14 - Special Circulations Flashcards
What are the blood supplies to the lungs?
- Bronchial: part of systemic circulation that meets the metabolic requirements of the lungs
- Pulmonary: blood supply to alveoli for gas exchange
What are the pressures in each part of the heart?
What are the features of the pulmonary circulation?
- Low pressure: MAP 12-15, MCP 9-12, MVP 5
- Low resistance: short wide vessels, lots of capillaries, little smooth muscle in arterioles
How are the lungs adapted for efficient gas exchange?
- High capillary density in alveoli so high SA
- Short diffusion distance
What is the venitlation/perfusion ratio?
- For effective oxygenation need to match ventilation of alveoli with perfusion
- Optimal V/Q = 0.8
- Ventilation/perfusion mismatch if unbalanced
How does the pulmonary system ensure optimal ventilation/perfusion ratio when ventilation falls?
- Hypoxic pulmonary vasoconstriction when alveolus hypoxia
- Poorly ventilated alveoli are less perfused to maintain optimal elsewhere
What happens during chronic pulmonary vasoconstriction and when can this condition occur?
- Right ventricular failure
- Increase in vascular tone so increase in vascular resistance leading to chronic pulmonary hypertension
- RV hypertrophy as increased afterload, leaded to RV faiure
- Occur at altitude or due to lung disease, e.g emphysema
What is the appearance of the capillaries in each part of the lung at rest?
How do the pulmonary vessels match the increased oxygen demand during exercise?
- Increased CO and increased pulmonary arterial pressure
- Opens apical capillaries so increased O2 uptake
- Blood flow increase so capillary transit time reduced from 1s to 0.3s without compromising gas exchange
What influences hydrostatic pressure in capillaries?
- Mainly venous pressure, especially in systemic circulation
- Therefore, if venous pressure increases so will hydrostatic in the capillary
What is the rate of formation of tissue fluid in the lungs?
- Hydrostatic pressure in capillaries is low
- Small amount of tissue fluid
How can oedema occur?
- Increase in capillary hydrostatic pressure, due to increased venouse pressure
How do the lungs prevent pulmonary oedema, and what conditions is pulmonary oedema likely?
- Pulmonary capillary pressure normally low (9-12) but can get oedema if left atrium rises to 20-25
- Mitral valve stenosis
- Left ventricular failure
Where in the lungs does pulmonary oedema normally occur and how do you treat it?
- Normally at base when upright, all over when lying down due to changes in hydrostatic pressure in gravity
- Use diuretics
- Treat underlying condition
How much of the O2 demand from the body does the brain hold?
- Recieves 15% CO and grey matter O2 consumption accounts for 20% of total body consumption at rest
How does the cerebal circulation meet the high demand for oxygen?
- High capillary density: SA and reduced distance
- High basal flow rate
- High O2 extraction (35% more as using lots of O2 so high conc gradient)
When do neurones become irreversibly damaged?
- After 4 minutes of hypoxia neurones undergo death
- Loss of consciousness in seconds
How is a secure blood supply to the brain established?
- Anastomoses between basilar and internal carotid arteries to form circle of Willis
- Myogenic autoregulation in hypotension
- Metabolic factors control blood flow
- Brainstem regulating other circulations
What is myogenic autoregulation in the brain?
- Cerebal resistance vessels response to changes in transmural pressure
- BP increade vasoconstriction
- BP decrease vasodilation to increase blood flow
How does metabolic regulation maintain cerebral blood flow in terms of a gas produced?
- Allows blood flow to be directed to regions with high metabolic activity
How can panic hyperventilation cause syncope?
- Hypocapnia
- Cerebral vessels constrict restricting O2 supply to blood vessels
What other metabolites can cause increase in blood flow to an area in the brain?
Increased:
- PCO2
- K+
- Adenosine (powerful dilator of cerebral arterioles)
Decreased:
- O2
How is Cushing’s reflex an example of the brain stem altering other circulations to allow cerebral blood flow to continue occuring?
- Increase in intracranial blood pressure impairs cerebral blood flow
- Decreased blood flow to vasomotor control regions (medulla oblongata) increases sympathetic vasomotor activity so vasoconstriction of peripheries
- Increased arterial B.P to maintain cerebral blood bflow
When does blood flow through the left coronary artery during the cardiac cycle?
- Normally during diastole as the in systolecontraction from the heart constricts the coronary vessels
- During diastole cardiac muscle relaxes enough to allow blood to flow through capillaries with no obstruction
What are the differences in the perfusion of cardiac and skeletal muscle and why?
- Cardiac needs more oxygen demand
- Continous production of NO in the coronary endothelium maintains high basal flow
How does the coronary blood flow meet the increase demand in oxygen by the myocardium in exercise?
- Vasodilation due to metabolic hyperaemia, adenosine, increased K+ and decreased pH
- Increases blood flow
Why are coronary arteries prone to narrowing and what are some factors that can narrow them?
- Functional end arteries, no anastomes
- Atheromas, stress and cold and thrombus can all cause narrowing of coronary arteries
Why would someone with a narrowed coronary artery experience angina on exercise?
- Increased O2 demand for body so heart rate increases
- Diastole gets shorter
- Increased O2 demand to heart but less time for coronary arteries to fill so ischemia occurring
What are the feautres of skeletal muscle circulation?
- Capillary density depends on muscle type, e.g higher in postural muscles
- High vascular done so lots of dilation
- Only half capillaries perfused at rest and pre-capillary sphincters open in exercise to perfuse
When the oxygen demand to the skeletal muscles increases, how does it’s circulation accomodate for this?
What is the main purposes of the cutaneous circulation?
- Temperature regulation
- Maintaining blood pressure
SKIN NOT VERY METABOLICALLY ACTIVE
What are artereovenous anastomoses (AVAs) and what are their purposes?
- In apical skin (high SA:V ratio)
- Under sympathetic vasoconstrictor control
- Decreased core temperature increases sympathetic tone in AVA so less blood flow to skin for shorter amount of time
- Increased core temperature opens AVAs by vasodilation so blood diverted to veins near surface
JORDANS GENIUS
