Physiology Study Guide for Week 3: Key Concepts and Definitions

What is the heart's pump rate (L/min)?

5L/min

Which side of the heart has higher pressure?

Left side (has to send blood throughout the entire body)

What is unique about capillaries compared to other blood vessels?

Do not have muscle, do not control vasodilation/constriction

Allow for nutrient/gas exchange here!

What makes veins unique in comparison to arteries?

Veins are larger, less muscular, and can store blood

Pressure also lower

Why does blood flow vary greatly between different organs/tissues?

Certain organs need continuous blood flow (ex. kidneys), while others have local mechanisms to control blood flow based on when they need it

What % of blood is the pulmonary circulation? What % is peripheral/systemic circulation?

9% pulmonary

84% systemic

Remaining % is blood going to the heart (in coronary arteries, etc.)

Describe pulmonary circulation.

Low pressure, low volume system

Systolic = 25 mmHg

Diastolic = 8 mmHg

Pulmonary capillary pressure = 7 mmHg

Describe systemic circulation.

Contains arteries, arterioles, capillaries, venules, and veins

Systolic = 120 mmHg

Diastolic = 80mmHg

Capillary pressure = 17 mmHg (will vary, dependent on tissues)

What is the significance of arterioles in the systemic circulation?

Arterioles are small, their diameter to resistance ratio is greatest --> can control BP the best

What does velocity of blood flow depend on?

Total cross-sectional area and the volume of blood flow

What is the velocity of aortic blood flow? Capillary blood flow? Why is this difference significant?

Aortic blood flow = 33 cm/sec

Capillary blood flow = 0.03 cm/sec

Aorta has less surface area --> will have faster velocity

Capillaries have larger surface area --> will have slower velocity, but this is good for nutrient exchange

Is pressure higher on the arterial end of capillaries, or the venous end?

Arterial end (35 mmHg vs 10 mmHg in venous end)

What is the MAP in the superior and inferior vena cava?

0 mmHg

Blood is continuously flowing and the IVC and SVC are not contracting so pressure will be 0

Why are veins 4x as big as arteries?

They also act as a storage reservoir for blood

What is the MAP in the aorta?

100 mmHg

High because the heart is continuously pumping blood into the aorta

How is blood flow controlled throughout the body?

By local tissue needs

Specifically, through microvessels that monitor and control dilation and constriction of local blood vessels to control blood flow

What is cardiac output?

The sum of all local tissue flow --> whatever flows in, has to flow out

How is arterial pressure regulated? How does arterial pressure relate to local flow and cardiac output control?

INDEPENDENT of local flow and CO control

Will rely on nervous reflex response to a fall in systemic BP (usually under 100 mmHg)

-Increased force of pumping

- Contraction of venous reservoirs to provide more blood to the heart

- Generalized constriction in arterioles so blood accumulates there

Over time, the kidneys will help regulate the blood volume by secreting pressure-controlling hormones

If you need to quickly increase blood pressure, what can you do to the cardiac output?

INCREASE it

What are the two determinants of blood flow?

1. Pressure difference between 2 ends of a vessel

2. Resistance (impediment to blood flow)

What is the equation that relates flow, pressure difference, and resistance?

F = deltaP / R

What causes resistance in blood vessels?

Friction between the flowing blood and the intravascular endothelium (vessel wall)

What is laminar flow?

Blood flows at a steady rate, characterized by smooth, parallel layers of fluid

Molecules that are closest to vessel wall will not travel as easily as molecules in the middle (due to resistance)

What is turbulent flow?

Fluid movement that is disorganized (think: white water rapids)

Characterized by blood flowing in all directions in the vessel and continuously mixing

What is the total blood flow in an average adult?

5000 mL/min (think CO is 5 L/min)

What is Reynold's number? What does it depend on?

The tendency for turbulent blood flow in a vessel

Directly depends on velocity, diameter, density, and is inversely proportional to viscosity

How does velocity change with respect to diameter?

Change in diameter = (change in velocity)^4

How do we determine the resistance of a vessel?

Calculated from: pressure difference / flow

What is the total peripheral resistance in the body?

