14 - Special Circulations

Question 1

What are the blood supplies to the lungs?

Answer 1

- Bronchial: part of systemic circulation that meets the metabolic requirements of the lungs

- Pulmonary: blood supply to alveoli for gas exchange

Question 2

What are the pressures in each part of the heart?

Answer 2

Question 3

What are the features of the pulmonary circulation?

Answer 3

- Low pressure: MAP 12-15, MCP 9-12, MVP 5

- Low resistance: short wide vessels, lots of capillaries, little smooth muscle in arterioles

Question 4

How are the lungs adapted for efficient gas exchange?

Answer 4

• High capillary density in alveoli so high SA
• Short diffusion distance

Question 5

What is the venitlation/perfusion ratio?

Answer 5

• For effective oxygenation need to match ventilation of alveoli with perfusion

- Optimal V/Q = 0.8

• Ventilation/perfusion mismatch if unbalanced

Question 6

How does the pulmonary system ensure optimal ventilation/perfusion ratio when ventilation falls?

Answer 6

- Hypoxic pulmonary vasoconstriction when alveolus hypoxia

• Poorly ventilated alveoli are less perfused to maintain optimal elsewhere

Question 7

What happens during chronic pulmonary vasoconstriction and when can this condition occur?

Answer 7

- 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

Question 8

What is the appearance of the capillaries in each part of the lung at rest?

Answer 8

Question 9

How do the pulmonary vessels match the increased oxygen demand during exercise?

Answer 9

• 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

Question 10

What influences hydrostatic pressure in capillaries?

Answer 10

• Mainly venous pressure, especially in systemic circulation
• Therefore, if venous pressure increases so will hydrostatic in the capillary

Question 11

What is the rate of formation of tissue fluid in the lungs?

Answer 11

• Hydrostatic pressure in capillaries is low
• Small amount of tissue fluid

Question 12

How can oedema occur?

Answer 12

• Increase in capillary hydrostatic pressure, due to increased venouse pressure

Question 13

How do the lungs prevent pulmonary oedema, and what conditions is pulmonary oedema likely?

Answer 13

• Pulmonary capillary pressure normally low (9-12) but can get oedema if left atrium rises to 20-25

- Mitral valve stenosis

- Left ventricular failure

Question 14

Where in the lungs does pulmonary oedema normally occur and how do you treat it?

Answer 14

• Normally at base when upright, all over when lying down due to changes in hydrostatic pressure in gravity
• Use diuretics
• Treat underlying condition

Question 15

How much of the O₂ demand from the body does the brain hold?

Answer 15

• Recieves 15% CO and grey matter O₂ consumption accounts for 20% of total body consumption at rest

Question 16

How does the cerebal circulation meet the high demand for oxygen?

Answer 16

- 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)

Question 17

When do neurones become irreversibly damaged?

Answer 17

• After 4 minutes of hypoxia neurones undergo death
• Loss of consciousness in seconds

Question 18

How is a secure blood supply to the brain established?

Answer 18

• 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

Question 19

What is myogenic autoregulation in the brain?

Answer 19

- Cerebal resistance vessels response to changes in transmural pressure

• BP increade vasoconstriction
• BP decrease vasodilation to increase blood flow

Question 20

How does metabolic regulation maintain cerebral blood flow in terms of a gas produced?

Answer 20

• Allows blood flow to be directed to regions with high metabolic activity

Question 21

How can panic hyperventilation cause syncope?

Answer 21

• Hypocapnia
• Cerebral vessels constrict restricting O₂ supply to blood vessels

Question 22

What other metabolites can cause increase in blood flow to an area in the brain?

Answer 22

Increased:

• PCO₂
• K+
• Adenosine (powerful dilator of cerebral arterioles)

Decreased:

• O₂

Question 23

How is Cushing’s reflex an example of the brain stem altering other circulations to allow cerebral blood flow to continue occuring?

Answer 23

- 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

Question 24

When does blood flow through the left coronary artery during the cardiac cycle?

Answer 24

• 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

Question 25

What are the differences in the perfusion of cardiac and skeletal muscle and why?

Answer 25

- Cardiac needs more oxygen demand - Continous production of NO in the coronary endothelium maintains high basal flow

Question 26

How does the coronary blood flow meet the increase demand in oxygen by the myocardium in exercise?

Answer 26

- Vasodilation due to metabolic hyperaemia, adenosine, increased K+ and decreased pH - Increases blood flow

Question 27

Why are coronary arteries prone to narrowing and what are some factors that can narrow them?

Answer 27

- Functional end arteries, no anastomes - Atheromas, stress and cold and thrombus can all cause narrowing of coronary arteries

Question 28

Why would someone with a narrowed coronary artery experience angina on exercise?

Answer 28

- 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

Question 29

What are the feautres of skeletal muscle circulation?

Answer 29

- 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

Question 30

When the oxygen demand to the skeletal muscles increases, how does it's circulation accomodate for this?

Answer 30

Question 31

What is the main purposes of the cutaneous circulation?

Answer 31

- Temperature regulation - Maintaining blood pressure SKIN NOT VERY METABOLICALLY ACTIVE

Question 32

What are artereovenous anastomoses (AVAs) and what are their purposes?

Answer 32

- 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