29.Microcirculation, exchange of substances, venous circulation Flashcards
What should be mentioned?
Microcirculation
Exchange of substances with diffusion
Factors influencing diffusion
Oxygen diffusion to tissues
diffusion of gases
Partial pressure
Exchange of Substances with Filtration/Reabsorption
Direction of plasma flow
Hydrostatic Pressure in the Capillary
Osmotic Forces in the Capillary
Starling forces in disease
Veins and circulation
Factors maintaining circulation: Veins
Microcirculation
There are two major types of material exchanges in this part of circulation: diffusion and filtration/resorption.
These determine the rate of delivery of nutrients to the tissues and the rate of the metabolite transport from the tissues.
Most of the exchange occurs via diffusion.
But Filtration/resorption also occurs adjusting the extracellular space (see the formation of interstitial fluid in the chapter of fluid spaces).
Exchange of substances with diffusion
Materials:
-Gas exchange
- Ion exchange
- small substances (glucose, with facilitated diffusion, etc.)
2types:
– flow limited (small substances)
– diffusion limited (bigger molecules)
Factors influencing diffusion
-Concentrationgradient
-Permeability
-Surfaceofthecapillary
(Timeavailableforthediffusion)
Diffusion and partial pressure
The extent of gas diffusion depends on Partial Pressure.
• That drops towards the end of the capillary and towards the distant cells.
– The higher the O2 consumption of a tissue, the faster
will be the drop of pO2 (partial pressure of oxygen).
- That will lead to hypoxia,
- which will result in the involvement of more (resting) capillaries (more capillaries are going to be open).
- This local autoregulation is a very important way of insuring an even distribution of gases (and of nutrients) in a tissue.
Oxygen diffusion to tissues
- That drops towards the end of the capillary
- Need for more oxygen: more capillaries are going to be open
Diffusion of Gases
- Exchange is transcellular, with diffusion. For O2 – CO2 exchange we have 1 sec (average capillary length is 1 mm, flow rate is 1 mm/sec).
- In rest even less time (2-3 msec) is enough for gas exchange.
- Higher the need for O2 in the tissue, faster the blood flow, so the average time spent in the capillary will be too short for optimal gas-exchange. That lack of O2 turns on regulatory mechanisms (see later), so more capillaries will be connected into the microcirculation, more sphincter will open, larger capillary space will be active.
What is Partial Pressure?
it’s the individual pressure exerted independently by a particular gas within a mixture of gasses. The air we breath is a mixture of gasses: primarily nitrogen, oxygen, & carbon dioxide.
So, the air you blow into a balloon creates pressure that causes the balloon to expand (& this pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about & collide with the walls of the balloon).
However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide.
That part of the total pressure generated by oxygen is the ‘partial pressure’ of oxygen (pO2), while that generated by carbon dioxide is the ‘partial pressure’ of carbon dioxide.
The partial pressure of a gas, therefore, is a measure of how much of that gas is present (e.g., in the blood or in alveoli).
Diffusion (summary) (picture)
Exchange of Substances with Filtration/Reabsorption
Major forces: hydrostatic pressure difference, permeability, the oncotic pressure and the pressure of the (surrounding) tissue.
- So the direction of the substance movement (where is the H2O going?) is determined by the Effective Filtration Pressure.
- (Results show that there are capillaries only for filtration, others only for reabsorption – so filtration and reabsorption do not (or not only) necessarily happen in the same capillary, however for didactic reasons the above model still can be used.)
Direction of plasma flow
The direction of the substance movement (where is the H2O going) is determined by the Effective Filtration Pressure.
- Oncotic pressure is constant in the capillary
- Hydro satic pressure,on the other hand,
constantly drops to the venous end of capillary – If the hydrostatic pressure is higher then oncotic:
net filtration
– If the hydrostatic pressure is lower then oncotic: net reabsorption
Hydrostatic Pressure in the Capillary
Hydrost pressure is lower in the capillary than in the artery, and gradually decreases while the blood flows through the capillary.
- Hydrostatic P at the arteriolar-end of the capillary is ±35 mmHg. At the venule-end is ±15 mmHg.
- The pressure profile of the capillary is linear.
- Hydrostatic pressure of the Interstitial Fluid (ISF) is considered to be 5 mmHg.
Osmotic Forces in the Capillary
-Wall of the capillary: is permeable for water and impermeable for plasma proteins, hence the macromolecules generate osmotic pressure. Moreover, plasma proteins are negatively charged, therefore they try to keep cathions within the plasma (Gibbs-Donnan effect), so they increase the osmotic gradient between the plasma and the interstitial space.
-The combination of the above osmotic and Gibbs-Donnan effects generates sucking force, which absorb H2O from ISF to the plasma.
– This is called Colloid Oncotic (Oncotic) pressure.
- Oncotic pressure is proportional with the protein concentration of plasma and of the ISF.
- Oncotic pressure of the plasma is ± 28 mmHg, while in the ISF
it is ± 3 mmHg. Therefore the net Oncotic P = 25 mmHg.
-That value is constant in all capillaries.
Veins
Function:reservoir
-Resistance is zero.
Veins
-Distended by increased pressure,enlargement is limited by collagen fibers.
-Venuletypes:
– Postcapillary venule (some pericytes)
– Collecting venule (continuous pericyte layer)
– Muscular venule (contractile elements are present)
Venous circulation
Characteristics of venous system: – Capacitance system (reservoir)
- 55 – 75 % of circulating blood reside in the veins. – In case it is needed: redistribution
- the vasomotor mechanism (symphatetic vasoconstriction through α-Receptors) may „redistribute” the blood
- (toward the resistance segment). – Distensibility is large
- but the collagen network sets the limit; there are only few elastic elements in the veins
- Function is determined by: structure of the wall and venous valves.
- The pressure in veins drop from the venules (10-15 mmHg) to the right atrium (0-3 mmHg) continuously.
- Both, pressure and flow rate change with certain rhythmicity in the veins, due to the valves and the change of tissue pressure (i.e. muscle pump), and gravitation
The structure of veins substantially differs from that of the arteries. Veins have very thin distensible walls and possess venous valves. This gives rise to the peculiar behavior described herein. The maintenance of venous blood flow is determined by the work of the heart and some additional forces (gravitation, “muscle pumping”). The pressure in veins drops from the venules (7- 10 mmHg) to the right atrium (about. 3 mmHg) continuously. Both, pressure and flow changes rhythmically in the veins. This pulsation is however not the result of the heart cycle but that of the changing pressure of muscles around the veins, presence of valves and gravitational forces.
