Capillary Exchange and Localised Control of Circulation

Question 1

What does capillary exchange refer to?

Answer 1

the exchange of materials from capillary blood vessels into the surrounding tissues. The processes that facilitate capillary exchange are principally by diffusion, bulk transport and transcytosis

Question 2

Why are capillary vessels and capillary beds ideal for the process of exchange?

Answer 2

• 8 um in diameter
• Thin walls – facilitate diffusion
• Large intercellular clefts
• Fenestrations (pores)
• Blood flow through capillaries is slow (low blood pressure)

Question 3

Give a brief overview of what is absorbed into the body cells in capillary exchange and what is reabsorbed into the blood?

Answer 3

• Water, oxygen, amino acids, glucose, ions etc. absorbed into the body cells
• Water, CO2, water molecules, ions ect. reabsorbed into the blood

Question 4

What are the cellular mechanisms for filtration and reabsorption in the capillaries?

Answer 4

• Filtration pores e.g. albumin
• Transcytosis e.g. fatty acids, albumin, insulin etc
• Diffusion: O2, CO2, steroids
• Intercellular clefts in capillary walls (bulk flow): glucose, ions, water
• Active transport

Question 5

What is the capillary hydrostatic pressure?

Answer 5

‘blood pressure’, adds a force to the wall of capillaries and tends towards ‘pushing; water and solutes out of capillaries into the interstitial fluid

Question 6

What is the blood colloid osmotic pressure (BCOP)?

Answer 6

osmotic forces arising from the interstitial fluid that oppose the CHP

Question 7

What is the net filtration pressure (the overall drive) determined by?

Answer 7

both hydrostatic and osmotic pressure values

Question 8

What is the equation to work out the net filtration pressure?

Answer 8

Net CHO - Net BCOP

Question 9

Capillary blood pressure declines from the arterial end to the venous end. What consequences does this have?

Answer 9

This has functional consequences related to the rates of filtration as blood passes along the length of a capillary

Question 10

What is the net hydrostatic pressure?

Answer 10

• The Net hydrostatic pressure contains the capillary hydrostatic pressure CHP (ranging from 35mmHg (arterial end) to 18mmHg (venous end))
• CHP is counteracted by a hydrostatic pressure generated by the interstitial fluid. Has a very low value and is somewhat negligible (usually valued as 0)
• net hydrostatic pressure is the difference between the capillary hydrostatic pressure and the interstitial fluid’s hydrostatic pressure
• Effectively the net hydrostatic pressure is the capillary hydrostatic pressure as the interstitial fluid pressure is negligible

Question 11

What is the net osmotic pressure?

Answer 11

• Net osmotic pressure = Blood colloid osmotic pressure (BCOP) – Interstitial Fluid Colloid Osmotic Pressure (ICOP)
• BCOP = 25 mmHg
• ICOP – interstitial fluid has few proteins in suspension therefore this is low and negligible
• So Net Osmotic pressure becomes equivalent to the blood colloid osmotic pressure

Question 12

How do you work out the net filtration pressure?

Answer 12

• Net filtration Pressure (NFP) = Net hydrostatic pressure (35 to 18 mmHg) – Net Colloid Osmotic Pressure (about 25 mmHg)
• Net filtration pressure = Net CHP – Net BCOP

Question 13

Give features of blood pressure in the capillary bed?

Answer 13

• Blood flow in capillaries is extremely slow due to low bp

* Declining blood pressure through capillary bed

Question 14

What happens at the arterial end of the capillary bed?

Answer 14

• the hydrostatic pressure will exceed the blood colloid osmotic pressure difference
• NFP = 35 mmHg – 25 mmHg = +10 mmHg
• CHP > BCOP
• Fluid forced out of the capillary
• Filtration

Question 15

What happens at the venous end of the capillary bed?

Answer 15

• the BCOP will exceed the hydrostatic pressure
• NFP = 18 mmHg – 25 mmHg = -7 mm Hg
• BCOP > CHP
• Fluid moves into capillary
• Net reabsorption

Question 16

What happens when the hydrostatic and osmotic pressures are equivalent?

Answer 16

• NFP = 25 mmHg – 25 mmHg = 0
• No net movement of fluid
• Takes place in venous end of capillary bed

Question 17

Why does more filtration take place across the capillary than reabsorption?

Answer 17

Because the max filtration pressure is greater than the maximal absorption pressure, the transition point (when hydrostatic and osmotic pressures are equivalent) is located towards the venous end of the capillary and not in the centre

Question 18

Why is elevated hydrostatic pressure problematic?

Answer 18

• High blood pressure results in poor capillary exchange
• If CHP is increases or if BCOP decreases fluid balance is altered there is net exit into peripheral tissue:
   Fluid collects in extremities
   Systemic Oedema

Question 19

What is different about pulmonary circulation?

Answer 19

• Pulmonary vascular resistance is very low
- Arterioles thinner, wider and shorter
- Arteries thinner walled, less elastin
• Pulmonary arterial BP is lower than the systemic BP – 10 mmHg vs 35 mmHg
• Increased distensibility means these vessels can readily accommodate increased cardiac output without significant increases in pressure
• Importantly, fluid can also be continually absorbed along the entire capillary length. This prevents any interference with gas diffusion

Question 20

What happens with high blood pressure in pulmonary circulation?

