Physiology #19: Respiratory Physiology #5
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What determines the partial pressures of oxygen and carbon dioxide?
- PO2 and PCO2 is determined by:
- blood flow to alveoliNOT by alveolar ventilation
What is the relationship between ventilation and perfusion?
- Ventilation/perfusion ratio
- need to be balanced for a proper working system
- Consider relative ratios: how much ventilation is there relative to perfusion?
When the V/Q is normal what happens?
- V/Q = 1
- balance between ventilation and perfusion such that optimal gas exchange is occurring
- * and equal V of air is meeting and equal V of blood over a period of time in the alveoli
- *O2 and CO2 are being exchanged as expected
What are the terms for deviation from the normal V/Q ratio?
- "V/Q mismatch" or "V/Q inequality"
- doesn't explain where the inequality is, just that there is one
What does a V/Q over 0 mean?
- for v/Q to be zero there must be no ventilation even though perfusion is continuous
- when there is no fresh air coming into a part of the lung, there can be no gas exchange
- Blood perfusing that part of the lung will return to the heart without releasing CO2 and picking up O2
- Shunting happens
There might be no perfusion to a lung if....
- there is
- a foreign body
- thickening of airway: through pneumonia or inflammation
- small airway collapse from mechanical compression
- atelectasis (absorption of O2, so pul venous blood flow without O2)
absorption of O2, so pul venous blood flow without O2
- Occurs when V/Q is 0
- blood is being shunted from the right side of circulation to the left side without any gas exchange occurring
what is "hypoxic pulmonary vasoconstriction"
Alveolar hypoxia (PAO2 < 70mmHg ie low alveolar P) induces constriction in the arterioles that supply the hypoxic alveoli
Response is activated by oxygen levels in the alveolus itself (not the blood returning to the heart) - occurs even in denervated or isolated lungs
- nitric oxide may be involved as mediator
- The net effect is to divert blood away from poorly ventilated areas of the lungs (as a result the shunt effect is MINIMISED and the PaO2 is maximized)
what happens when you climb a mountain
- too much shunting
- get edema because there is too much alveolar pressure so fluid gets pushed out
What happens when V/Q = infinity
- There is no perfusion even though ventilation continues
- when there is no new blood coming into a part of the lung there is no gas exchange
- air that is being moved in and out of the lung through normal ventilation will no pick up CO2 from the blood and its O2 will no be absorbed
- this is referred to as "dead space ventilation" sice a V of gas is being moved in an out of the body without any gas exchange occurring
why might there be no perfusion (v/q = 0)
- blocked blood vessels
- blood clot
- profound hypotension (low BP)
"Dead space ventilation"
- V/Q = infinity
- a V of gas is being moved in and out without any gas exchange occurring
- physiological dead space ie resp zone
- no perfusion
Explain what V/Q > or < 1 means
- V/Q less than 1 means ventilation is lower, but perfusion is still adequate
- V/Q greater than 1 means perfusion is lower, but ventilation is still adequate
- both extremes and everything inbetween are normally occurring all the time in different part of normal, healthy lungs ie due to gravitational effects of body position
V/Q zones in the lung
- Dorsal >1 more ventilation air is lighter than blood
- Ventral <1 more perfusion blood pools due to gravity
- Use concepts of zones
describe the zones of the lungs via blood flow distribution
- Zone 1 (at apex): low blood flow, dead space ventilation, PA>Pa>Pv
- Zone 2 (middle): medium blood flow (perfusion), Pa>PA>Pv )
- zone 3 (at base): highest blod flow (perfusion too much), Pa>Pv>PA, shunt blood
What size of animal experiences more gravitational effects?
- Large animals
- more V/Q mismatch
- exponentially important
How can you deal with gravitational effects in large animals? ie V/Q mismatch
- try to get lungs and chest wall functioning as a unit again (drain air or fluid, fix diaphragmatic hernia etc..)
- Provide supplemental O2 to improve oxygenation ie during 'field' conditions with injectables
- support cardiovascular systems with fluids, drugs: imrove bp and cardiac output to 'drive' blood into under perfused areas
- ventilate the patient if it is not breathing on own
- consider changing body positions (if possible)
- consider use of inhaled bronchodilators
What does A-a tell you?
O2 gas exchange if it increases then this is getting worse
What is the cardiovascular systems primary function?
