Week 5: Capillaries & Veins/ special circulations and exercise

Question 1

What is a metarteriole? when are they useful? What are precapillary sphincters?

Answer 1

A vessel that emerges from an arteriole connecting directly to a venule with the distal portion of the metarteriole having no smooth muscle. This is particularly useful for when artierioles constrict to prevent blood flow, but since all tissues require oxygen, there is still passive flow to the tissue.

• The precapillary sphincters are basically rings of smooth muscle that surround the entrance of capillary beds constricting and relaxing based off of blood supply requirements

Question 2

What are the main characteristics of capillaries?

Answer 2

• Walls are very thin (single layer of endothelial cells) which helps with efficeincy of exchange)
• Narrow lumen (blood cells travel in single file allowing for as much of the blood to participate in exchange)
• extensive branching with Incredible surface area that allows for the slow velocity of blood allowing for more time for exchange of materials

Question 3

How is the structure of a capillary vessel important to its function? How do water and lipdsoluble substances move in and out?

Answer 3

• Endothelial cells fit together like a jigsaw with the gaps that are inbetween the cells acting as pores for exchange of water soluble substances from the blood to the interstitial fluid and vice versa, whereas lipid soluble substances move down concentration gradients through the cell membranes of capillary walls.

Question 4

Why is it good to have plasma proteins stay in the plasma and not participate in exchange?

Answer 4

Plasma proteins attract osmotic pressure. If they were to escape from the capillary you would end up with large concentration gradient change causing alot more fluid to move out from the blood and into the tissues causing an oedema.

Question 5

What is a good example of a lipid soluble molecule that is very important to physiological function? how is it transported?

Answer 5

oxygen, it is transported straight through the endothelial cells as it is lipid soluble.

Question 6

What are some examples of water soluble molecules? how are they transported?

Answer 6

sodium, potassium, glucose, amino acids:

They are transported through the pores in between endothelial cells. as they cannot dissolve through lipids.

Question 7

How are proteins exchanged? (breiflly answer)

Answer 7

exchangable protiens are moved in and out of the plamsa using vesicles from endothelial cells (vesicular transport)

Question 8

What are the two extremes of cappilary pore presence and size? and what would “standed” capillary pores be able to transport?

Answer 8

At one extreme brain capillary endothelial cells are joined by tight junctions with no pores whereas on the other extreme in the liver the pores in the capillaries are so large that even proteins can be exchanged.

Standard capillary (continuous capillary) pores would be able to permit the movement of small water soluble molecules like amino acids, glucose and ions.

Question 9

What are fenestrated capillaries? how do they differ from continuous capillaries?

Answer 9

They differ from capillaries in that thier endothelial cells have numerous fenestrae (pores) where the cytoplasm is absent.

• Found in villi of small intestine, inside bownmans capsure in nephrons, endocrine glands and other places where there are higher rates of exchange taking place

Question 10

What is the effect of histamine on capillary permeability?

Answer 10

Histamine (released from mast cells) is through to be responsible for increasing capillary permeablilty by triggering contractile (actin-myosin) responses in endothelial cells. Essentially it widens the pores so that proteins can leak out.

Question 11

Explain the capillary situation within the brain?

Answer 11

• The brain is sheilded from harmful changes in the blood by the blood brain barrier (BBB)
• In these capillaries, cells are joined by tight junctions that stop all molecules from passing through the JUNCTIONS.
• the only way to exchange materials is if the materials are lipid soluble (like oxygen) or if they are vesicularly transported.

Question 12

how can we calculate the net rate of diffusion?

Answer 12

Net rate of diffusion = ( Concentration gradient x surface area x diffusion coefficeint) divided by the distance

Question 13

Explain the term “bulk flow”?

Answer 13

A volume of protein-free plasma filters out of the capillary and mixes with the surrounding interstitial fluid and is then subsequently reabsorbed due to hydrostatic or osmotic pressure gradients.

