Week 8 - Blood Pressure Reg & Capillary Exchange

Question 1

What is blood pressure?

Answer 1

The force exerted upon vessel walls as blood flows through

Question 2

How is blood pressure calculated?

Answer 2

Blood pressure = cardiac output x total peripheral resistance

Question 3

Describe the autoregulation of blood pressure when local blood pressure is too low

Answer 3

• cardiovascular centres in CNS activated
• short term increase in blood pressure achieved via increase of cardiac output and vasoconstriction
• homeostasis achieved

Question 4

How is vasoconstriction or vasodilation controlled?

Answer 4

Vasomotor centre activation (constriction) or inhibition (dilation)

Question 5

What is the chemoreceptor reflex to a rise in CO2?

Answer 5

• respiratory centre activation (increased respiratory rate)
• vasodilation of cerebral vessels (increased blood flow to brain)
• vasoconstriction of peripheral vessels

Question 6

What is the Endocrine response to low blood pressure?

Answer 6

ADH
- released by pituitary gland
- increases vasoconstriction
- increases thirst

Erythropoietin
- released from kidney
- increases blood volume
- incenses vasoconstriction

RAAS (renin-angiotensin-aldosterone system)
- liver releases angiotensinogen
- kidney releases renin
- forms angiotensin I
- angiotensin converting enzyme released from lung converts this to angiotensin II (increases vasoconstriction)
- adrenal glands convert angiotensin II to aldosterone (increases fluid retention - increases blood volume)

Question 7

What mechanisms would be activated after a severe haemorrhage?

Answer 7

• SNS activation
• Barorector + chemoreceptor reflex
• endocrine mechanisms
• autoregulation

(Every mechanism except parasympathetic NS)

Question 8

What is the function of capillary exchange?

Answer 8

• so cells can obtain nutrients and oxygen
• so cells can remove metabolic wastes (e.g. CO2)

Question 9

How are capillaries designed to aid 2-way exchange?

Answer 9

• short diffusion distance (1 micrometer)
• blood flows slowly through capillaries
• large surface area for exchange

Question 10

What are the 3 types of capillary?

Answer 10

Continuous
- least permeable
Fenestrated
- has water-filled pores called fenestrations used for rapid exchange of water and solutes
Sinusoid
- has large clefts between endothelial cells and incomplete basement membrane - free exchange of water + larger solutes (most permeable)

Question 11

What are the 3 main transport mechanisms in capillary exchange?

Answer 11

• diffusion (directly through endothelial cell membrane or ion channels)
• bulk flow (through clefts/pores)
• transcytosis ( via vesicular transport)

Question 12

What is the net filtration pressure?

Answer 12

NFP = net hydrostatic pressure - net osmotic pressure

Question 13

How is net hydrostatic pressure calculated?

Answer 13

NHP = capillary hydrostatic pressure (CHP) - interstitial fluid hydrostatic pressure (IHP)

Question 14

How is net osmotic pressure calculated?

Answer 14

NOP = Blood bolloid osmotic pressure (BCOP) - interstitial fluid colloid osmotic pressure (ICOP)

Question 15

What is the difference between hydrostatic and osmotic pressure?

Answer 15

Hydrostatic pressure is a ‘push’, in which it exerts force on fluids to push them out or in to the capillary
Osmotic pressure is ‘pull’, in which it exerts force on fluids by pulling them in or out of the capillary

Question 16

What happens is net hydrostatic pressure is greater than net osmotic pressure (i.e. net filtration pressure has a positive value?)

Answer 16

Filtration occurs - fluid is pushed out of capillary

Question 17

What happens is net osmotic pressure is greater than net hydrostatic pressure (i.e. net filtration pressure has a negative value?)

Answer 17

Reabsorption occurs - fluid is moved back into the capillary

Question 18

What occurs at the arteriole and venous ends of a capillary? (In ref to capillary exchange)

Answer 18

• arteriole - substances move out of capillary to tissues
• venule - substances move into capillary from tissues

Question 19

Why happens to NFP during hypertension (unusually high blood pressure)

Answer 19

• CHP increases (increases blood pressure)
• overall NFP increases
• fluid collects in extremities - causes systemic oedema

Question 20

What happens to NFP during haemorrhage?

Answer 20

• CHP decreases massively (blood pressure decreases)
• NFP becomes negative - BCOP is a greater value than NFP
• this means fluid is then recalled from tissue into bloodstream

Question 21

What happens to NFP during dehydration?

Answer 21

• BCOP increases
• BCOP > CHP - therefore NFP has a negative value
• fluid recalled from tissue to bloodstream as there is a higher osmotic pressure in the blood

Question 22

What happens to NFP during tissue damage?

Answer 22

• plasma proteins leak into interstitial fluid, increases ICOP
• causes local swelling (oedema)

Question 23

How is capillary exchange different in pulmonary circulation?

Answer 23

• pulmonary circulation must enable gas exchange to occur constantly
• CHP < BCOP all the way along the capillary - fluid absorbed along entire capillary
• if CHP exceeds 25mmHg, fluid leaks into alveoli - causes pulmonary oedema

Question 24

How is capillary exchange different in coronary circulation?

Answer 24

• coronary blood flow is affected by the cardiac cycle (systole + diastole)
• coronary flow restricted during systole as left coronary artery is compressed
• coronary flow is highest during diastole due to arterial elastic recoil