LECTURE 7 (Microcirculation) Flashcards
What are capillaries?
The sites for exchange of materials between blood and tissue cells
What are the adaptations of capillaries that enhance diffusion?
- Capillaries consist of a single layer of flat endothelial cells = short diffusion distance
- Each capillary is NARROW so RBC squeeze through in a single file = slows diffusion
- Extensive capillary branching = short diffusion distance + increases surface area
Why is the blood flowing through any level of the system equal to the Cardiac Output?
because the circulatory system is a closed system
Why does velocity of blood flow vary with different segments of the vascular tree?
Since velocity of blood flow is inversely proportional to the cross-sectional area of all vessels at any given level
Which part of the vascular tree has the largest cross-sectional area?
Capillaries
Explanation: Large cross-sectional area is where the velocity of blood is greatly reduced. Slow velocity allows adequate time for exchange of nutrients and metabolic end products between blood and tissue cells.
What is the difference between Flow rate and Velocity of flow?
Flow rate = volume of blood per unit of time flowing through a given segment of the circulatory system
Velocity of flow = speed that blood forward through a given segment of the circulatory system
Describe the composition of capillary pores, from tightest to leakiest
- (TIGHTEST) = brain capillaries are joined by “tight junctions” so pores are non-existent -> forms blood-brain barrier
- Most capillaries allow small molecules (e.g O2, ions, glucose) but not plasma proteins
- Kidneys and intestine capillaries have FENESTRATIONS (larger holes) which function in rapid fluid movement across capillaries
- (LEAKIEST) = Liver cells are discontinuous with SINUSOIDS (large channels) that allow proteins to freely pass
Are smooth muscle cells present in capillaries? (YES/NO)
NO
What is the result of the enlarged pores of capillaries?
The affected capillary wall is leakier resulting in normal retained plasma proteins escaping into the surrounding tissue where they exert an OSMOTIC EFFECT
Where do capillaries usually branch from?
Directly from an arteriole or a METARTERIOLE (a thoroughfare channel that runs between an arteriole and a venule)
What are the properties Precapillary sphincters?
- Not innervated
- High degree of myogenic tone
- Sensitive to local metabolic changes
- Act by controlling blood flow through capillary that each one guards
How do precapillary sphincters function?
Tissue metabolic activity increases -> Local metabolic changes bring about relaxation of pre capillary sphincters in the vicinity -> Increases number go open capillaries -> Tissue activity decreases -> Local precapillary sphincters contract -> blood bypasses the capillary bed and flows only through the metarteriole
What is blood flow through a particular tissue regulated by?
- Degree of resistance offered by the arterioles in the organ regulated by SYMPATHETIC ACTIVITY and LOCAL METABOLIC FACTORS
- Number of open capillaries controlled by action of the same local factors on PRECAPILLARY SPHINCTERS
Exchanges between blood and surrounding tissues is accomplished in which ways?
- Passive diffusion
- Bulk flow
What is Bulk Flow?
When a volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid and is then reabsorbed
Explanations: called “bulk flow” because various substances of fluid are moving in bulk in contrast to diffusion with individual solutes
What is Ultrafiltration?
When pressure inside the capillary exceeds pressure on the outside so fluid is pushed out through the pores
What are the forces that influence bulk flow?
- Capillary blood pressure (Pc) = fluid/hydrostatic pressure exerted on the inside of the capillary walls by blood -> forces fluid OUT of capillaries
- Plasma colloid osmotic pressure (πp) = oncotic pressure is force caused by colloidal dispersion of plasma proteins -> forces fluid INTO capillaries
- Interstitial fluid hydrostatic pressure (Pif) = fluid pressure exerted on the outside of the capillary wall by interstitial fluid -> forces fluid INTO capillaries
- Interstitial fluid-colloid osmotic pressure (πif) = essentially zero but when plasma proteins pathologically leak into interstitial fluid (e.g histamine release) -> forced fluid OUT of capillaries
What is Reabsorption?
When inward-driving pressures exceed outward pressures across the capillary wall, net inward movement of fluid from the interstitial fluid into capillaries takes place through the pores
How do you calculate the Net exchange pressure?
Net exchange pressure = (Pc + πif) - (πp + Pif)
A positive net exchange pressure = ultrafiltration
A negative net exchange pressure = Reabsorption
What is the function of Bulk flow?
It regulates the distribution of ECF between plasma and interstitial fluid -> maintains the balancing of forces across capillary walls to maintain blood pressure homeostasis (so blood pressure is not too high or too low)
What is the function of the lymphatic system?
An accessory route by which fluid can be returned from the interstitial fluid to the blood
Describe how the arrangement of endothelial cells in an initial lymphatic function
1) Fluid pressure on the outside of the vessel pushes the endothelial cell’s free edge inward -> allowing the entrance of interstitial fluid
2) Fluid pressure on the inside of the vessel forces the overlapping edges together so that lymph cannot escape
Which mechanisms direct lymph from tissues towards the venous system in the thoracic cavity?
- lymph smooth muscles contract rhythmically due to MYOGENIC ACTIVITY + sympathetic nervous system stimulation
- contraction of muscles squeezes lymph out of vessels
What are the functions of the lymphatic system?
- Return of excess filtered fluid
- Defense against disease
- Transport of absorbed fat
- Return of filtered protein
What are the 4 causes of oedema?
- A reduced concentration of plasma proteins decreases
[allows excess fluid to filter out] - Increased permeability of the capillary walls
[allows more plasma protein to filter out -> water moves out by osmosis] - Increased venous pressure
[creates a “backlog” since less blood moves out of capillaries into veins] - Blockage of lymph vessels
[excess filtered fluid is retained in interstitial fluid than returned to blood]
What is Blood?
The vehicle for long-distance, mass transport of materials between the cells and external environment or between the cells themselves
What are Erythrocytes (RBCs)?
Plasma membrane-enclosed bags of haemoglobin that transport O2 in the blood
What are Leukocytes (WBCs)?
The immune system’s mobile defense units transported in the blood to sites of injury or of invasion by disease-causing microorganisms
What do the liquid plasma elements consist of?
- Erythrocytes (red blood cells)
- Leukocytes (white blood cells)
- Platelets (thrombocytes)
What are the functions of ions?
- Membrane excitability
- Osmotic distribution of fluid between the extracellular fluid (ECF) and the cells
- Buffering pH changes
What are the other elements that plasma consists of?
- Nutrients (glucose, amino acids, lipids and vitamins)
- Waste products (creatinine, bilirubin and urea)
- Dissolved gases
- Hormones
What are the functions of plasma proteins?
- Establish an osmotic gradient between the blood and the interstitial fluid
[primary force preventing excessive loss of plasma from capillaries into interstitial fluid -> helps maintain plasma volume] - Buffer changes in pH
- Albumins = contribute extensively to colloid osmotic pressure + bind poorly soluble substances
- Fibrinogen = blood clotting
Where are Plasma proteins synthesised?
Liver
[except of antibodies which are produced by lymphocytes]
What are the properties of the different types of globulins (alpha, beta and gamma)?
- Some alpha and beta globulins bind poorly water-soluble substances
- Many factors involved in blood-clotting are alpha and beta globulins
- Some plasma proteins are inactive, circulating precursor molecules -> activated by specific regulatory inputs
- Gamma globulins are antibodies/immunoglobulins
