P: Physiology of capillary veins Flashcards
What vessels participate in microcirculation?
Arterioles, capillaries and venules (smallest ones)
Name different types of capillaries and their characteristics + location
- 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
Capillaries branch from ___ and ___
Terminal arterioles and metarterioles
Metarterioles branch at ___ from terminal arterials and have ___ fibres that encircle the vessel at intermittent points
- right angles
- smooth muscle
Functions of true capillaries
- Regulate entry of blood to capillary
- Participate in delivery of nutrients to/removal of wastes from cells and deliver blood into venules = nutritional flow
What delivers blood directly into venules?
Preferential channels
Some tissues (fingertips, ears) also have ___ which ___ capillaries entirely = non nutritional flow
- arteriovenous shunts
- bypass
What increases transmural pressure?
Increase of blood volume in capillaries
Intrinsic regulation of blood flow into capillary beds:
Increased transmural pressure triggers ___ of terminal arterioles while decreased transmural pressure triggers ___ –> ___ regulation
- contraction
- relaxation
- myogenic
____ of upstream vessels overrides local vasomotion
Extrinsic neural regulation
Through which processes does transcapillary exchange occur and what’s the most important one?
- Diffusion (most important)
- Filtration
- Pinocytosis
Explain the limitations of diffusion at capillaries
- 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
___ ends of capillaries have higher density of pores and permeability is increased here.
Venous
What determines the direction and rate of movement of O2 and CO2?
PO2 and PCO2
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?
- Hydrostatic forces and osmotic (oncotic) forces
- Starling forces
___ of arterioles and ___ of venules will raise hydrostatic pressure
- Vasodilation
- constriction
What increases and decreases capillary hydrostatic pressure?
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
Explain what happens when moving from a lying to a standing position regarding hydrostatic forces
- 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
Which is bigger: [protein] in plasma or in tissue fluid? What does this cause?
Bigger in plasma –> creates osmotic (oncotic) pressure –> movement of water (absorption) out of tissue fluid into capillaries
80% of the total oncotic pressure of plasma results from ___ and 20% from ___
- albumin
- globulins
What is the name of the equation that determines the direction of fluid movement and how does it do so?
- 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)
Explain from where does the plasma that goes into lymphatic system come from
- 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
What causes the fact that in some tissues, capillary beds only filter or absorb
- Regulation of capillary hydrostatic pressure by arteriolar vasoconstriction or vasodilation
- Magnitude of Pc (capillary hydrostatic pressure) is the determining factor
How does pinocytosis work and what is its function?
- 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
Adjoining endothelial cells form ___ which allow interstitial fluid and suspended particles into lymphatic capillaries
One way valves
What is oedema and what causes it?
- 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
What is pulmonary oedema?
- 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)
Reduction in central venous pressure (CVP) increases ___ to the right atrium –> impact on cardiac output (___ mechanism)
- venous return
- Frank-Starling
Increase in ___ and a reduction in ___ of the veins increase overall venous pressure
- Blood volume
- Compliance
Explain effect of gravity on venous pressure when standing upright without moving for a long time
- 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
Explain effect of gravity on venous pressure when going from lying down to standing position
- 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
Explain varicose veins and what happens on standing/following muscular activity
- 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
What is at origin of capillaries?
Precapillar sphincter
