Chemoreceptors, located in the carotid arteries and the aorta, monitor the chemical composition of the blood, specifically focusing on levels and pH. An increase in metabolic activity leads to higher concentration, which lowers blood pH, triggering a reflex to increase heart rate and expedite gas exchange.
Baroreceptors (or pressure receptors) are sensitive to the stretching of blood vessel walls caused by blood pressure changes. If blood pressure rises too high, these receptors send impulses to the inhibitory center of the medulla to trigger a reduction in heart rate, preventing damage to delicate vessels.
To increase heart rate (e.g., during exercise), the body follows a specific sequence: High is detected by chemoreceptors, which send impulses via sensory neurones to the acceleratory center in the medulla. This center then sends impulses down sympathetic neurones to the SAN, where noradrenaline increases the frequency of electrical waves.
To decrease heart rate (e.g., high blood pressure), the process reverses: Baroreceptors detect high pressure and signal the inhibitory center in the medulla. Impulses travel via parasympathetic neurones (vagus nerve) to the SAN, where acetylcholine is released to reduce the frequency of heartbeats.
| Feature | Sympathetic Pathway | Parasympathetic Pathway |
|---|---|---|
| Condition | High , Low pH, Low BP | Low , High pH, High BP |
| Medulla Center | Acceleratory Center | Inhibitory Center |
| Neurotransmitter | Noradrenaline | Acetylcholine |
| Effect on SAN | Increases impulse frequency | Decreases impulse frequency |
Identify the Stimulus First: When answering questions about heart rate changes, always start by identifying whether the stimulus is chemical (/pH) or mechanical (pressure). This determines which receptor and which medulla center you must discuss.
Use Precise Terminology: Always specify the neurotransmitter involved (noradrenaline vs. acetylcholine) and the specific node being affected (SAN). Avoid saying 'the brain' generally; use 'medulla oblongata' or 'cardioregulatory center'.
Check the Direction: A common mistake is swapping the effects of the ANS branches. Remember that 'Sympathetic' is for 'Stress' (increasing rate), while 'Parasympathetic' is like a 'Parachute' (slowing you down).
Cardiac Output is the total volume of blood pumped by one ventricle per minute and is a product of heart rate and stroke volume. It is calculated using the formula: .
Stroke Volume refers to the volume of blood pumped out of a ventricle during a single contraction. During exercise, both heart rate and stroke volume typically increase to maximize the delivery of oxygenated blood to working muscles.