- deliver blood to tissues provide essential nutrients to the cells for metabolism
- remove waste products from the cells
- Heart: a pump that generates P to drive blood through a series of vessels
- Vessels: are conduits that carry blood from the heart to the tissues of the body
Hemostatic function the CV system
- delivers endocrine hormones from glands to tissues
- adjustments to altered physiological states such as hemorrhage, exercise, postural changes
- The rate at which blood is pumped from either ventricle
- two sides of the heart operate in series so cardiac output of the left heart = cardiac output of the right heart
- cardiac output = HR x stroke V cardiac output is simultaneously distributed to the body by a parallel system of arteries
- The rate at which blood returns to the heart is called "venous return"
- -venous return to L and R sides is equal (if not blood is backing up somewhere)
In the CV what happens in a steady state
Cardiac output = venous return
How much % of cardiac output goes to the:
Liver and GIT
- Liver and GIT: 25%
- Kidneys: 25%
- skeletal muscle: 25%
- brain: 15% (LOTS compared to weight)
- heart: 5%
- skin: 5%
T or F: % of cardiac output to various organs is fixed
- F! % of cardiac output to various organs is not fixedIf CO is constant can adjust flow to an rgan by altering resistance (blood flow to one organ is increased at the expense of another)
- If % distribution to each organ is held constant, can adjust flow to all organs by increasing overall CO (blood flow to all organs is increased) ie exercise increases cardiac output
Both: can incease CO as well as alter % distribution to organs by altering resistance to flow (ie working really hard)
How do vessels change blood flow to different organs
by constricting or dilating vessels change resistance to blood flow through the system, regulating flow to different organs
Which blood vessel has the most elastic tissue?
which blood vessel does not have smooth muscle or elastic tissue?
- Arteries function is to transport oxygenated blood to the body have thick walls with extensive development of elastic tissue, smooth muscle, connective tissue
- receive blood under high P from the heart
- blood flows rapidly through their lumens
- The smallest branches of the arterial system
- have diameters less than 200 mu
- site of highest resistance to blood flow lots of smooth muscle
- (allows for alterations in resistance to flow)
- innervated by adrenergic nerve fibers
- Alpha1 (most organs)= constriction and Beta2 (skeletal muscle)= dilation receptor activation = constriction or dilation
- so controlled by sympathetic tone
what vessel has the highest resistance to blood flow?
- Thin-walled structures with a single layer of endothelial cells
- Site where gases, water, solutes (ions), and nutrients are exchanged between blood and tissues.
- Not all capillaries are perfused at all times: selective perfusion is determined by the tone on arterioles and pre-capillary sphincters (smooth muscle bands that lie "before" capillaries)
Venules and Veins
- drain capillary beds and carry blood black to heart
- Function is to collect blood from capillaries and carry it back to the heart
- Thin-walled structures that are composed of elastic tissue, smooth muscle, and connective tissue
- less elastic tissue in walls than in arteries (greater capacity to hold blood)
HUGE compliance: change in V over a large change in P
Division of body's total blood V to the systemic and pulmonary circulation
Systemic circulation carries ~80% of the body's total blood V at any one time: ~20% is in the pulmonary circulation and the heart
How much of the blood V is in the venous system?
So what would you do if your body demands more circulation?
- 2/3 of blood V is in the venous system
- ie sp of need to mobilize blood (ie hmorraging) constrict the veins to push this V of blood back into circulation to meet body demand
What is veins special feature?
- veins have "valves" that encourage one-way flow back to the heart
- Movement and skeletal muscle contraction compresses the veins so blood is directed towards the heart
- veins lie between 2 muscle bellies as the skeletal muscles contract they shorten and squeeze on the vein (valves prevent backup) and blood gets pumped up
- viens have huge compliance, a small change in P get a huge change in V
- arteries can not stretch very much so increased P just pushes blood through and doesn't dilate or stretch
What is considered under hemodynamics
- Blood Pressure
- Blood Flow
*What 2 factors determine blood flow
- 1. Pressure difference between the two ends of the vessel
- 2. Resistance of the vessel to blood flowPressure difference is the driving force of blood flow
- Resistance is the impediment to blood flow
- Blood flow = Pressure difference / resistance
- (P difference = Pstat - Pend of tissue bed)
and relationship to resistance
****what is the major mechanism for altering changing blood flow
- Speed of blood flow is proportional to the maginitude of the pressure difference (bigger difference results in more flow)
- Direction of blood flow is determined by the direction of the pressure gradient (always from high to low P*)
- Blood flow is inversely proportional to resistance (ie vasoconstriction of a vessel will decrease flow to the area downstream)
- ****Altering resistance in blood vessels is the major mechanism for changing blood flow in the CV system ************
Total peripheral resistance
how to measure
total peripheral resistance: aka "systemic vascular resistance" (SVR)
Is not easily measured: but can calculate by measuring BP and cardiac output (flow through the heart)
- SVR = (MAP -CVP) / Cardiac Output MAP- mean arterial pressure
- CVP- central venous pressure (in right atrium)
Normal Arterial Blood Pressure
- Heart ejects blood into the aorta such that blood pressure fluctuates between ~80 and ~120 mmHg
- -diastolic (expansion) 80
- -systolic (contraction) 120
- Have systolic arterial pressure (SAP), diastolic arterial P (DAP), and mean arterial pressure (MAP)
Systolic arterial Pressure
vs Diastorlic arterial P
vs Mean AP
- SAP- systolic arterial P: HIGHEST arterial pressure measured during a cardiac cycle (during ventricular contraction when blood is being ejected from heart)
- DAP- diastolic arterial P: LOWEST arterial P measured during a cardiac cycle (during ventricular relaxation when NO blood is being ejected from the heart)
- SAP - DAP = pulse pressure
- MAP- mean arterial P: average pressure during a complete cardiac cycle
Where does the driving F for blood flow come from?
