Gas exchange and the lymphatic system Flashcards
Features of capillaries
- Thinwalled; presents a small diffusion barrier
- Small diameter; big SA:Volume ratio
Structure of capillaries (types)
Continous
- no clefts/pores ( brain)
- clefts only ( muscle)
Fenestrated
- clefts + pores ( intestine)
Discontinous
-clefts + massive pores ( liver)
Diffusion and carrier mediaed transport in Gas exchange
Diffusion
- self regulating
- non saturable
- non polar substances acorss the membrane
- polar substances via clefts/channels
Carrier mediated transport
-eg glucose transporter
Only few places (BBB) prevent diffusion of polar substances such as glucose = protein carrier
Bulk flow in capillaries - STarling forces
- Bulk flow is the exchange of fluid
- describes the balance between hydrostatic pressure and osmotic pressure.
- hydrostaatic pressure pushing fluid out through leaky capillaries which builds up an osmotic pressure which draws fluid back in
Function of lymphatic system
-excess filtered fluid drains into lymphatic vessels > enters circulation
What is an oedema?
-accumulation of excess fluid in extravascular space
What can an oedema be caused by?
Lymphatic obstruction (elephantiasis) -due to filariasis, surgery
Raised CVP (high hydrostatic pressure) -due to ventricular failure
Hypoproteinemia ( low osmotic pressure)
-due to nephrosis, liver failure, nutrition
Increased capillary permeability ( low osmotic pressure)
-inflammation ( rheumatism)
What vessels control BF
- mainly by arterioles as they have smooth muscle + are resistance vessels
- dilation/constrict
- radius of arteriole has effect on resistance and hence flow
Thhe effect of dilation of arterioles on TPR
- Reducing resistance of a vasucular bed will increase flow through the vascular bed but it will reduce TPR + also reducing mean arterial pressure
- Hence, arteriolar radius affects flow through individual vascular beds and mean arterial pressure
Compensation
Control over smooth muscles surrounding arterioles:
Intrinsic + extrinsic mechanisms
Intrinsic + Extrinsic mechanisms
Intrinsic mechanisms
-meeting the selfish needs of each individual tissue
Extrinsic mechanisms
- ensures that the TPR ( MAP) of whole body stays in the right ball park
Neural extrinsic control
SYMP NERVES
- releases noradrenaline
- binds to A1 receptors
- causes arteriolar constriction
- thereflow, less flow to that tissue, and increases TRP
Parasympathetic nerves
-no effect
- this is happening throughtout the body so main thing is to increase in TPR ( which increases MAP)
Extrinsic control ( hormonal)
Adrenaline
- releases froma drenal medulla
- binds to A1 receptors
- causes arteriolar constriction
- decreases BF tp tossue, and increases TPR
BUT
- in some tissues (Skeletal/cardiac) it also activates B2 receptors which causes arteriolar dilation
- therefore, increase BF through that tissue which decreases TPR
- A1 is most important in increase TPR
- B2 will help redirect blood flow to heart and SM during flight/fight reaction
Angiotensin 2
- released by kidneys?
- produced in response to Low BV
- causes arteriolar constriction, therefore increase TPR
Vasopressin ( ADH )
- released in repsonse to low BV
- causes arteriolar constriction, increasing TPR
Atrial natriuretic factor
- released in response to high BV
- causes arteriolar dilation, hence decreases TPR
Local intrinsic controls
- Active ( metabolic) hyperaemia
- high metabolic activity causes increased concentration of metabolites which triggers release of EDRF/NO ( paracrines)
- this causes arteriolar dilation
- increased flow to wash out metabolites
- adaptation to match blood supply to metabolic needs of that tissue - Pressure ( flow) autorefulation
- low MAP causes low BF
- metabilites accumulates which triggers release of EDFR/NO
- arterioles dilate + flow is restoe
Local intrinsic controls
- Active ( metabolic) hyperaemia
- high metabolic activity causes increased concentration of metabolites which triggers release of EDRF/NO ( paracrines)
- this causes arteriolar dilation
- increased flow to wash out metabolites
- adaptation to match blood supply to metabolic needs of that tissue - Pressure ( flow) autorefulation
- low MAP causes low BF
- metabilites accumulates which triggers release of EDFR/NO
- arterioles dilate + flow is restored to normal ( or could be myogenic)
- an adaptation to ensure that a tissue maintains its BS despite changes in MAP - Reactive hyperaemia
- occlusion of BS causes subsequent increase in BF
- an extreme version of pressure autoregulation - Injurty response
- aids delivery of blood born leukocytes… to injured area
- arteriolar dilation; inc BF/permeability
Special areas
- coronary circulation
- BS is interupted by systole but still has to cope with increased demand during excercise
- shows excellent active hyperaemia
- expresses many B2 receptors
- these swamp any sympathetic arteriolar constriciton
Cerebral circulation
- needs to be stable
- shows good pressure autoregulation
Pulmonary circulation ( pul. shunt)
- lows O2 causes arteriolar constriction
- opposite response to most tissues
- ensures that blood is directed to best ventilated parts o lung
Renal circulation
- main function is filtration which is dependent on pressure
- changes in MAP affects BV
- good pressure auto regulation
