L21 Special Circulation: Cerebral & Pulmonary

Question 1

Anatomy of Cerebral circulation + BBB

Answer 1

Receives a steady BF of about 15% of CO

NO LYMPHATICS

BBB limits what gets into the brain cells - H+ cannot pass – CO2, O2, glucose, steroids, ethanol can pass

Question 2

Autoregulation effects on BF

Answer 2

Maintains constant BF to brain regardless of change in BP. But, active areas of the brain get more BF than non-active areas.

Cerebral Perfusion Pressure = Mean Arterial Pressure - Intracranial Pressure

MAP = how hard the blood is pushing to get out

ICP = how hard the brain is pushing on the vessels.

CPP = a balance of the force pushing blood into the brain and the force pushing it out

Can also be seen as the force from the brain tissue onto the vessels

If CPP drops, there is less pressure on the vessels and they dilate. If it goes up, the vessels constrict

Question 3

Cushing’s Response

Answer 3

Accident and head trauma –> turn on Symp activity.

Thus increased BP

Elevation of ICP due to head trauma turns on Parasymp and decreases the HR

HIGH BP - LOW HR - probable cerebral ischemia

Question 4

Metabolism effects on BF

(The main regulator of Cerebral Circulation)

Answer 4

Cerebral BF is sensitive to PCO2.

CO₂ can cross the BBB. When it’s in the brain tissue, it undergoes metabolic change to H+, which cannot cross the BBB. The H+ is a metabolite and causes vasodilation.

• Hypoventilate* = Increase CO₂= more acidodic = more vasodilation = INCREASE BF
• Hyperventilate *= decrease CO2 = more alkalotic = less vasodilation = DECREASE BF (like for edema)

The brain is not sensitive to metabolic acidosis. Only respiratory acidosis, which works via the CO2 pathway

O₂

Brain isn’t as sensitive to oxygen; more oxygen gets to regions that are more active. Lack of oxygen (therefore lack of BF) may cause metabolite buildup and therefore vasodilation

Adenosine, K⁺, and NO

These are all **vasodilators. **NO –> increase cGMP –> increase PKG –> increase P of MLCK –> turns kinase off, therefore inactivating the myosin –> vasodilation, no contraction

Question 5

Neural effects on BF

Answer 5

Neural effects are very low.

Symp: minimal constriction

Parasymp: minimal vasodilation

Question 6

Tissue Pressure effects on BF

Answer 6

Since the brain is inside a rigid cranium, the tissue pressure has a big effect. Any increase in tissue pressure causes compression of vessels, and BF takes an alternate route. Can lead to ischemia.

Reduction in CPP can be from reduced MAP (force pushing blood into brain - like hypovolumic shock) or from increasing ICP (force that resists blood from entering the cerebral vessels - like tumor, hematoma, hydrocephalus - these all create an increase in P in the cranium, which means less blood can get it)

The Munro-Kellie Doctrine highlights this - what goes in must come out. Any volume increase inside means something else inside has to decrease.

Increase in CSF P = more Resistance = LESS BF – this would induce vasodilation to maintain BF. If CSF gets TOO HIGH, towards the MAP, then no amount of vasodilation can help relieve the effects

Question 7

General anatomy and pressures within Pulmonary Circulation

Answer 7

Pulmonary arteries are 7 TIMES more compliant than systemic – due to minimal smooth muscle and low P

Pressure- LOW

Resistance - LOW

BF - HIGH

Mean pulmonary arterial P (P that pushes blood into lungs from RV) = 14mmHg

Left atrial P (P that sucks oxygenated blood back into the LA) = 8 mmHg

—- Therefore, 6mmHg pulls blood in the correct direction through the lungs

Capillaries surround the alveolar walls - they thus control the vascular resistance

EXERCISE: you recruit more capillaries, which decreases the R and increases BF and P

Question 8

Respiratory effects on BF

Answer 8

When you inhale, the alveoli expand and this squeezes the alveolar vessels shut (more R), while expanding the extra-alveolar vessels (less R).

The opposite happens with exhalation

Normal respiration - these changes don’t have that much of an effect on pulmonary resistance and BF

Forced respiration, intense exercise, etc. -

inspiration = increased resistance in alveolar

expiration = increased resistance in extra-alveolar all due to changes in pressure

Question 9

Gravitational and “Waterfall” effects on BF

Answer 9

Bottom of the lung experiences more hydrostatic P, therefore results in more BF, and more O₂ exchange

Generally zones 2 and 3 of the lung are present under normal conditions.