Question 14

What are hydrostatic forces (in relation of capillary exchange) ?

Answer 14

BHP - Blood Hydrostatic Pressure:

• Moves fluid out of capillaries into interstitial fluid
• Usually 30mmHg near arteriole end and 15mmHg at venule end

Interstitual Fluid Hydrostatic Pressure:

• Opposes BHP
• Wants to move fluid from interstitual space into capillaries
• Usually a very small value around 0mmHg

Question 15

What are Osmotic forces (in relation to capillary exchange) ?

Answer 15

BOP: Blood Osmotic Pressure:

• A colloid osmotic pressure exerted due to plasma proteins
• Tends to attract fluid back into capillaries
• about 28-30mmHg

OP: Interstitial Osmotic Pressure:

• A colloid osmotic pressure exerted due to a small ammount of escaped proteins in the IF
• It tends to pull fluid out of the capillaries
• About 6mmHg

Question 16

How can we calculate the effective filtration pressure? (P_eff)

Answer 16

P_eff = (BHP + IFOP) - (IFHP + BOP)

Question 17

Calculate the Arterial end and venous end pressures of the capillaries using the following values, also state whether net flow is in or out.

Arterial end:

BHP = 30, IFHP = 0, BOP = 28, IFOP = 6

Venous end:

BHP = 15, IFHP = 0, BOP = 28, IFOP = 6

Answer 17

Arterial End:

P_eff = (30 + 6) - (0 + 28)

P_eff = 8mmHg

Possitive value = Net fluid movement out of the capillary and into the tissue

Venous End:

P_eff = (15 + 6) - (0 + 28)

P_eff = -7mmHg

Negative value = Net fluid movement into capillary and out of the tissues

Question 18

What happens to capillary exchange if the BP suddenly falls? e.g. a haemorrhage

Answer 18

When blood volume is reduced during a hemorage, BP is reduced which therfore means that capillary BP lowers aswell which will therefor alter the hydrostatic forces in the capillary bed. Because the pressure is lower in the capillaries than the IF, fluid will be reabsorbed by the capillaries from the IF until homeostatic margins are reached or until medical attention resolves the problem.

Question 19

Explain how capillary exchange can aid hypertension?

Answer 19

Capillary exchange can help to lower the volume in the capillaries to help reduce venous return, and therfore lower CO which in turn will assist in lowering BP. Due to the pressure being much higher within hypertensive capillaries net blood flow will be within the interstial fluid, as IF will contain the lower end of the pressure gradient (ultrafiltration).

Question 20

How is lymph flow accomplished?

Answer 20

1. The “squeezing” effect of skeletal muscles
2. Lymph vessels beyond lymph capillaries are surrounded by smooth muscle

Question 21

What are the main characteristics of lymph?

Answer 21

• 3L of flow per day
• Lymph nodes filter bacteria and destroy them
• transports absorbed fat from small intestine
• returns filtered proteins back to CVS

Question 22

What is an Oedema?

Answer 22

Excess fluid accumulation in the tissue

Question 23

What are the 4 major causes of Oedema?

Answer 23

1. Reduced concentration of plasma proteins
2. Increased permeability of capillary walls (e.g. histamine causing a widening of he pores)
3. Increased venous pressure; Seen with Congestive Heart Failure (inneficient blood pumping due to narrowed walls causing high BP and therefor higher ammounts of fluid drainage)
4. Blockage of lymph vessel (parasite etc.)

Question 24

What is a venule? What is the structural difference between the section of the venule closest to the capillaries vs the section closest to the veins?

Answer 24

When several capillaries come together they form a venule which then drains into veins.

Closest to capillaries:

• Tunica interna (endothelium) and Tunica Externa (CT)

Closest to Veins:

• Tunica interna (endothelium), TUNICA MEDIA (smooth muscle), and Tunica Externa (CT)

Question 25

explain the term “veins are vessels of capacitance”

Answer 25

Easily distend to accomodate additional volumes of blood with only a small increase in venous pressure

• veins are very complient - they have great stretchability

Question 26

How is blood “promoted” back to the heart? what are the 5 different mechanisms that aid venous return?

