Structure of Vessels
What are venules?
- Venules are the smallest subtype of veins. They connect directly to capillaries and carry blood to medium-sized veins, which carry blood to the large veins that return it to the heart.
What is the function of the capillaries?
Capillaries facilitate the exchange of nutrients and other solutes between the blood and the tissues
What are Starling forces?
At the level of microcirculation, Starling forces govern the movement of water and permeable solutes between capillaries and the surrounding tissue.
The net direction of flow is determined by the balance between hydrostatic pressure and osmotic pressure.
What are the 3 methods of local control of blood flow?
1. Metabolic regulation
2. Autoregulation (myogenic regulation)
3. Shear stress induced vasodilation
What are the three equations for cardiac output?
CO = HR x SV
CO = (MAP - CVP) / TPR
CO = HR x (EDV-ESV)
Name the 3 basic layers of a blood vessel, from outermost to innermost
3. Tunica adventitia (outermost)
2. Tunica media
3. Tunica intima (innermost)
What are the 3 types of capillaries?
1. Continuous capillaries -- complete endothelial lining (no gaps)
2. Fenestrated capillaries -- characterized by small pores in the endothelial lining
3. Sinusoidal capillaries -- have gaps between adjacent endothelial cells
Complete the sentence: at the arteriole end of a capillary bed, fluid/solutes move [INTO/OUT OF] the capillaries because CHP is [GREATER THAN/LESS THAN] BCOP.
At the arteriole end of a capillary bed, fluid/solutes move [INTO/OUT OF] the capillaries because CHP is [GREATER THAN/LESS THAN] BCOP.
Which type(s) of local regulation is/are endothelium-independent? Which type(s) is/are endothelium-dependent?
Endothelium-dependent: shear stress induced vasodilation, metabolic regulation
Endothelium-indepedent: myogenic regulation
(if anyone is confused about one or more of these processes, try to explain them to each other!)
Your patient has a heart rate of 75 beats per minute and ejects 60 mL of blood with each heartbeat. What is their cardiac output?
CO = 75 beats/min x 60 mL/beat = 4500 mL/min
Name 3 differences between arteries and veins.
Arteries have:
- Thick tunica media
- Internal and external elastic membranes present
- Round shape
- Smaller lumen
Veins have:
- Thinner tunica media
- NO internal/external elastic membranes
- 'Flattened' shape
- Larger lumen
Which substances can be exchanged through continuous capillaries?
Continuous capillaries permit diffusion of water, small solutes, and lipid-soluble materials; they do not allow transport of large plasma proteins or blood cells.
The movement of fluid out of capillaries and into tissues is called ____. The movement of fluid from the issue into capillaries is called ___.
Filtration; reabsorption
Transmural pressure most directly affects which type of blood-flow regulation?
Myogenic regulation!
--> Increased transmural pressure leads to vasoconstriction via the vascular smooth muscle of the tunica media
What is the difference between velocity and flow of blood?
- Velocity (v): rate of displacement of blood per unit time (cm/s)
- Flow (Q): volume of blood passing through a space per unit time (mL/sec)
1. Secretory and modulatory function: secrete substances that regulate the tone and growth of vascular smooth muscle and maintain platelet function
2. Metabolic function: involved in the production of vasoactive factors like ANGII and bradykinin
3. Plasticity: can undergo angiogenesis (production of new blood vessels) to create new capillaries in response to injury or ischemia
Which type(s) of capillaries is/are found in endocrine organs?
(Probably also some continuous capillaries)
What are the two pressures whose relative strengths govern the movement of nutrients/waste between capillaries and the surrounding tissue?
1. Capillary hydrostatic pressure (CHP) -- the pressure of fluid inside of the capillaries; pushes fluid/permeable solutes out of the capillaries, into the cells of the surrounding tissue
2. Blood colloid osmotic pressure (BCOP) -- the osmotic pressure of fluid moving into the capillaries to dilute the impermeable solutes in the vessels
What is myogenic regulation?
- Myogenic regulation is a means of local control of blood flow stimulated by increased transmural pressure
- When the vessel experiences a increased "outward push" from blood within the lumen, the tunica media contracts and "pushes back" to maintain a relatively stable BP
Explain the variables and relationships outlined by the equation Q = ∆P/R
Flow through a vessel = change in longitudinal pressure across the length of vessel ÷ resistance in the vessel
This tells us that:
--> Q and ∆P: the greater the difference in pressure between the start and end of a vessel, the greater the flow of blood down into concentration gradient
--> Q and R: the greater the resistance in a vessel, the smaller the flow of blood
Name 3 substances released by vascular endothelial cells, and describe their functions.
- Nitric oxide (NO) and prostacyclin (PGI2) -- vasodilators
- Endothelin -- vasoconstrictor
- Anti-aggregatory substances for platelets
- Anti-mitogenic signals for vascular smooth muscle
- Angiotensin-converting enzyme (ACE) -- used in the production of angiotensin II (a vasoconstrictor)
Name four places where fenestrated capillaries are found.
- Choroid plexus
- Endocrine organs
- Kidneys
- Intestinal tract
Within a portion of a capillary bed, you observe a CHP of 16 mmHg and a BCOP of 25 mmHg. Which way can you expect fluid to flow? At which end of the capillary bed can this region most likely be found?
Since blood colloid osmotic pressure (BCOP) > capillary hydrostatic pressure (CHP), we can expect fluid to flow into the capillaries. This corresponds to the end of the capillary bed closest to the venules.
What 2 factors contribute to vascular tone?
Vascular tone (the level of constriction/dilation in a vessel) is influenced by:
1. Myogenic tone -- intrinsic contraction level of and influences from of the vessel itself
2. Neurogenic and humoral factors: external factors like hormones, signals from the nervous system, etc
Which of the following correctly relates the following variables to describe cardiac output?
(a) CO + MAP = CVP
(b) CO = (CVP/TPR) + MAP
(c) CO = (MAP-CVP) / TPR
(c)
~ Make sure you know what all of the variables represent! If you have extra time (or if anyone in the group is confused), try explaining it to each other.