph forward into the collecting lymphatic instead of backward through the cell junctions.

Question 51

What are the 2 primary factors that determine lymph flow?

Answer 51

1. The interstitial fluid pressure
2. The activity of the lymphatic pump

Question 52

Local blood flow control can be divided into 2 phases - which are they and what differs them from each other?

Answer 52

1. Acute control: achieved by rapid changes in local vasodilation or vasoconstriction of the arterioles, metarterioles and precapillary sphincters occurring within seconds to minutes to provide very rapid maintenance of appropriate local tissue blood flow.
2. Long term control - means slow controlled changes in flow over a period of days, weeks or even months.

Question 53

How much does an increase in metabolism that is up to eight times compared to normal increase the blood flow?

Answer 53

It increases the blood flow acutely about fourfold.

Question 54

What happens to the blood flow in tissues whenever the availability of oxygen to the tissues decreases?

Answer 54

The blood flow increases markedly