Ch. 16 Day 2 Flashcards
Chemoreceptors
Automatic control of breathing influenced by feedback from chemoreceptors - monitor pH of fluids in brain and pH, PCO2, and PO2 in blood
- Central chemoreceptors in medulla
- Peripheral chemoreceptors in carotid and aorta arteries
Chemoreceptors in Medulla
Increased CO2 in fluids of brain decrease pH - sensed by chemoreceptors in medulla, –> increased ventilation
Senses CO2, not H+ which does not cross the BBB
Takes longer, but responsible for 70-80% of increased ventilation
CO2 readily diffuses
Peripheral Chemoreceptors
Aortic and carotid bodies respond to rise in H+ due to increased CO2 levels
–chemoreceptors monitor CO2, not O2
Respond faster than medullary chemoreceptors
Effect of Blood PO2 on Ventilation
Indirectly affects ventilation by affecting chemoreceptor sensitivity to PCO2
Low blood O2 makes carotid bodies more sensitive to CO2
Hypoxic Drive - carotid bodies respond directly to low oxygen dissolved in plasma (below 70mmHg)
Major control by chemoreceptors is achieved by monitoring ___, not O2.
CO2
Gas Exchange
Occurs at 3 levels
- Atmosphere and lung
- Lung and blood
- Blood and cells
Partial pressure oxygen changes with?
Altitude and location
Causes of Low Alveolar PO2
Inspired air has abnormally low oxygen content
–altitude is major factor influencing atmospheric oxygen - as altitude increases, TOTAL atmospheric pressure decreases
Alveolar ventilation is inadequate
- -Decreased lung compliance: e.g. fibrotic, restrictive pulmonary diseases, lack of surfactant
- -Increased airway resistance: narrowing/obstruction by mucus, bronchoconstriction
- -CNS depression: slows breathing rate, decreases depth of breathing (e.g. alcohol poisoning, drug overdose)
Once you decrease PO2 in the alveoli in the lungs, does everything else fall or rise?
Fall
Respiratory System Bulk Flow
Entire mixture of gases is moving
Flow from regions of higher to lower pressure
Muscular pump creates pressure gradients
Resistance to flow
Partial Pressure of Gases in Blood
Alveoli and blood capillaries quickly reach equilibrium for O2 and CO2
- a) this helps maximize the amount of gas dissolved in fluid
- b) the amount of gas that can dissolve in liquid depends on:
- -1) partial pressure of the gases - major determining factor
- -2) solubility of the gas in the liquid (constant)
- -3) temperature of the fluid (more gas can dissolve in cold liquid); for blood T = constant
Pulmonary Circulation - High Flow, Low Pressure System
R. ventricle –> pulmonary trunk –> pulmonary arteries –> lungs –> pulmonary veins –> L. atrium
BP is low, but resistance to flow is VERY low, so flow through pulmonary circulation is very high
Pulmonary blood flow is equal to cardiac output and is controlled by the factors regulating cardiac output
Important to match blood perfusion and ventilation in the lung
Pulmonary arterioles constrict when alveolar pO2 is low and dilate when pO2 is high
–blood flow to alveoli is increased when they are full of O2 and decreased when not (this is how we match profusion and ventilation)
Hypoxic Vasoconstriction
Local mechanism for regulating the distribution of pulmonary blood flow away from hypoxic alveoli
Normal Perfusion of Blood
Normal perfusion of blood past alveoli is matched to alveolar ventilation to maximize gas exchange
Ventilation-Perfusion Mismatch
Caused by under-ventilation alveoli
If ventilation decreases in a group of alveoli, PCO2 increases and PO2 decreases. Blood flowing past those alveoli does not get oxygenated.
Local Control Mechanisms to Try to Keep Ventilation and Perfusion Matched
ex: Hypoxic vasoconstriction
Decreased tissue PO2 around underventilated alveoli constricts their arterioles, diverting blood to better alveoli
- -low oxygen depolarizes smooth muscle cells of the arteriole wall by inhibiting outward flow of K+
- -opens voltage-gated Ca2+ channels, which stimulate contraction
- -the response of pulmonary arterioles to low oxygen levels makes sure that ventilation (O2 into lungs) matches perfusion (blood flow)
What is perfusion?
Blood flow
Diffusion and Solubility Influence Gas Exchange
Constants (under normal conditions):
- -surface area
- -membrane thickness
- -diffusion distance (occurs best over a small distance)
- -temperature
Concentration Gradient:
–primary factor affecting gas exchange
Exchange Surface of Alveoli
At basal heart rates, RBCs spend about 0.75 seconds in the pulmonary capillaries - 2-3 times the time needed to completely equilibrate w/ alveolar gases
Gas Solubility Affects Amount of Dissolved Gas that can be Carried by Plasma
Oxygen solubility is low –> very little oxygen can be carried dissolved in plasma
NOTE: even though PO2 is the same in both air and water, [O2} is NOT!
Concentration depends on the gas’s SOLUBILITY
Carbon dioxide is __ times more soluble in water than is oxygen.
20 times
____ transports most of the oxygen in the blood.
Hemoglobin
More than __% of the oxygen in blood is bound to hemoglobin in RBCs, and less than __% is dissolved in plasma.
98%; 2%
At rest, cells need?
50 mL O2/min
Not enough O2 to meet tissue need if all we could rely on was what’s soluble in the plasma
The amount of oxygen bound to hemoglobin (Hb) depends on?
- Plasma O2, which determines % saturation of Hb
- The amount of hemoglobin which determines total number of Hb binding sites calculated from (Hb content per RBC) * (Number of RBCs)
(% saturation of Hb) * ( total number of Hb binding sites)
PO2 determines oxygen-hemoglobin binding.
97% saturation; 75% saturation
–amount O2 unloaded to tissues…the remainder is a reserve which can be unloaded to tissues if need arises - e.g. during exercise so we can keep aerobic respiration instead of having to go to anaerobic respiration
Several factors influence oxygen-Hb binding (5).
- pH
- -as pH decreases, affinity decreases, and Hb gives up O2 (Bohr effect) - Temperature
- -as temp. increases, affinity for O2 decreases - PCO2
- -as PCO2 increases, affinity for O2 decreases - 2,3-DPG (2,3-diphosphoglycerate)
- -as 2,3-DPG increases (chronic hypoxia [anemia, high altitude]), affinity for O2 decreases - Changes in hemoglobin structure (fetal Hb)
- -has higher affinity for O2 than maternal Hb
Decreased pH, increased temperature, increased PCO2 decrease O2 bound to hemoglobin
CO2 Transport
Dissolved: 7%
Converted to bicarbonate ion: 70%
Bound to hemoglobin: 23%
- -hemoglobin also binds H+
- -Hb and CO2: carbaminohemoglobin
