Mircocirculation
- comprises arterioles, capillaries, venules
- The surface area of the microcirculation is huge, comprising ~95% of the total surface area of all blood vessels in circulation
The capillaries
- The capillaries are the only vessels where gas exchange occurs
- To facilitate gas exchange, the capillary wall is very thin (0.5um)
- Made up of just single endothelial layer - Capillaries are the smallest vessels in the body but they make up the largest cross-sectional area
- Estimated 40 billion capillaries in the human body compared to ~10million arterioles - Capillary pressure is very low
- Capillary flow rate is very slow (due to cross sectional area)
Blood flow and blood flow velocity
- Blood flow refers to the movement of blood through the vessels, It is measured as volume/time
- Blood flow velocity (or rate) is a measure of the speed that the blood is moving. It is measured as distance/time
- Blood flow velocity is inversely related to the total cross-sectional area of the blood vessels
- As total cross-sectional area increases, the velocity of flow decreases - This is the case moving down the vascular tree
- Blood flow velocity is slowest through capillaries
The microcirculation (in depth) - arterioles, metarterioles
Arterioles contain a relatively high proportion of smooth muscle;
- thick walls, i.d. ~30um 9rnage 15-300um);
- resistance vessels.
- They are densely innervated by sympathetic nerves
Metarterioles contain some smooth muscle, and serve either as arterio-venous shunts (esp. in skin), or give rise to capillaries
- No diffusion taking place = is not a capillary
- Is a bypass system which come into effect when there’s not a great need for oxygen delivery to a tissue = so a tissue that is not highly metabolically active
The microcirculation (in depth) - capillaries, precapillary sphincters, venules
Capillaries consist only of a single layer of endothelial cells i.d. ~8um;
- exchange vessels.
- They form a complex network
- density of capillaries varies in different tissues, according to the normal metabolic activity of that tissue (e.g. high in skeletal muscle, brain, glands, musch less in bone or cartilage)
Precapillary sphincters (band of smooth muscle) at the origin of capillaries.
- These open in response to increase in metabolite concentration
- Wraps around the beginning of the capillary; it can contract = when it contracts it closes off the capillary = is a way of regulating blood flow through capillaries (can turn it on or off) by the contraction or relaxation of these sphincters
- Not innervated by the sympathetic nervous system → only way they contract or relax has to do with the metabolite concentration (how much CO2 is present)
- E.g. during exercise, the number of open capillaries in skeletal muscle may increase 20-fold
Venules have an i.d. of ~20um;
- mainly consist of connective tissue
Increasing metabolic activity
- Increasing metabolic activity relaxes precapillary sphincters and more capillaries open
- If the no. of open capillaries is increased, then:
- The average diffusion distance for exchange is reduced
- The average time for exchange within the capillaries is increased
4 ways that exchange takes place across capillary walls: 1: simple diffusion, 2: facilitated diffusion
1 and 2: Diffusion
- The rate of diffusion is dependent on 3 factors:
- The concentration gradient across the capillary wall of the substance
- Surface area available for exchange
- Permeability of a particular molecule - Substances move down concentration gradient
- [high] → [low]; e.g. glucose and O2 diffuse out of blood, CO2 and metabolites diffuse into blood - With lipid-insoluble molecules of increasing size, the rate of diffusion becomes progressively less, until at a molecular weight of 60 000 the diffusion is nearly zero
4 ways that exchange takes place across capillary walls: 3. transcytosis
3: Transcytosis
- Endothelial cells encapsulate a substance on one side of the membrane (endocytosis), transport the vesicle across the membrane and release the material on the other side (exocytosis).
- Examples include insulin, fatty acids
4 ways that exchange takes place across capillary walls: 4. Filtration and Reabsorption
- Movt of water + solute across the capillary wall
- Bulk flow = driving of blood flow through the circulation
- Large amounts of fluid
- Driving force is pressure gradient, not concentration gradient
- Difference b/w bulk flow and diffusion: you can easily observe bulk flow in some way whereas diffusion is much more subtle - don’t necessarily know that it is going on
Filtration and Reabsorption
- Filtration: when pressure inside the capillary exceeds pressure outside → fluid is pushed out of capillary
- Reabsorption: when pressure outside the capillary is greater than pressure inside → fluid moves into capillary
- Osmosis: this process of water moving from areas where there is a low concentration of solute to areas where there is a high concentration of solute
- Filtration and reabsorption are regulated by balance between:
- Capillary hydrostatic pressure (PC) = pressure due to the presence of a fluid which is blood in the capillary
- Interstitial fluid hydrostatic pressure (PIF) =
- Capillary colloid osmotic pressure (πC) = caused by the presence of proteins in the blood
- Interstitial fluid colloid osmotic pressure (πIF) = pressure that draws water into the interstitial fluid = the osmosis effect caused by proteins in the interstitial fluid - [filtration coefficient (k)]
- Colloid = essentially referring to proteins
- Normally the filtration forces are slightly greater than the reabsorption forces
Starling’s hypothesis
states that:
- Fluid flow OUTWARDS = k (sum (outward forces) - sum (inward forces))
- Therefore, fluid flow OUTWARDS = k ((Pc + piIF) - (PIF + piC))
POSITIVE = filtration
NEGATIVE = reabsorption