Answer 26

Because veins have a pressure greater than that of the Atrium (which is 0mmHg) this promotes blood towards the heart.

5 Factors that help:

1. Skeletal Muscle pump
2. Sympathetic activity
3. Respiratory Activity
4. Cardiac Suction pump
5. Valves

Question 27

How does sympathetic activity enhance venous return?

Answer 27

Vasoconstriction moderately elavates venous pressure helping to drive it towards the heart by reducing volume.

Question 28

How does the skeletal muscle pump assist venous return?

Answer 28

Because large veins usually lie between large muscle blocks, when these muscles contract, they increase the resistance in the vessel and therefor force the blood to go back towards the heart by “milking” it.

Question 29

How do valves assist in venous return?

Answer 29

With valves being at 2-4cm intervals this means that with some increased pressure or skeletal muscle pump assistance, that blood will move upwards when the pressure increases enough within a section of a vessel. The valves stop blood from moving backwards.

Question 30

How does the effect of respiratory activity assist in venous return?

Answer 30

On inspiration, pressure within the thoracic cavity is reduced subatmospherically to fill the lungs with air causing a negative pressure in the lungs. This pressure gradient assists in bringing back blood to the heart.

Question 31

How does the cardiac suction pump assist in venous return?

Answer 31

During contraction of the heart, the AV valve rings are drawn down creating a negative pressure gradient causing venous return to happen more efficiently.

Question 32

What is recruitment in pulmonary circulation?

Answer 32

When pulmonary pressure increases causing the RECRUITMENT of new capillaries due to them being forced open by the pressure increase.

Question 33

What is the response of the pulmonary arterioles to local hypoxia?

Answer 33

increase PCO₂ and decrease in pH causes the opposite response to that present in the systemic response. Hypoxia causes local vasoconstriction of blood vessels so that blood flow is directed AWAY from areas of poor lung ventilation with the aim of improving gas excachange ratios.

Question 34

“the heart has a low capacity to develop O₂ Debt” explain

Answer 34

The heart is very greedy in that it requires alot of oxygen and nutrients therefore it cannot afford to have this supply compromised. To compesate for importance the heart contains many anastomoses in the event that blockages or clots were to occur.

Question 35

During excercise what has to happen to coronary circulation?

Answer 35

During exercise coronary blood flow needs to increase substatially as the heart removes a large percentage of the O₂ from haemoglobin. Adenosine (Potent vasodilator) is released by myocytes in response to decreasing O₂ levels. Adenosine is the major contributor for the increased coronary vasodilation. Sypathetic nervous stimulation of coronary blood vessels also cause vasodilation (beta receptor effect), whilst the alpha activity (vasoconstrictor) is weak.

Question 36

What are the main characteristics of cerebral circulation?

Answer 36

- The flow rate remains constant over a wide range of activities.

• Capillaries of the brain are relatively impermeable due to the protective nature of the BBB
• interruption to the blood flow to the brain for 5-10 seconds results in loss of conciousness and a loss for more than 3 or 4 minutes will cause brain damage
• The ANS has little to no affect on cerebral blood flow
• Myogenic and metabolic auto-regulation are important controlling mechanisms

Question 37

How do myogenic mechanisms assist in cerebral circulation?

Answer 37

Myogenic mechanisms maintain a constant blood flow when MAP fluctuates between 50-150mmHg

Question 38

How do metabolic auto-regulatory mechanisms assist cerebral circulation?

Answer 38

Metabolic auto-regulatory mechanisms maintain a constant O₂ supply to regions of the brain depending upon their activity.

Question 39

“cerebral arterioles are sensitive to changes in PCO₂ or acidity” explain how this works?

