P: Physiology of capillary veins

Question 1

What vessels participate in microcirculation?

Answer 1

Arterioles, capillaries and venules (smallest ones)

Question 2

Name different types of capillaries and their characteristics + location

Answer 2

• Continuous capillaries: adjacent endothelial cells tightly joined together, intercellular clefts allow passage of ions and small molecules, not permeable to proteins, muscle + lungs + adipose tissue
• Fenestrated capillaries: wider intercellular pores, permeable to small proteins, kidney + endocrine gland + intenstinal capillaries
• Discontinuous (sinusoidal) capillaries: large and leaky capillaries with pores, allow RBCs + WBS + various serum proteins to pass, liver + spleen + bone marrow

Question 3

Capillaries branch from ___ and ___

Answer 3

Terminal arterioles and metarterioles

Question 4

Metarterioles branch at ___ from terminal arterials and have ___ fibres that encircle the vessel at intermittent points

Answer 4

• right angles
• smooth muscle

Question 5

Functions of true capillaries

Answer 5

• Regulate entry of blood to capillary
• Participate in delivery of nutrients to/removal of wastes from cells and deliver blood into venules = nutritional flow

Question 6

What delivers blood directly into venules?

Answer 6

Preferential channels

Question 7

Some tissues (fingertips, ears) also have ___ which ___ capillaries entirely = non nutritional flow

Answer 7

• arteriovenous shunts
• bypass

Question 8

What increases transmural pressure?

Answer 8

Increase of blood volume in capillaries

Question 9

Intrinsic regulation of blood flow into capillary beds:
Increased transmural pressure triggers ___ of terminal arterioles while decreased transmural pressure triggers ___ –> ___ regulation

Answer 9

• contraction
• relaxation
• myogenic

Question 10

____ of upstream vessels overrides local vasomotion

Answer 10

Extrinsic neural regulation

Question 11

Through which processes does transcapillary exchange occur and what’s the most important one?

Answer 11

• Diffusion (most important)
• Filtration
• Pinocytosis

Question 12

Explain the limitations of diffusion at capillaries

Answer 12

• Flow-limited transport: if blood flow is slow, small molecules pass rapidly through pores and accumulate at arterial end of capillary (increase in blood flow or in capillary density increases supply of diffusable solutes)
• Diffusion-limited transport: big molecules can’t diffuse OR if capillary density is low/during oedema, increased distance between capillaries and tissue limits efficient exchange

Question 13

___ ends of capillaries have higher density of pores and permeability is increased here.

Answer 13

Venous

Question 14

What determines the direction and rate of movement of O2 and CO2?

Answer 14

PO2 and PCO2

Question 15

Filtration at capillaries: direction and magnitude of movement of fluid between capillaries and interstitium is determined by relative strength of which 2 opposing forces? What are they known as together?

Answer 15

• Hydrostatic forces and osmotic (oncotic) forces
• Starling forces

Question 16

___ of arterioles and ___ of venules will raise hydrostatic pressure

Answer 16

• Vasodilation
• constriction

Question 17

What increases and decreases capillary hydrostatic pressure?

Answer 17

Increases capillary hydrostatic pressure:
- Dilation of arterioles –> increased inflow
- Increased venous pressure –> reduced outflow
- Increase in arterial or venous pressure

Decreases capillary hydrostatic pressure:
- Increase in arteriolar resistance

Question 18

Explain what happens when moving from a lying to a standing position regarding hydrostatic forces

Answer 18

• Initial movement of blood to lower extremities raises arterial and venous pressure –> increases capillary hydrostatic pressure
• Increased filtration and oedema results if not corrected by myogenic regulation

Question 19

Which is bigger: [protein] in plasma or in tissue fluid? What does this cause?

Answer 19

Bigger in plasma –> creates osmotic (oncotic) pressure –> movement of water (absorption) out of tissue fluid into capillaries

Question 20

80% of the total oncotic pressure of plasma results from ___ and 20% from ___

Answer 20

• albumin
• globulins

Question 21

What is the name of the equation that determines the direction of fluid movement and how does it do so?

Answer 21

• Starling equation
• It depends on capillary hydrostatic pressure (Pc) and plasma oncotic pressure (πp) –> largest one determines net filtration
• If fluid movement (Qf) is positive –> filtration, if negative –> absorption
• Net filtration at arterial end, net absorption at venous end (idealized capillary)

Question 22

Explain from where does the plasma that goes into lymphatic system come from

Answer 22

• Most capillaries, hydrostatic and oncotic pressures equilibrate quickly due to rapid movement of water
• 2% of plasma is filtered and 85% of that is immediately reabsorbed
• In entire capillary circulation, net filtration occurs
• 15% excess of the 2% goes into lymphatic system

Question 23

What causes the fact that in some tissues, capillary beds only filter or absorb

Answer 23

• Regulation of capillary hydrostatic pressure by arteriolar vasoconstriction or vasodilation
• Magnitude of Pc (capillary hydrostatic pressure) is the determining factor

Question 24

How does pinocytosis work and what is its function?

Answer 24

• Section of endothelial cell membrane engulfs plasma, membrane buds off (endocytosis) to form intracellular vesicle –> vesicle transported crosses cell, fuses with cell membrane (exocytosis) releasing contents in interstitial fluid
• Exchange of large molecules

Question 25

Adjoining endothelial cells form ___ which allow interstitial fluid and suspended particles into lymphatic capillaries

Answer 25

One way valves

Question 26

What is oedema and what causes it?

Answer 26

- Excess fluid accumulation in extracellular spaces - Caused by excessive capillary filtration and/or poor lymphatic drainage: increased capillary filtration coefficient, increased capillary hydrostatic pressure, decreased plasma oncotic pressure

Question 27

What is pulmonary oedema?

Answer 27

- Left ventricular failure --> build up of blood in pulmonary circulation --> pulmonary hypertension and increased pulmonary Pc - Excessive fluid accumulation in lung capillaries (life-threatening, excess fluid interferes with gas exchange)

Question 28

Reduction in central venous pressure (CVP) increases ___ to the right atrium --> impact on cardiac output (___ mechanism)

Answer 28

- venous return - Frank-Starling

Question 29

Increase in ___ and a reduction in ___ of the veins increase overall venous pressure

Answer 29

- Blood volume - Compliance

Question 30

Explain effect of gravity on venous pressure when standing upright without moving for a long time

Answer 30

- Gravity causes pooling of blood to lower extremities of arterial and venous systems - Higher compliance of veins causes greater pooling and distension --> venous pressure gets higher the lower you go in the body - Reduces venous return --> reduces CO - Fainting

Question 31

Explain effect of gravity on venous pressure when going from lying down to standing position

Answer 31

- Lying down: venous blood pools in veins in thoracic cavity - Standing up: blood will shift immediately to lower extremities (venous pooling) --> drop in venous return and CO - Drop in arterial pressure = orthostatic hypotension --> dizziness - Immediately corrected by baroreceptor reflex

Question 32

Explain varicose veins and what happens on standing/following muscular activity

Answer 32

- Damage to venous valves due to prolonged increases in venous pressure stretch - On standing/muscular activity: downward movement of blood into ankles --> venous + capillary pressures become very high --> leakage of fluid from capillary = constant oedema + swelling in feet and ankles

Question 33

What is at origin of capillaries?

Answer 33

Precapillar sphincter