Physiological Control of the Cardiovascular system

Cardiovascular control centre

Location: Brain Stem

Function:

• Regulates heart rate

• Contractility of ventricles (how forcefully they contract)

• Blood vessel diameter (vasoconstriction/vasodilation)

Output of the cardiovascular system

Parasympathetic and sympathetic nerve impulses sent to the heart and blood vessels

• Vagus nerve

• Sympathetic cardiac nerves

Vagus nerve

Nerve within the brain stem that sends signals to the heart via the parasympathetic nervous system.

• Decreases heart rate

  • decrease in heart rate is achieved by decreasing the rate of pacemaker potentials in the SA node

Sympathetic cardiac nerves

Nerve within the brain stem that sends signals to the heart via the sympathetic nervous system.

• Innervate the SA node & myocardium

  • achieved by increasing the rate of pacemaker potentials in the SA node

• Increases heart rate and force of ventricular contractions (contractility)

  • Causes an increase in stroke volume

Input of the cardiovascular control centre

• Input from higher brain centres that travel down to the brain stem

• Input from pressure receptors (baroreceptors)

Baroreceptors

Pressure receptors

Location:

• Aorta

• Large blood vessels around the heart (e.g carotid sinus)

Function:

• An increase in blood pressure, causes an increase in action potentials and signals sent to the brain, and vice versa

Control of heart function

1. Autonomic nervous system (sympathetic & parasympathetic)

2. Endocrine system

3. Intrinsically

Intrinsic mechanism that controls the heart

Increased venous return

• results in more blood in the ventricles, which stretches the heart muscle also increasing the force of contraction and stroke volume

Endocrin system & Heart function

An increase in sympathetic outflow (fight or flight response) to the body causes the adrenal medulla to release the hormones epinephrine and norepinephrine, which also increase heart rate and force of ventricular contraction

Controlling blood vessel function

1. Sympathetic nervous system

2. Intrinsically (locally)

Sympathetic nervous system

• Sympathetic cardiac nerves of the cardiac control centre

Sympathetic nerve activity causes the arteries and veins to contract, in arterioles this can reduce blood flow to an organ, and in veins it can help propel blood towards the heart

• Fight or flight response causes vasoconstriction in skin arterioles so blood can be diverted to skeletal muscles that might need to contract (e.g if you are fleeing a dangerous situation/fighting)

Intrinsic mechanisms controlling blood vessel function

Localised chemical conditions in organs can regulate blood flow (intrinsic mechanism).

Often occurs in response to increased metabolism. Causes a need for increased oxygen and increased nutrient supply

The following local chemicals promote vasodilation:

• decreased oxygen

• increased carbon dioxide

• decreased pH

• increased extracellular potassium

• nitric oxide released from the endothelium (inner layer of blood vessels)

Short term blood pressure control:

Baroreceptor reflex

Activated whenever blood pressure decreases or increases

• More sensitive to decreases in blood pressure

• More sensitive to sudden changes in blood pressure than gradual changes

Baroreceptor Reflex Scenario:

Standing up from lying down

1. When moving quickly from lying to standing, gravity pulls blood downwards away from the heart

2. Reduced venous return and cardiac output, decreasing arterial pressure (causing dizziness as less blood flows to the brain)

3. Decreased arterial pressure reduces stretch on sensory fibres in baroreceptors, decreasing the frequency of action potentials to the cardiovascular control centre

4. Control centre responds by increasing sympathetic nervous system output and decreasing parasympathetic output

5. As a result, heart rate increases, force of contraction of ventricles increases (increases stroke volume), and arterioles vasoconstrict to further increase pressure

6. Altered heart function (increase in heart rate and ventricular contractility) increases cardiac output

7. Vasoconstriction of arterioles increases peripheral resistance

   • Both of these changes act to increase arterial pressure (within seconds)

Long term blood pressure control:

Kidneys

Function:

• Control blood volume

• Control degree of arteriole vasoconstriction

  1. Decreased blood pressure sensed

  2. Renin is produced

  3. Renin production gives rise to angiostenin II, which is a chemical that causes blood vessels to constrict (rapid reaction)

  4. Angiostenin II also causes the release of aldosterone from the cortex of the adrenal gland

  5. Aldosterone causes kidneys to reabsorble water and NaCl, increasing blood volume. This takes hours to days to come into effect.