L23. Cardiovascular Integration Flashcards
(15 cards)
Resting arterial blood pressure?
MABP = CO x TPR
- Resting arterial blood pressure is (MABP) is close to 90mmHg
- This provides sufficient pressure to overcome the resistance of the vessels while flowing at the desired rate
- Need to maintain adequate perfusion (e.g. fainting)
- Blood pressure is regulated (seconds) –> avoid fluctuation
Reflexes maintaining normal arterial pressure?
- Multiple subconscious special nervous control mechanisms operate all the time to maintain arterial pressure at or near normal
- Almost all of these are negative feedback reflex mechanisms;
- Arterial baroreceptors
- Carotid and aortic chemoreceptors
- Cardiopulmonary baroreceptors (located in the atria, ventricles and pulmonary vessels - also called “low pressure receptors”)
- Central chemoreceptors (located in medulla)
Arterial baroreceptors?
- Sensors are the aortic and carotid sinus baroreceptors
- Their afferent fibres travel in vagus and glossopharyngeal nerves to CVS centres in brainstem (nucleus tractus solitarius in the medulla)
- There is tonic activity in both the afferent nerves from the baroreceptors, and in the efferent parasympathetic and sympathetic nerves
- This means that the system can respond readily to either a fall or an increase in arterial pressure
E.g. a pressure rise will increase activity in baroreceptor afferent fibres –> parasympathetic activity to the heart increases –> sympathetic activity to the heart and blood vessels decreases –> heart rate, cardiac contractility, and total peripheral resistance will fall, and vasodilation will occur
Hypertensive baroreceptors?
Would have persistent firing
Baroreceptors as a “pressure buffer” system?
- Dog with intact baroreceptors have narrow operating range in pressure
- When baroreceptor nerves denervated pressure has wide distribution
Baroreceptors (baroreflex) play a key role in controlling minute-minute pressure changes at normal physiological state
How posture affects blood pressure?
Because blood vessels are not rigid, and because gravity is a determinant of the pressure that blood exerts on the walls of blood vessels, veins in the dependent (below heart) region of the body tend to be distended
- When you lie down, most of the vessels are close to heart level; when you stand, some vessels may be almost 1 metre below the heart
Orthostatic hypotension?
Decrease in CVP –> decrease right stroke volume –> decrease left ventricular filling pressure –> decrease left stroke volume –> decrease arterial pressure –> decrease cerebral blood flow (O2 lack) –> symptoms of cerebral underperfusion = dizziness and visual fade
Reflex to preserve cerebral perfusion?
Decrease in arterial blood pressure
–> decrease input from low pressure receptors; decrease input from high pressure receptors –> increase sympathetic drive to:
- SA node (and decrease vagal)
- Myocardium
- Resistance vessels
- Capacitance vessels
Exercise - vasoconstriction?
- Cardiac output can increase from 5L/min to 35L/min
- Most of the increase in cardiac output goes to exercising muscles
- Vasoconstriction in abdominal organs, kidneys and other non-exercising muscles (as they don’t need a lot of blood during exercise), so there is little vasoconstriction for brain, heart, and limbs as they need the increased blood flow
Changes to heart due to exercise?
- Decreased parasympathetic and increased sympathetic activity to SA node
- Increased contractility due to increased sympathetic activity
- Increased systolic pressure
- MABP increased preload
- Decreased TPR
Blood loss/haemorrhage and the effect on blood pressure?
- Tightly regulated (5-6L, 70ml/kg) for optimal CVS function
- Changes in the blood volume can have profound effects on BP
- 10% blood loss - standard blood donation - no significant threat
- 20-30% blood loss - clinical shock - cardiac output falls followed by arterial pressure
- > 40% blood loss - may cause severe and sometimes irreversible shock - reduced cerebral and coronary perfusion
Blood loss response?
Body responds in three steps:
1. Rapid - within seconds - baroreceptor
2. Intermediate - within minutes - fluid reabsorption
3. Long-term - days - kidneys
Blood volume post hemorrhagic?
- The cells are not fully replaced for several weeks, but the circulating volume can be rapidly restored from the interstitial fluid (ISF)
- As far as the CVS function is concerned, the appropriate fullness of the vessels is far more important than having an ideal blood composition
Two important components:
1. The transfer of fluid from the interstitial space into the circulation to restore circulating volume (autotransfusion)
2. The slower replacement of lost volume (salt and water) (endocrine and renal)
Autotransfusion?
Haemorrhage –> decreased blood volume –> decreased capillary hydrostatic pressure –> decreased absorption from the interstitial fluid
Long term response - fluid replacement?
- Even if BP maintained still have to replace lost volume
- Decreased renal perfusion triggers renin/angiotensin II
- Angiotensin II: vasoconstrictor, reduces renal perfusion; less urine (+ADH), reduces renal Na loss, stimulates aldosterone, stimulates thirst
- Over the next few hours-days total body lost water and salt are replaced (increase intake/reduce losses)
- Replacement of red blood cells by bone marrow and albumin by liver; takes around 6 weeks