1 PRU

Rate of blood flow in humans = 5L/min, which is roughly 100 mL/s

Pressure difference from arteries to veins is 100 mmHg --> 100/100 = 1 PRU

What is 1mmHg equivalent to in cm H2O?

1.36cm H2O

What is conductance?

Ease with which blood flows through a circulation at a given pressure difference

Equal to blood flow through a given vessel over given pressure difference

RECIPROCAL to resistance (if resistance is higher, conductance will be lower --> will be more difficult for the blood to flow)

How does conductance change with respect to a change in diameter?

C proportional to (diameter change)^4

This means a small increase in diameter could lead to a large change in conductance

What plays the greatest role in determining blood flow and conductance?

Diameter

What is Poiseuille's law?

Flow = [Pressure Difference x (pi radius^4)] / (8Length*Viscosity)

What is the main controller of systemic vascular resistance? Why?

Arterioles (smallest vessel besides capillary which do not control blood flow, a slight increase in the diameter has LARGE changes to blood flow in comparison to a larger vessel)

How does resistance to blood flow in series work?

R(total) = R1 + R2 + R3 + ...

All vessels run in series to one another (capillaries are parallel to one another), so the total peripheral resistance will be the sum of the individual resistances

How does resistance to blood flow in parallel work?

1/R(tot) = 1/R1 + 1/R2 + 1/R3 + ...

Total resistance will be less --> parallel flow allows for greater surface area which means there are more ways for the blood to flow, and thus, less resistance

How does hematocrit affect flow?

Increase in hematocrit increases the blood viscosity --> will decrease the flow

As viscosity increases, resistance will also increase, causing blood flow to decrease

What range does blood flow stay constant in? How does it do so?

70-175 mmHg

By autoregulation!

What is autoregulation?

Automatic adjustment of blood flow to each tissue in proportion to its requirements to maintain consistent blood flow during changes in blood pressure from 70-175 mmHg

DOES THIS THROUGH ADJUSTING RESISTANCE!!!

How does autoregulation maintain constant blood flow?

1. Increase in arterial BP causes increase in vascular resistance (to bring down flow to normal level)

2. Decrease in arterial BP causes decrease in vascular resistance (to bring flow back to normal)

What do vasoconstrictors do to blood flow, and what are some examples of them?

Decrease blood flow

Examples include norepinephrine, angiotensin II, vasopressor, and endothelin

What does sympathetic stimulation do to blood flow? How is local control implicated?

Sympathetic stimulation will induce vasoconstriction of vessels

Within minutes, local tissues will take over and restore blood flow back to its original level

What is the major factor in vascular resistance?

Vessel diameter

What is vascular distensibility?

The ability of a blood vessel to distend (stretch)

How well blood vessels can accommodate for pulsatile blood flow --> provides smooth continuous blood flow

Which are more distensible: veins or arteries?

Veins (by 8x!!)

Why? Because they need to store blood

What happens to the elasticity of our blood vessels as we age?

Reduces!

The pulse pressure (difference between systolic and diastolic pressures) will increase

What is vascular compliance?

How much blood can be stored for each 1 mmHg

Compliance = distensibility x volume

Compliance is trying to keep your pressure at baseline

What is delayed compliance and how does it work?

Sharp increase in blood volume will cause sharp increase in pressure

Delayed stretching of the smooth muscle in the vessel wall allows pressure to return to normal levels over time --> the new blood has been accommodated for

What is an aortic incisura?

Closing of the aortic valve

What is the pulse pressure?

Systolic pressure - diastolic pressure

(usually 40 mmHg)

What are 2 factors affecting pulse pressure?

1. Stroke volume

2. Arterial compliance

How does stroke volume affect pulse pressure?

Greater SV, greater amount of blood that must be accommodated for -->

Greater rise of systolic (high pressure before distension) and greater fall of diastolic (low pressure after distension) --> greater pulse pressure

Describe the pressure contour of arteriosclerosis.

Arteries have stiffened --> non compliant

Pulse pressure will rise because systolic pressure would rise due to the greater resistance

Describe the pressure contour of aortic stenosis.