Answer 20

the opposite dynamic dominates and fluid build up affects respiration as fluid builds up in the lung tissues (alveoli) as opposed to the extremities
- Leads to pulmonary oedema

Question 21

How much CO does the brain consume for what % of body mass?

Answer 21

• Consumes 12% of CO for 2% body mass

* Flow rate 750 ml/min

Question 22

What happens with the brain and blood flow?

Answer 22

• Four arteries used to supply brain and anastomoses inside the cranium
• Blood flow is constant and preserved even in emergencies
• Under extreme conditions peripheral circulation will be constricted but cerebral circulation will be maintained (vasodilation)
• So during any temporary loss of arterial supply, CNS will be unaffected
Coronary Blood Flow

Question 23

What happens with coronary blood flow?

Answer 23

• Coronary blood flow varies throughout the cardiac cycle influenced by heart rate
• Blood flow through the coronary artery is interrupted curing cardiac contraction as a result of compression
• In order to compensate
- High capillary density leading to a high capacity (70%) for O2 extraction
- Capillaries have few arterial collateral interconnections
• Adrenaline promotes vasodilation of coronary vessels, increases heart rate and the strength of contractions of the myocardia
- Elsewhere in the body adrenaline is a vasoconstrictor
- Coronary blood flow increases where elsewhere in the body vasoconstriction predominates

Question 24

What does blood flowing through the capillaries slowly allow time for?

Answer 24

diffusion or active transport of materials across the capillary walls

Question 25

How can water, ions and small organic molecules (e.g. glucose) usually enter or leave the bloodstream?

Answer 25

by diffusion between adjacent endothelial cells or through the pores of the fenestrated capillaries

Question 26

When are large water-soluble compounds able to leave the bloodstream?

Answer 26

at fenestrated capillaries such as those of the hypothalamus, the kidneys, many endocrine organs, and the intestinal tract

Question 27

How can lipids and lipid-soluble materials cross the capillary walls?

Answer 27

by diffusion through the endothelial plasma membranes

Question 28

Where is the only place plasma proteins can cross the endothelial lining?

Answer 28

the sinusoids

Question 29

What happens in capillary filtration?

Answer 29

water and small solutes are forced across a capillary wall leaving larger solutes and suspended proteins in the bloodstream

Question 30

What does the osmotic pressure of a solution represent?

Answer 30

the pressure that must be applied to prevent osmotic movement across a membrane. The higher the solute concentration of a solution the greater the solution’s osmotic pressure

Question 31

What do suspended proteins that cannot cross capillary walls create?

Answer 31

an osmotic pressure called blood colloid osmotic pressure (BCOP) otherwise known as oncotic pressure

Question 32

What is bulk flow and what does it do?

Answer 32

the continuous net movement of water out of the capillaries, through peripheral tissues and then back to the bloodstream by way of the lymphatic system: - it ensures that plasma and interstitial fluid communicate and exchange material constantly - It accelerates the distribution of nutrients, hormones and dissolved gases throughout tissues - It assists in the transport of insoluble lipids and tissue proteins that cannot cross the capillary walls and enter the bloodstream - It flushes out bacteria toxins and other chemicals carrying them to the lymphatic tissues

Question 33

What is the net hydrostatic pressure the difference between?

Answer 33

the hydrostatic pressure inside the capillary and outside the capillary

Question 34

What are the conditions that limit the heart's ability to increase it's output?

Answer 34

- Permanent restrictions or blockage of coronary vessels | - Tissue damage

Question 35

What happens with oxygen content and the alveoli?

Answer 35

When an alveolus contains plenty of oxygen its blood vessels dilate. Blood flow then increases, promoting the absorption of oxygen from air inside the alveolus. When the oxygen content of air is very low the vessels constrict. They shunt the blood to alveoli that contains more oxygen. This mechanism is the opposite from other tissues where a decline in the O2 level causes local vasodilation rather than vasoconstriction

Question 36

What happens as you begin light exercise?

Answer 36

- Extensive vasodilation occurs as skeletal muscles consume oxygen more quickly. Peripheral resistance decreases, blood flow through the capillaries increases and blood enters the venous system at a faster rate - The venous return increases as skeletal muscle contractions squeeze blood along the peripheral veins and faster breathing pulls blood into the vena cavae by the respiratory pump - Cardiac output rises in response to increased venous return and atrial stretching

Question 37

What happens during heavy exercise?

Answer 37

• When doing heavy exercise the cardiac and vasomotor centres activate the sympathetic nervous system increasing cardiac output - Also severely restricts blood flow to nonessential organs (e.g. the digestive system) - Only blood supply to the brain remains unaffected

Question 38

How does regular exercise lower the risk of cardiovascular disease?

Answer 38

lowers cholesterol by stimulating enzyme that help move low density lipoproteins from the blood to the liver. In the liver cholesterol is converted to bile and excreted from the body