Answer 39

If PCO₂ increases from 40 to 100mmHg (hypercapnia), this induces vasodilation: 2x Blood flow.

If PCO₂ Falls from 40 to 20mmHg (hypocapnia), this induces vasoconstriction: 0.5x cerebral blood flow

These effects are designed to increase or decrease the oxygen supply to the active brain tissue

Question 40

How does the skeletal muscles O₂ consumption differ from resting to responding to maximum exercise rates? What mechanism controls this?

Answer 40

At rest = recieves 20% of total CO (20% of O₂)

At Max exercise: recieves 90% of total CO (90% of O₂)

This is controlled by the metabolic regulation mechanism that causes vasodilation to tissues that have a high/important metabolic activity. Initially adrenaline also binds to beta receptors to help with initial vasodilation.

Question 41

What are the main characteristics involved with renal circulation?

Answer 41

• Recieves about 20% CO through a low resistance vascular bed
• Renal functions are independent of fluctuations in MAP because of myogenic autoregulation of the arterioles THIS IS BECAUSE FILTRATION RELIES ON A PRESSURE GRADIENT = MUST BE SOMEWHAT STABLE
• ^ can be overriden by SNS and adrenaline acting on alpha receptors when requied (during exercise or stress response…..)

Question 42

How does the cutaneous circulation help maintain a constant body temperature? how does the body respond to hot and cold temperatures?

Answer 42

Thermoregulation: Temp monitored by hypothalamus

• When body temperature increases, a decrease in sympathetic stimulation to the alpha receptors results in vasodilation. This is enhanced by bradykinin which is produced from sweat glands in response to hypothalamic activation.
• When body temperature decreases this induces initial vasoconstriction.

Question 43

What is splanchnic circulation? what are the main characteristics?

Answer 43

Visceral organs within abdominal cavity

• recieves 25% CO at rest the liver recieves 75% of its bloof from the hepatic portal vein and 25% from hepatic artery.
• Increase in glandular activity in muscosal and submucosal layers increase release of bradykinin causing local vasodilation
• Splanchnic circulation is controlled by autoregulatory mechanisms: during exercise Sympathetic stimulation to alpha receptors of the splanchnic arterioles and veins mediates vasoconstriction and venoconstriction.

Question 44

What is bradykinin?

Answer 44

An inflammatory mediator, acts as a vasodilator which therefore lowers blood pressure.

Question 45

Explain what happens in relation to the circulatory response in exercise?

Answer 45

exercise produces an increased blood flow to active muscles. Apart from the increase in CO, this is due mostly to vasodilation induced by local factors, decrease in the pressure of oxygen and an increase in the pressure of carbon dioxide etc. Capillaries open up allowing the tissues to participate in a greater extraction of oxygen from blood. Due to the Bohr effect, skeletal muscle is capable of removing 90% of oxygen from haemoglobin at maximum exercise levels.

Question 46

Why does skin circulation increase essentiall with exercise but then rapidly decreases?

Answer 46

Essentially the body tries to cool off by getting rid of heat as sweat, but as exercise increases the skeletal muscle has such a high demand for oxygen that circulation can not be “wasted” in vasodilating so it becomes more passively controlled by the circulatory system.

Question 47

What happens to regional circulation during exercise in the follwing areas:

Heart, gut and kidneys, skin, and brain?

Answer 47

Heart: coronary circulation increases due to the binding of adrenaline onto the coranary artery beta receptors; this is to meet the extra deman of the increased CO

Gut and kidneys: Gut and kindey arterioles are vasoconstricted by the alpha receptors

Skin: Essentially the body needs to get rid of heat so vasodilation to skin occurs

Brain: Remains constant; no change in metabolic rate

Question 48

What happens to cardiac output during exercise?

Answer 48

CO increases due to venoconstriction, vasodilation and increases myocardial contractility as well as the action of working muscles on leg veins.