Diameter of aortic valve opening has reduced significantly --> aortic pulse pressure will decrease significantly due to diminished blood flow through the stenotic valve

Describe the pressure contour of patent ductus arteriosus.

Patent ductus arteriosus = failure of the ductus arteriosus to close after birth

50% or more of blood pumped into the aorta by the left ventricle will flow immediately backwards through the open ductus

This allows diastolic pressure to fall very low --> increases pulse pressure

Describe the pressure contour of aortic regurgitation.

Aortic valve is absent or does not close completely

After each heartbeat, the blood that has just been pumped into the aorta flows immediately back into the left ventricle --> aortic pressure will fall to 0 between beats.

No incisura because no aortic valve to close

What is dampening of pressure pulses and where do we see it in transmission of pressure pulses?

Very little variation between systole and diastole, we see it in arterioles and capillaries

What is the clinical method of measuring BP and how does it work?

Auscultatory method where we listen to Korokoff sounds

BP cuff occludes the vessels --> causes turbulence, which is what we hear

Why is mean arterial BP not average?

We do not spend equal time in systole and diastole. Roughly 40% in systole and 60% in diastole

What is the equation for mean arterial blood pressure?

MAP = systolic(0.4) + diastolic(0.6)

Why is right atrial pressure considered the central venous pressure?

Blood from all systemic veins flows into the right atrium

What is the normal central venous pressure?

0 mmHg

What regulates central venous pressure?

1. Pumping ability (ability of heart to pump blood out of R atrium and ventricle into lungs)

2. Blood flow from the periphery (tendency of blood to flow from peripheral veins to right atrium)

Why is venous pressure close to 0 near the heart when a person is standing?

The heart is pumping any blood that accumulates in the right atrium OUT as soon as it gets there --> no pressure

What is the value of venous resistance (theoretically)? And what is it actually?

Theoretical = close to 0 (large veins usually have no resistance to blood flow)

Actual = 4-6 mmHg (due to compression by surrounding tissues, will be occluded and cause resistance to blood flow)

How does gravitational (hydrostatic) pressure affect venous pressure?

Gravitational pressure causes the pressure in the legs to be +90 mmHg (due to the gravitational weight of the blood between the heart and feet)

Though theoretically the venous pressure in the legs should be +90 mmHg, it rarely ever is. It usually is around +20 mmHg when not stationary. Why?

In a walking adult, every time the legs move the muscles tighten and compress the veins, causing blood to be propelled forward (pressure will not build up as much)

Veins have one way valves --> when blood is propelled it will move forward towards the heart

How quickly can venous pressure in the legs rise to +90mmHg if a person is standing completely still?

Within 30 s

What would happen if the venous pressure in the legs was actually +90 mmHg?

Pressure in the capillaries would also rise, signaling fluid to leak from the circulatory system into the tissue spaces

Causes:

- Leg swelling

- Decrease in blood volume

- Possible fainting (when enough blood pools and is not going back to the right side, blood won't get to the brain --> syncope)

What are varicose veins?

Bulging veins, usually as a result of faulty venous valves

Can cause buildup of fluids

What are some organs that can act as blood reservoirs?

- Venous system

- Spleen

- Liver

- Large abdominal veins

- Venous plexus beneath skin

How does the liver act as a blood reservoir?

Spleen filters blood --> will trap RBCs in the venous sinuses/pulp until they are needed --> will be expelled when the sympathetic nervous system is activated

Can raise hematocrit by 1-2%

What is the nervous system compensation if blood is lost?

Nervous system will be activated to return blood from the reservoirs to restore lost blood and raise BP

Which factor will NOT increase right atrial pressure?

a. Increased blood volume

b. Increased large vessel tone

c. Hemorrhage

d. Dilation of arterioles

Hemorrhage

What is microcirculation?

Determines how much blood is supplied to any given tissue, main purpose is for the diffusion of substances

Transport of nutrients to tissues and removal of wastes from tissues

How do small arterioles control blood flow to the tissues?

Local conditions will control the diameters of the arterioles (will cause them to dilate or constrict as needed by the local tissue to increase/decrease blood flow)

What is the precapillary sphincter?

