VASCULAR: CAPILLARIES, LYMPH, AND VEINS Flashcards
Function of capillaries
Exchange site for gases, nutrients, water, and waste
Where is capillary density greatest and lowest?
In tissues with high O2 consumption such as cardiac muscle, skeletal, glands, brain
Low density: cartilage and subcutaneous
At rest, how many capillaries are open?
20%
What regulates capillary opening?
Capillary opening is regulated by small arteries, arterioles, metarterioles
Describe this type of capillary: continuous capillary
- Most common
- Junctions 10-15 nm wide
- Blood brain barrier has tight junctions
Describe this type of capillary: fenestrated capillary
- Fenestrations: membrane lined holes through cell
- 20-100 nm wide
- Can be closed by diaphragm
- Exist in the intestine, glomerulus, and exocrine glands
Describe this type of capillary: sinusoidal (discontinuous) capillary
- Large gaps between cells
- 100-1000 um
- Facilitate exchange
- Exist in the liver, bone marrow, and spleen
_________ form at the ends of endothelial cells which prevents the leakage of _________ from capillary. This makes it appear as if their membranes are _____
- Tight junctions
- macromolecules
- fused
List the mechanisms of transcapillary exchange?
- Diffusion (gases, small solutes)
- Filtration
- Bi-directional vesicular transport
a. transcytosis of macromolecules
b. transendothelial channels (stack of fused endocytotic vesicles across cell)
State diffusion equation
J = ([solute]out - [solute]in)PA
J: flux : quantity moved per unit time
P: permeability coefficient
A: capillary surface area
What can diffuse during capillary exchange?
- Gases - direct diffusion across endothelial membrane
- Small solutes - diffusion through small pores and clefts
- Polar molecules have decreased permeability due to poor lipid solubility
- Large molecules have no diffusion above 60 kDa (ex: albumin can’t diffuse)
What are the two forces that govern filtration at the capillary?
- Hydrostatic pressure - blood pressure
- Oncotic or plasmoid osmotic pressure - blood proteins
What is the net filtration pressure?
The net filtration pressure is 0.3 mmHg or 2-3 litres a day
Name where capillaries mainly filter and mainly absorb
renal glomerulus
intestinal mucosa
precapillary sphincters
regulate flow of blood to capillaries depending on demand.
Describe how injury to the capillaries, severe burns, or inflammation will disturb the net filtration balance
If you injure capillaries, severe burns, or inflammation, this will all result in damage or increased leakiness of the capillaries → we lose plasma protein concentration in the blood → oncotic pressure decreases → increased filtration → increased interstitial fluid → edema
Generally the lymphatic tissues can come with increase in interstitial fluid but if it can’t, it leads to edema. What are some of the causes of capacity exceeding?
- Motionless (lymph doesn’t circulate)
- Lymph glands are removed
- Lymph is blocked by tumors
How can pregnancy disturb the net filtration balance
With pregnancy, there is an increase in blood volume but the same amount of blood proteins which result in dilute blood protein concentration → oncotic pressure decreases → increased filtration → increased interstitial fluid → edema
How can hypertension disturb the net filtration balance
Hypertension → increase in hydrostatic pressure → increase in filtration → increase in interstitial fluid → edema
How can standing disturb the net filtration balance
Standing → increase in venous pressure and pooling of blood in legs → increase in filtration → increase in interstitial fluid → edema
How can dehydration disturb the net filtration balance
Dehydration lowers plasma volume → plasma protein concentration increases → increase in oncotic pressure → decrease in filtration
Where does lymph on the left side of the head and neck drain into?
thoracic duct which drains into left subclavian vein at the junction with left internal jugular
Where does lymph on the right side of the head and neck drain into?
Right head and neck drain into the right subclavian vein at junction with right internal jugular
Describe the closed ends of the lymphatic system
Differs from capillary beds as it has closed ends. The ends can open when interstitial pressure increases. These ends are valve-like inter-endothelial junctions
What anchors lymph capillaries to surrounding tissue?
Fine filaments anchor lymph capillaries to surrounding tissue
Describe the expansion phase of lymphatic flow
- Pinterstitial > Lymphatic
- Interendothelial “valves” allow interstitial fluid to enter initial lymphatic
Describe the compression phase of lymphatic flow
- Movement of tissue e.g. skeletal muscle pump
- Lymphatic compression
- Plymphatic > P interstitial
- Interendothelial valves close
- Lymph forced downstream past secondary valve
What regulates lymphatic flow?
- Interstitial pressure: increase net efflux from capillaries → increase in interstitial pressure and increase in lymphatic flow
- Compression forces: skeletal muscle contraction propels lymph towards central areas
- Myogenic tone: stretch causes smooth muscle contraction and lymphatic constriction; moves lymph towards central areas
What is the average flow of lymph and why is it important?
There is a flow of 2-3 liters of lymph which is slow, unidirectional, and required to prevent edema.
What can removal of lymph nodes cause?
Removal of lymph nodes can cause lymphedema of hand, arm, back, breast, and trunk (e.g. breast cancer)
Describe elephantiasis
Elephantiasis: extensive swelling of lower half of body due to obstruction of lymph flow by parasitic roundworms
List the 3 layers of veins
tunica intima, tunica media, tunica adventitia
Why is it important the veins are highly compliant and very distensible
High capacitance
Allows for venous pooling
At rest, 70% of blood is on the venous side of circulation
What helps to ensure unidirectional flow in the veins?
Valves
Compare the compliance of veins and arteries
Arteries are highly compliant and distensible
- Important for their role in receiving high pressure blood flow
- Allows arteries to reduce pulsatility and act as pressure reservoirs
Compliance of veins varies with blood pressure
- compliance is very high at low blood pressure
- high capacitance: large ∆V with small ∆P
- capacitance allows veins to act as volume reservoirs
- compliance decreases at high blood pressures
define: passive capcitance
Passive capacitance: passive changes in venous volumes (e.g. flow change)
define: active capacitance
Active capacitance: active changes in venous volume (e.g. sympathetic vasoconstriction)
How can sympathetic stimulation affect the vascular capacitance and cardiovascular control
Sympathetic stimulation (e.g. hemorrhage) → active constriction of venous VSM → decreased venous volume → increased venous return → increased cardiac output
Sympathetic stimulation → vasoconstriction of arteriole VSM → decreased flow into vascular bed → decreased venous flow → passive recoil of veins → decreased venous volume → increased venous return → increased cardiac output
Fill in the blank: capacitance changes in the peripheral veins are ______ . The veins are responding to what is flowing into them ______
passive
passively
How did the splanchnic circulation experiment prove that splanchnic circulation shows active capcitance
If we keep inflow rate constant, there are changes in capacitance in the venous bed of splanchnic circulation with sympathetic circulation. Thus splanchnic circulation shows active capacitance
Describe the symptoms, complications, and treatment associated with varicose veins
Symptoms:
- Heavy/aching legs
- Ankle swelling
- Skin discolouration (build up of metabolites)
Complications
- Predisposition to syncope (fainting)
- Intolerance to standing
- Eczema, dermatitis
- Thrombophlebitis (PE, DVT, stroke, MI)
Treatment
- Compression stockings
- Elevation of legs
- Anti-inflammatory/anti-coagulant drugs
- Removal of the vein
Describe how the peripheral vascular system and the splanchnic vascular system differ in terms of capacitance response with sympathetic stimulation
peripheral vascular system has only a passive capacitance response when there is sympathetic stimulation. Splanchnic vascular system also has an active capacitance response that accounts for 10% of the capacitance response (other 90 is passive. )
