Capillary Exchange Flashcards
What is the primary function of capillary exchange?
To obtain nutrients and oxygen, and to remove metabolic wastes like CO2
What is required to exchange substances across capillary networks?
A minimum pressure, known as capillary hydrostatic pressure
Why are capillaries good sites for substance exchange?
- Short distance for diffusion
- Thin walls (1µm)
- Small diameter (8µm)
- Close proximity to cells
- Blood flows slowly
- Large cross-sectional area of capillary network
- Large surface area for exchange (600m²)
What are the three types of capillaries?
- Continuous (most capillaries)
- Fenestrated - water-filled pores = rapid exchange in solutes (endocrine organs, intestines, kidneys)
- Sinusoid - large cleft between endothelial cells and incomplete BM = free exchange of water and larger solutes e.g. plasma proteins (endocrine organs, liver, bone marrow, spleen)
What is the main mechanism of substance exchange in capillaries?
Diffusion (down a conc gradient)
Bulk flow (down pressure gradient)
Transcytosis (by vasicular transport through endothelial cells)
What types of molecules pass through capillary walls by diffusion?
- Lipid-soluble gases (e.g., O2, CO2)
- Ions (e.g., Na+, K+, Ca2+, Cl-)
- Small water-soluble molecules (e.g., water, glucose, urea, amino acids)
What influences fluid and solute movement across capillary walls?
Net Filtration Pressure (NFP)
What are the four forces that influence fluid movement in capillaries?
- Blood Colloid Osmotic Pressure (BCOP)
- Interstitial Fluid Colloid Osmotic Pressure (ICOP)
- Capillary Hydrostatic Pressure (CHP)
- Interstitial Fluid Hydrostatic Pressure (IHP)
What happens to net filtration pressure (NFP) during hypertension?
Increased filtration and fluid collects in extremities, leading to systemic oedema
What is the typical NFP value at normal conditions?
10 mmHg
How does dehydration affect net filtration pressure?
Increases reabsorption, recalling fluid from tissues into the bloodstream
What is a significant characteristic of pulmonary circulation?
Pulmonary vascular resistance is very low
What occurs if pulmonary capillary hydrostatic pressure exceeds 25 mmHg?
Fluid leaks into alveoli, causing pulmonary oedema
How does coronary circulation respond to increased cardiac output?
Coronary artery vasodilation is promoted by SNS/adrenaline
What is the flow rate of blood to the brain?
750 ml/min
Fill in the blank: The process by which substances move down concentration gradients in capillaries is called _______.
Diffusion
True or False: Capillary exchange occurs at the site of capillary networks only.
True
What happens to coronary blood flow during systole?
Coronary flow is restricted due to compression of the left coronary artery
What is the typical capillary hydrostatic pressure (CHP) at the arterial end?
35 mmHg
What type of capillary has large clefts between endothelial cells for free exchange?
Sinusoid capillaries
What is the effect of tissue damage on net filtration pressure?
Increased filtration due to plasma proteins leaking into interstitial fluid
Describe the structure of a capillary
Designed for two-way process of exchange
- short distance for diffusion = thin walls (1 um), small diameter (8 um) and close proximity to cells
- blood flows slowly = large cross-sectional area of capillary network (lots of exchange)
- large surface area for exchange = 10+ billion capillaries (so greater diffusion)
What is net filtration pressure?
Bulk flow between capillaries and interstitial fluid is determined by the net pressure difference across capillary walls
- Capillary hydrostatic pressure = Pushes fluid OUT
- Interstitial fluid hydrostatic pressure = pushes fluid IN
- Blood colloid osmotic pressure = pulls fluid IN (from interstitial fluid)
- Interstitial fluid colloid osmotic pressure = pulls fluid OUT
How is net filtration pressure calculated?
Net hydrostatic pressure - net osmotic pressure
How is net hydrostatic pressure calculated?
Favours “pushing” fluid from capillary to IF = filtration
Capillary hydrostatic pressure (CHP) - Interstitial fluid hydrostatic pressure (IHP)
CHP decreases along capillary
IHP is generally negligible (0mmHg)
How is net osmotic pressure calculated?
Favours “pulling” fluid into capillary from IF = reabsorption
Blood colloid osmotic pressure (BCOP) - Interstitial fluid colloid osmotic pressure (ICOP)
BCOP is affected by blood volume
ICOP is generally negligible
What are the special circumstances in pulmonary circulation?
- In lungs, arterioles constrict in regions of low O2 to shunt blood flow to O2-rich areas
- Enhances O2 absorption
- In other organs, vessels dilate when O2 falls
- Enhances O2 delivery
Pulmonary circulation supplies >300 million alveoli
SO
- Pulmonary vascular resistance is very low = arterioles are shorter, wider and have thinner walls
- CHP is lower than in systemic circulation = 10 mmHg rather than 35 mmHg
- Arteries are more distensible = can accommodate increased CO with little increase in pressure
What are the special circumstances in coronary circulation?
A wide range of molecules regulate myocardial perfusion
- SNS/Adrenaline promotes coronary artery vasodilation
- Coronary flow increases when vasoconstriction dominates elsewhere
- Coronary flow is restricted during systole due to compression of left coronary artery
- Coronary flow is highest during diastole enabled by arterial elastic recoil
TO COMPENSATE
- Cardiomyocytes have high O2 reserves
- The myocardium has high capillary density = increases O2 extraction
What are the special circumstances in cerebral circulation?
- Blood flow to the brain must be preserved at all times
- In emergencies, there is vasodilation of cerebral vessels while there is vasoconstriction in the periphery
THE BRAIN
- consumes 12% CO for 2% body mass
- flow rate = 750 ml/min
- neurons have poor metabolic reserves
- 4 arteries used to supply the brain which anatosome inside cranium
- flow can be maintained if there is distruption
