Mechanisms

Question 1

Frank-starling

Answer 1

Greater end diastolic volume leads to a greater preload leads to a greater force/more pressure on chamber walls, causing greater stretch, then greater force exerted on blood, greater stroke volume and hence greater cardiac output

Question 2

Baroreceptor reflex (high BP)

Answer 2

-Increased arterial pressure causes more stretching of the baroreceptors
-results in more action potentials are sent to the medulla in the brainstem
-Medulla response:
-Increased Parasympathetic activity: enhances vagal tone to slow down the heart rate
-Decreased sympathetic activity reducing vasoconstriction and heart rate
-Leads to decreased cardiac output and vasodilation to reduce the blood pressure

Question 3

Baroreceptor reflex (low BP)

Answer 3

-lower arterial pressure causes less stretching of the baroreceptors (peripheral ones in the aortic and carotid body)
-leads to decreased afferent firing so fewer signals are sent to the medulla in the brainstem
-Medulla response:
-Decreased Parasympathetic activity:reduces vagal tone, allowing heart rate to increase
-Increases sympathetic activity, increase vasoconstriction and heart rate
-Results in an increased cardiac output and vasoconstriction to raise BP

Question 4

Homeostatic control of respiration

Answer 4

Chemoreceptors play a critical role in the homeostatic control of respiration by continuously monitoring blood and cerebrospinal fluid levels of CO2, O2, and pH. The central chemoreceptors (in blood brain barrier detect CSF ) respond primarily to CO2 and pH changes in the CSF, while the peripheral chemoreceptors 9in aortic and carotid bodies) respond to changes in CO2, O2, and pH in the blood. These chemoreceptors send signals to the medullary and pontine respiratory centers to adjust the rate and depth of breathing, ensuring stable levels of blood gases and maintaining acid-base balance.

Question 5

The Hering-Breuer Reflex

Answer 5

1.Lung Inflation: During inspiration, the lungs expand and stretch the lung tissues.
2.Stretch Receptor Activation: Stretch receptors in the bronchi and bronchioles are activated by this expansion.
3.Afferent Signal Transmission: These receptors send signals via the vagus nerve to the respiratory centers in the brainstem (medulla oblongata and pons).
4.Central Processing: The brainstem processes these signals and sends inhibitory signals back to the respiratory muscles.
5.Inhibition of Inspiration: These inhibitory signals reduce inspiratory activity, preventing further lung inflation.
6.Promotion of Expiration: As inspiration is inhibited, the lungs begin to deflate, leading to expiration.
7.Termination of Reflex: As the lungs deflate, the stretch receptors are no longer activated, stopping the inhibitory signals and allowing the next inspiration to occur.

Question 6

RAAS (Renin-Angiotensin-Aldosterone System)

Answer 6

1. Specialised cells in the juxtaglomerular apparatus (juxtaglomerular cells) detect low BP and/or low Na+, and secrete the hormone Renin
2. Renin converts pepsinogen (produced in the liver) into Angiotensin I
   3 Angiotensin I is converted into Angiotensin II by Angiotensin-converting Enzyme (ACE)
3. Angiotensin II stimulates the release of Aldosterone from the adrenal glands, and causes vasoconstriction of the efferent arteriole.
4. Aldosterone acts on the distal convoluted tubules and collecting ducts of the kidneys, promoting the reabsorption of sodium and the excretion of potassium.
5. Increased sodium reabsorption leads to increased water reabsorption, resulting in an increase in blood volume.
6. The increase in blood volume and vasoconstriction caused by angiotensin II contribute to the restoration of blood pressure to normal levels.
7. Negative feedback mechanisms come into play as the restored blood pressure and increased sodium levels inhibit the release of renin.

Question 7

Micturition reflex (pissing)

Answer 7

Involuntary steps:
1. Receptors in the wall of the bladder detect stretch and send a signal to spinal cord (pons)
2. Detrusor muscle surrounding bladder contract, causing bladder to contract and squeeze urine outer
3. The internal sphincter relaxes, enabling urine to exit the bladder and enter the urethra
Voluntary steps:
4. An afferent signal is sent to the brain which processes the response and sends efferent signal to urethra
5. The external sphincter relaxes or contracts (depending on the desired response) leading to urination or urine withholding