18.3 - Maintenance of Blood Pressure

How is short-term maintenance of BP established?

• Nervous System

• Hormones of Endocrine system

What part of the Nervous System maintains homeostasis?

• ANS = Maintains homeostasis via SNS and PNS

• Has immediate effects on BP

Sympathetic NS effects on BP:

• Neurotransmitters involved

  • Immediate changes

• Stimulates changes that cause increase in BP

• Norepinephrine (NE) and Epinephrine (E)

  • Increase HR = Increase CO = Increase BP

  • Increase Contractility = Increase CO = Increase BP

  • Vasoconstriction of vessels = Increase Resistance = Increase BP

    • Upon SNS stimulation:

      • PNS vessels constrict (not necessary); minimizes O2 + nutrients to unneeded regions

      • SNS vessels dilate; increases O2 + nutrients to heart + sk. m.

Parasympathetic NS effects on BP:

• Neurotransmitters involved

  • Immediate changes

• Stimulates changes that decrease BP:

  • Acetylcholine (ACh): Released by vagus nerve onto cardiac pacemaker cells = slows HR = reduce contractility = decrease CO = decrease BP

    • SA and AV Node affected

What does the release of NE/E in the SNS cause:

Opening of Na+ Channels = Increase MP = AP easily conducted = Increase HR = Increase BP

What does the release of ACh in the PNS cause?

Opens K+ channels = K+ efflux = Repolarization = Decrease HR = Decreased BP

Do the PNS innervate vessels?

No, but it inhibits SNS activity to allow vessels to vasodilate.

What are Baroreceptors?

• Location

• Function

• Specialized mechanoreceptors from the ANS that monitor ANS.

• Found in common carotid arteries, aorta, carotid and aortic sinus.

• Fires action potentials in response to pressure exerted on arterial walls.

Baroreceptors if BP Increases:

Increase BP --> Baroreceptors fire AP faster --> Stimulates PNS --> Release ACh--> Decrease Resp. Rate --> Decrease HR --> BP decreases to normal rate

Baroreceptors if BP decreases:

BP decreases --> Baroreceptor fires AP slower --> Stimulates SNS --> Release NE/E --> Vasoconstriction --> Increase Resp. Rate --> Increase HR --> BP Increases back to normal range

What control center do baroreceptors send their impulses to?

Medulla Oblangata of brainstem

What are Chemoreceptors?

Receptors that detect presence of certain chemicals

What are the 2 population of chemoreceptors?

• Function

• Location

1. Peripheral Chemoreceptors = Respond to O2 in the blood.

   • If O2 is low --> SNS stimulation (vasoconstriction) --> Increase HR --> BP Increases

2. Central Chemoreceptors = Respond to H+ ions (pH) in blood.

   • If pH is low --> Increase SNS stimulation (vasoconstriction) --> Increase BP

What hormones and neurotransmitters are involved in the short-term changes that the Endocrine System creates on BP?

Affects Cardiac Output

1. Norepinephrine & Epinephrine; NE/E (vasoconstrict or vasodilate)

2. Thyroid Hormone

   
   

Affects pressure resistance:

1. NE/E

2. Angiotensin II (Vasoconstrictor = Increase BP)

3. Atrial Natriuretic Peptide (ANP); ANP (Vasodilator = Decrease BP)

How does NE, E, and Thyroid Hormone of the Endocrine System affect Cardiac Output?

• NE/E = released from SNS = Vasoconstriction = Increased HR = Increase CO

• Thyroid Hormone = Cardiac Muscle produce more receptors for NE/E

How does NE, E, Angiotensin II, and Atrial Natriuretic Peptide (ANP) of the Endocrine System affect Pressure Resistance?

• NE/E = SNS stimulation --> Vasoconstriction --> Increase resistance --> Increase BP

• Angiotensin II = Vasoconstrictor --> Increase resistance --> Increase BP

• Atrial Natriuretic Peptide (ANP) = Vasodilation of vessels supplying kidney --> decreased resistance --> Lower BP

What are the hormones of the Endocrine System that cause long term changes in BP?

1. Atrial Natriuretic Peptide (ANP)

   • Increase BP --> Secrete ANP --> Kidneys excrete more water and Na+ efflux --> Blood Volume Decrease --> Decrease BP

2. Angiotensin-II

   • Decrease BP --> Secrete Aldosterone --> trigger Na+ influx and water retention in kidney --> Increase Blood Volume --> Increase BP

3. Antidiuretic hormone (ADH)

   • Decrease BP --> Secrete ADH --> Trigger thirst + water retention in kidney --> Blood Volume Increase --> BP Increase

4. Renin (Enzyme in kidneys)

   • Decrease BP --> Secrete Renin --> Activates Angiotensin II --> triggers thirst +water retention in kidney --> Blood Volume Increase --> BP Increase

How does the Urinary System of the Endocrine System cause long term changes in BP?

• If BP Increases

  • Increase urine production = Decrease Blood Volume = Decrease BP

• If BP Decreases

  • Decrease urine production = Retain Water = Increase Blood Volume = Increase BP

How do capillaries contribute to maintenance of BP?

• Autoregulation: Local regulation of blood flow at cappillaries

  • Tissue Perfusion: Blood flow at capillaries.

What are the 2 types of autoregulation?

1. Myogenic Mechanism

2. Metabolic Mechanism

What is the Myogenic Mechanism of Autoregulation (capillaries)?

• If BP Increases = Vasoconstriction @ vessel that has high pressure = Decrease blood flow into capillaries.

• If BP Decreases = Vasodilation @ vessel with low pressure = Increased blood flow into capillaries

What is the Metabolic Mechanism of Autoregulation (capillaries)?

• If tissue is inactive, then there is excess O2 and not enough CO2:

  • Vasoconstriction of precapillary sphincters = Blood passes through capillaries = No gas/nutrient exchange = Decreased blood flow to tissue.

• If tissue is active, then there is not enough O2 and too much CO2:

  • Vasodilation of precapillary sphincters = Blood enters capillaries = Increase gas/nutrient exchange = Increased blood flow to tissue.