L24. Specialised Circulation Flashcards
(20 cards)
Generic circulation for blood?
Each organ and tissue receives a blood supply that flows through a circuit
- Blood leaves the heart –> travels to the vascular bed of each organ –> perfuses the organ –> delivers O2/removes CO2 –> blood returns to the heart/lungs
Blood flow regulators?
- Myogenic control (e.g. arterioles)
- Neural control
- Metabolic requirements
Fetal circulation?
- Distribution of blood flow is dependent of local requirements
- Organs not yet active (e.g. lungs) are by-passed
- O2 blood comes from placenta because lungs are deflated
- Blood bypasses liver
- Placenta is already filtering the blood
Placenta functions?
Placenta serves as:
- Intestine (nutrient uptake)
- Kidney (waste removal)
- Lungs (uptake of O2)
- Receives a large fraction of CO; is a low resistance circuit
Fetal blood?
Is oxygenated at the site of the placenta via the maternal blood supply – (relatively hypoxic = 80% saturated)
Right-to-left shunt via:
- Foramen ovale i.e. ‘hole’ in the heart
Liver/kidneys also by-passed:
- Ductus venosus
Fetal lung bypassing?
The lungs are by-passed!
- Extreme pulmonary vasoconstriction
- High resistance to blood flow
- Airways are collapsed
- Ductus arteriosus
Ductus arteriosus?
A blood vessel in a fetus that bypasses pulmonary artery by connecting the pulmonary artery directly to the ascending aorta
Ductus venosus?
Bypasses the liver and kidney since they aren’t needing blood for use
Pathway of blood flow through fetal circulation?
- Placental arterial blood has highest O2 saturation (but low compared to adult)
- By pass liver and connect to inferior vena cava
- Slightly contaminated with circulating placental venous blood
- Saturation = 67%
- Folds in vena cava “guide” blood flow through: right atrium –> foramen ovale –> left atrium
- Flow is laminar!
- This projected blood flow “crosses” pathways with venous blood from superior vena cava –> right ventricle
At birth switching from placenta?
- Newborn must be able to survive independent of placenta
- Placental role in nutrient support and waste removal
- Abruptly removed
- Lungs are CRITICAL!
- Stimulus for breathing = increased CO2
- Inspiratory motions
- Generate negative thoracic pressure
- Draws blood out of placenta
- Inflates lungs (VE)
- Decreased pulmonary vascular resistance; increased systemic resistance
- Blood flows into lungs
Ductus arteriosus closure?
- Change in blood flow direction through DA
- Favouring blood flow through the pulmonary circulation
- Reduction in prostaglandins causes ductus arteriosus to close
- Reduction in right atrial pressure and increase in left atrial pressure
- Left-to-right pressure gradient
- Causes folds of foramen ovale to close (‘mostly’ fuses completely with time)
Main pulmonary blood vessels?
Main pulmonary trunk - originating directly from the right ventricle, before dividing into the left and right main axial artery - supplies blood to the left and right lung
Pulmonary circulation?
Each axial artery branches into:
1st-20th generation of branching - the number of branches increases (size decreases) to ensure adequate perfusion of the alveoli for gas exchange
Pulmonary vascular bed?
The pulmonary circulation receives 100% of the cardiac output and it is a vascular bed characterised by:
- A low resistance circulation (10-15 fold lower than systemic vascular resistance) achieved by:
* High number of parallel vessels (some are recruited during increased blood flow)
* Larger diameter vessels
* Vessels are shorter in length
- A high compliance:
* Vessels are able to easily distend
* Hence able to accomodate an increase in cardiac output without a proportional increase in pressure
Pulmonary pressure?
A low arterial pressure; an increase in pulmonary pressure can lead to:
* Impaired ejection of blood (increased afterload)
* Right heart failure
* Pulmonary edema
Regulators of pulmonary blood flow?
- Gravity
- Hypoxia (low O2)
- Endothelial control
- Sympathetic nervous system
Gravity effecting pulmonary blood flow?
- The pulmonary vessels are subject to a low-pressure system with high compliance. Therefore, gravity influences regional blood flow within the lung
- The effect of standing:
*Lower regions of the lung = increases hydrostatic pressure in vessels, vessels easily distend (high compliance) which decreases resistance and increases blood flow to the lower regions of the lung
*Upper regions of the lung = reduced intravascular pressure, vessels partially collapse, increases resistance, decreases blood flow to the upper regions of the lung but blood flow to the upper regions increase/are recruited during exercise
Hypoxia effecting pulmonary blood flow?
- The pulmonary vasculature constricts in response to hypoxia
- The mechanisms behind the hypoxic pulmonary vasoconstriction (HPV) are unknown but are likely to involve various endothelial-derived vasoactive modulators
- HPV optimises the ventilation-perfusion matching for optimal gas exchange
- Decreases blood flow (longer time for gas exchange)
*Directs blood away from poorly ventilated alveoli
Endothelial control effecting pulmonary blood flow?
- The endothelium of the pulmonary vasculature releases various vasoactive peptides that regulate pulmonary vascular tone including:
- Nitric oxide - a potent vasodilator
- Endothelin-1 - a potent vasoconstrictor
Sympathetic nervous system effecting pulmonary blood flow?
- The pulmonary vasculature is innervated with:
- Alpha adrenoreceptors - located primarily in the large conduit vessels. Stimulation causes pulmonary vasoconstriction, an increase in resistance, an increase in pulmonary pressure
- Beta adrenoreceptors - located primarily in the smaller resistance vessels.
Stimulation causes pulmonary vasodilation, a decrease in resistance, a decrease in pulmonary pressure - Consequently, both a and B effects tend to negate each other; therefore the SNS has only a minor role in regulating pulmonary blood flow
