CV Compensation Flashcards
What type of nerves supply the heart?
Autonomic
What effect does innervation have on the heart (2)?
Moderate heart rate and contractility
Describe sympathetic innervation of the heart.
- Sympathetic fibers originate from the thoracic spinal cord segments (T1–T4) and extend to the heart.
- They innervate the sinoatrial (SA) node, atrioventricular (AV) node, and ventricular myocardium.
Describe parasympathetic innervation of the heart.
- Parasympathetic fibres reach the heart primarily via the vagus nerve (cranial nerve X).
- The vagus nerve mainly innervates the SA node and AV node but has limited direct influence on the ventricular myocardium.
Which vessels are most responsible for altering blood flow to a tissue?
Arterioles
Describe the reaction of vessels to sympathetic innervation.
- Most vessels constrict
- Coronary and working skeletal muscle arterioles dilate
Describe the reaction of vessels to parasympathetic innervation.
Only genital and coronary BVs have para innervation = dilation.
What are the parasympathetic receptors for heart and blood vessels?
Cholinergic
What are the sympathetic receptors for heart and blood vessels?
Blood vessels = α1 α2 β2 adrenergic +
Heart = β1 adrenergic
What is the common effect of the parasympathetic NS on cardiac output and BP?
Reduce CO and BP
What is the common effect of the sympathetic NS on cardiac output and BP?
Increase cardiac output and BP
What do cholinergic receptors respond to and from where?
Acetylcholine from the presynaptic neurone
What do adrenergic receptors respond to and from where?
- Noradrenaline from the presynaptic neurone
- Circulating noradrenaline/adrenaline from the adrenal gland
What is the overall effect of sympathetic activation and why?
Prepares the body for “fight or flight” by increasing cardiac output and redistributing blood flow to essential organs like the heart, lungs, and skeletal muscles.
How does parasympathetic stimulation reduce heart rate?
- Occurs via the vagus nerve
- Releases acetylcholine that binds to cholinergic receptors (M2) on the SA and AV nodes.
What five systems manage BP during exercise?
- Metabolic autoregulation of blood flow
- Psychogenic response
- Exercise reflex
- Baroreflex
- Skeletal muscle and respiratory pumps
What does the psychogenic response do?
Increases sympathetic activity/ decreases parasympathetic activity.
What is the exercise reflex?
Receptors in muscles and joints sense movement and chemical changes, signalling the brain to increase heart activity and blood flow to active muscles and to boost breathing.
What is the respiratory pump?
When you inhale, your chest expands, and pressure in the chest drops, while pressure in the abdomen rises. This pressure difference pushes blood from the veins in the abdomen up toward the heart.
What is the skeletal muscle pump?
When muscles contract they squeeze nearby veins pushing blood upward. Valves in the veins prevent blood from flowing backward, so each muscle contraction moves blood closer to the heart (increases preload).
What is the baroreflex?
Baroreceptors (located in the carotid arteries and aorta) detect changes in BP. If blood pressure rises, the baroreflex signals the heart to slow down and blood vessels to relax, lowering pressure. If blood pressure drops, it signals the heart to beat faster and blood vessels to constrict, raising pressure.
Define hypovolaemia.
Decreased volume of circulating blood.
What can hypovolaemia lead to?
Decreased preload as less venous return –> decreased cardiac output —> hypotension (drop in blood pressure) –> perfusion failure & tissue hypoxia –> organ dysfunction –> multi organ failure
What can cause hypovolaemia?
Haemorrhage, severe dehydration, sequestration of blood
How does our body notice hypovolaemia?
Perceived by atrial volume receptors and arterial baroreceptors –> initiates baroreflex.
How else might the body respond to hypovolaemia?
- Splenic contraction
- Starling’s law of capillaries; more absorption of fluid from interstitial space into capillaries.
- RAAS if hypovolaemia persists –> vasoconstriction, more water reabsorption at kidneys due to ADH release, makes you feel thirsty, sodium retained at kidney (aldosterone).
Why does PCV and TP reduce after a haemorrhage?
Replacement of fluid (through RAAS etc) dilute blood.
How is PCV and TP replenished?
- BM replaces cells within a few weeks.
- Protein synthesis in liver in a few days.
What is forward heart failure and what signs are there?
Systolic failure (failure of output): reduced cardiac output –> reduced mean arterial pressure (BP) –> signs of reduced perfusion
What are signs of reduced perfusion?
- Pale gums
- Poor capillary refill time
- Lethargic
What is backwards heart failure?
Inability of heart to cope with preload (input - blood is backing up); combination of excessive preload and failing heart.
What is the effect of backwards/congestive heart failure?
Atrial pressure increases –> leads to increased venous pressure –> increased capillary pressure –> oedema
What can cause backwards heart failure?
Degenerative AV valve disease
What happens with left CHF?
Lung oedema
What happens with right CHF?
Tend to get free fluid forming in abdomen/pleural effusion/ jugular pulse
What is the link between forward and backward heart failure?
One leads to the other!
How does Starling’s mechanism compensate for backwards heart failure?
Increased venous/atrial pressure –> increased preload –> increased CO
What are the negative effects of Starling’s mechanisms for backwards heart failure?
- Atrial stretch - reduced atrial systole, mechanical pressure = cough
- Oedema/effusions
What compensates for forward heart failure but what can this lead to?
Baroreflex - myocardial remodelling –> fibrosis
What is the problem of compensatory mechanisms?
Fail to correct cardiac output! Vicious circle.
What do natriuretic peptides do and what are they an indicator of?
- Causes sodium loss at the kidney –> loses water with the sodium
- Peripheral vasodilation
- May reduce renin & aldosterone
Can test - indicator of myocardial stretch.