Opens and closes the entrance into the capillary, controls the flow through tissues

What is the significance of having slightly more capillaries on the venous end of the capillary bed?

Contributes to a decrease of capillary pressure on the venous side, drop in P allows for pressure difference across capillary bed --> flow and exchange of nutrients

Describe the structure of a capillary wall.

Unicellular layer of endothelial cells, basement membrane and intercellular clefts

What is the significance of the intercellular cleft in the capillary wall?

Allows for free diffusion of water, water soluble ions will also be able to pass through

What are caveloae?

Located in the endothelial cell, are vesicles whose proposed function is endocytosis and transcytosis of macromolecules across the interior of endothelial cells

May form vesicular channels

What are examples of some special "pores" in various organs?

- Tight junctions in the brain (allow extremely small molecules to pass in and out of brain tissue)

- Clefts in the liver (allows all dissolved substances in the plasma, including plasma proteins, to pass through)

- Pores of GI membranes

- Fenestrae of glomerulus (allows tremendous amounts of small molecular & ionic substances to be filtered across glomerulus)

Why are the capillaries in the kidneys different?

Are larger to allow for protein exchange across the kidney

Describe the mechanism of intermittent blood flow through the capillaries.

1. Precapillary sphincter intermittently contracts/dilate (supplying a continuous flow of nutrients to tissues since there are so many capillaries)

2. Oxygen concentration (when rate of O2 usage by tissues is high, tissue O2 concentration will decrease --> intermittent period of capillary flow will increase to bring more O2 to the tissues)

How are substances exchanged between the blood and interstitial fluid?

- Lipid soluble substances (O2 and CO2) can go directly through cell to the tissues, do not need pores

- H2O soluble substances (NaCl, glucose) go through gaps between the cells (intercellular clefts, pores)

Size and concentration difference very important for diffusion!

What is interstitium?

Semisolid structure that contains collagen fibers (to provide tensional strength of tissues) and proteoglycan filaments (to form fibrous mat of interstitial fluid)

Also has gel component that is fluid between the proteoglycan filaments, and free fluid

What are free fluids in the interstitium?

Free of the proteoglycan filaments and can flow freely (similar to plasma, but low plasma protein concentration because these cannot diffuse easily through pores)

1. Rivulets

2. Vesicles

What are the 4 pressures that drive fluid filtration in the capillaries?

1. Capillary hydrostatic pressure (17.3 mmHg)

2. Interstitial fluid hydrostatic pressure (depends on tissue, usually negative)

3. Plasma colloid osmotic pressure (28 mmHg)

4. Interstitial fluid colloid osmotic pressure (8 mmHg)

What is capillary hydrostatic pressure?

Forces fluid outward through the capillary membrane

What is interstitial fluid hydrostatic pressure?

Usually negative value so will cause increased filtration OUT of the capillary

What is plasma colloid osmotic pressure?

By osmosis, water will move towards the direction of more particles (concentration is higher inside capillary than in the interstitium) --> water will flow inwards towards the capillary

What is interstitial fluid colloid osmotic pressure?

Tends to cause osmosis of fluid OUT through capillary membrane

What is the equation for net filtration pressure?

NFP = Pc - Pif - pip + piif

Pif will usually be negative value so the minus will really be a plus

If net filtration pressure is positive, then:

Net fluid filtration is out of the capillaries

If net filtration pressure is negative, then:

Net fluid filtration is going in the direction from the interstitium into the capillaries

On the arterial end of the capillary, are the forces driving filtration out or into the capillary?

OUT!

Total outward force = 41

Total inward force = 28

Net outward force = 13

On the venous end of the capillary, are the forces driving filtration out or into the capillary?

INTO

Total outward force = 21

Total inward force = 28

Net outward force = -7

What is the net outward force at the capillaries? Why?

0.3 mmHg (means slightly more filtration than reabsorption in the capillaries)

Allows for 2mL/min of net filtration throughout the body

What would happen if the net filtration throughout the body was higher than 2 mL/min?

Can cause edema (lymphatic system will accommodate)