Driving F for blood flow in the arteries comes from the ARTERIES
CALCULATION FOR MAP
MAP = DAP + 1/3pulse pressure
Does the heart spend more time in diastolic or systolic
Pulse Pressure is affected by
- Pulse pressure is affected by:
- -left ventricular stroke V
- -velocity of blood flow
- -compliance of the arterial system
- The greater the volume ejected by the heart (stroke V) the greater the V of blood that muse be accommodated in the arteries with each contraction: As a results pulse pressure will increase!
Of the vessels which has the most P and when does resistance increase?
- arteries = highest P
- Arterioles = greatest resistance so R increases and P decreases
Central Venous Pressure
- (Pulmonary) CVP
- ability of right ventricle to pump the lungs
- Right atrial P is affected by venous return and the ability of the right ventricle to eject blood into the pulmonary circulation
- Right Atrial P is commonly referred to as "central venous pressure" and is measured by advancing a catheter from a peripheral vein (ie jugular vein) to an intrathoracic location (into the chest)
- *Ideal location is the level of the tricuspid valve*
- Normal CVP ranges between +5 to -5 mmHg
What does inhaling and exhaling have to do with the amount of blood returned to the heart?
- Breath in: intrathoracic P decreases and enhances blood return to the heart
- Breath out: intrathoracic P increases and decreases the blood return to the heart
What is CVP used for?
- ***USED to balance fluid requirements with performance of the heart***
- CVP of 0 is normal
How do you use CVP to balance fluid requirements with performance of the heart ?
- -especially useful when expanding the blood V of a hypovolemic patient with heart failure (use CVP)
- CVP is LOW in patients with decreased venous return (eg blood loss, fluid deficits from vomiting, diahhrea) - get CVP -5 to -7
- CVP is HIGH in patients with heart failure (cant pump blood forward like it should) - get CVP 10
- CVP is commonly measured in critical care, emergency, anesthesia, and cardiac patients
- ******measuring venous return to help decide fluid requirements******ALWAYS DO THIS DO IT DO IT DO IT
Blood flow through the various types of vessels
- High flow/speed in arteries
- slow flow in capillaries*: together they have an enormous cross section - want to slow flow down here so that there is lots of time for RBCs and O2 and nutrients to get exchanged ****
- speeds up again when goes into veins (but arteries is still fastest)
Peripheral Venous Pressure (PVP)
- Large veins offer little resistance to flow when they are distended
- Most veins are compressed at multiple sites (through muscles) : venous P is so low that atmospheric P on the outside of the body causes them to collapse
Why is there no detectable pulse pressure in veins?
there is no detectable pulse pressure in veins since they have such high compliance : the more compliant the vessel the more V that can be added to it without causing an increase in P ***************
How does body position affect P in the veins?
- Pressure is higher in veins below the heart and is lower (or even negative) in the veins above the heart
- DUE TO EFFECT OF GRAVITY
- as a result, negative P can exist in large veins above the heart and air can be entrained into circulation during surgery or catherization
What is an "air embolus"
- air from the catheter goes into vein and then to the heart -- makes air bubble -- get VQ mismatch
- heart doesn't like bubbly blood can go into cardiac arrest
- Iv set to run FLUIDS through not air
Regulation of BP
- Maintained over a normal range by changes in cardiac output and systemic vascular resistance
- autonomic nervous system and baroreceptors play a role in this moment-to-moment regulation of BP
- Long term regulation of BP depends on the control of fluid balance by the kidneys, adrenal cortex, and CNS to maintain a constant blood V
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