CVS session 8: the special circulations Flashcards
What are the two circulations of the lungs?
Bronchial: part of systemic circulation; meets metabolic requirements of the lungs (parts not regularly perfused with oxygen)
Pulmonary: blood supply to alveoli, required for gas exchange, in series with the rest of the systemic circulation. Must accept the entire cardiac output
Pressure values for the heart, pulmonary artery and aorta
systole/diastole where relevant
Right atrium: 0-8 mmHg Left atrium: 1-10 mmHg (slightly higher than RA as more resistance) Right ventricle: 15-30 mmHg/0-8 mmHg Left ventricle: 100-140 mmHg/1-10 mmHg Pulmonary artery: 15-30 mmHg/4-12 mmHg Aorta: 100-140 mmHg/60-90 mmHg
Pressures and resistances in the pulmonary circulation?
- Low pressure: MAP ~12-15 mmHg, mean capillary pressure ~9-12 mmHg, mean venous pressure ~5 mmHg
- Low resistance: short, wide vessels, many capillaries (so many parallel elements), arterioles have relatively little smooth muscle
Adaptations for efficient gas exchange in the pulmonary circulation
- high density of capillaries in alveolar wall: large surface area
- short diffusion distance as very thin layer of tissue that separates gas from plasma
What is the ventilation-perfusion ratio?
Need to match ventilation of alveoli with perfusion of alveoli in order to allow efficient oxygenation. Optimal V/Q ratio is 0.8. This is achieved when:
- ventilation ~4L/min
- perfusion (cardiac output) ~5L/min
Maintaining this means diverting blood from alveoli which are not well ventilated
What is the most important mechanism regulating pulmonary vascular tone? How does it work?
Hypoxic pulmonary vasoconstriction
Maintains an optimal V/Q ratio: alveolar hypoxia causes vasoconstriction of pulmonary vessels (opposite of in systemic where metabolites cause vasodilation to increase perfusion). Poorly-ventilated alveoli are less perfused, which prevents deoxygenated blood from returning to the left side of the heart
What can chronic hypoxic vasoconstriction lead to and why?
Right ventricular failure
At altitude or due to a lung disease (e.g. emphysema), chronic increase in vascular resistance causes chronic pulmonary hypertension. The high after load on the right ventricle can therefore lead to right ventricular heart failure
Effects of gravity on the lungs
Strong influence due to the low pressure of the vessels:
- upright (orthostasis) there is a greater hydrostatic pressure on vessels in the lower part of the lung, so vessels are distended
- at the level of the heart the vessels are continuously patent
- vessels at the apex collapse during diastole
Effect of exercise on the pulmonary circulation
Increased CO increases flow to lungs ,causing a small increase in pulmonary arterial pressure which opens apical capillaries which have a faster transmit time as blood flow increases. This increases oxygen uptake by the lungs
Describe how Starling forces regulate tissue fluid locations
No1. Capillary hydrostatic pressure: pushes fluid out of capillary into interstitium normally (as is normally greater than tissue hydrostatic pressure). Main influence is by the venous pressure
- Tissue hydrostatic pressure
- Capillary plasma oncotic pressure: large molecules e.g. plasma proteins within draw fluid into the capillary normally
- Tissue interstitial oncotic pressure: usually lower than capillary oncotic pressure
In normal situations, tissue fluid is kept at a constant level by the capillary hydrostatic pressure pushing fluid out and the capillary oncotic pressure drawing fluid in
Does hypertension result in peripheral oedema?
No
No symptoms: silent killer
Oedema caused by heart failure due to increased pressure at venous ends of capillaries
How does a low capillary pressure minimise the formation of lung lymph?
Oncotic pressure of TF in lungs > in periphery
Capillary hydrostatic pressure in lungs
How does pulmonary oedema form?
Increased capillary pressure (usually venous end) resulting in filtration > reabsorption
Caused by left-sided heart failure, as blood backs up into the lung vessels
Left atrial pressure increased due to mitral valve stenosis of left ventricular hypertrophy: left ventricle can’t pump out as much so its harder for blood to move from LA to LV and from PV to LA
This causes capillary hydrostatic pressure to be increased, so fluid moves out of capillaries into interstitium
Gas exchange is therefore impaired
Use diuretics to relieve symptoms (reduce blood volume and venous pressure), treat underlying cause if possible
How does posture affect oedema of the lungs?
When upright it forms mainly at the bases so doesn’t cause as severe symptoms
When lying down, gravity causes redistribution of fluid when lying down so it is present throughout the lungs. Gravity increases venous pressure so increases capillary hydrostatic pressure causing fluid to move out into lungs
Leads to shortness of breath
What is the oxygen demand of the brain?
~15% of cardiac output even though only ~2% of body mass
O2 consumption of grey matter (cell bodies) accounts for ~20% of whole body oxygen consumption at rest, so secure oxygen supply is vital (neurones irreversibly damaged by hypoxia in ~4 minutes)
How does the cerebral circulation meet its demand?
High capillary density: large SA for gas exchange
Low diffusion density
High basal flow rate: x10 average for whole body
Blood supply secured by anastomoses between the basilar and internal carotid arteries: circle of Willis
