Control on BP, Blood flow & Hypertension

What sympathetic neurones act on the heart from the ANS?

Cardiac accelerator nerves

• increase contractility and HR

What parasympathetic nerve acts on the heart form the ANS?

Vagus nerve

• decreases HR

What are vasomotor nerves

Nerves to the smooth muscle of blood vessel walls

What is vasomotor tone?

Moderate state of tonic contraction or vasocontraction

• sets the resting levels of systemic vascular resistance

What are two main receptor reflexes for the neural control of BP?

• Baroreceptor reflex

• chemoreceptor reflex

Where are the major baroreceptors located?

• Aorta (aortic reflex)

• internal carotid arteries (carotid sinus reflex)

Which cranial nerve mediates the carotid sinus reflex?

Glossopharyngeal (CN IX)

What BP does the aortic reflex mediate?

Systemic blood pressure

Which cranial nerve mediates the aortic refelx?

Vagus nerve (CN X)

What effect does a fall in blood pressure have on baroreceptors?

• Baroreceptors = stretched less = send impulses to medulla at decreased rate

• CV centre increases sympathetic and decreases parasympathetic stimulation

• Adrenaline & noradrenaline is also secreted

OVERALL = increase in HR and contractility of the heart = increase in systemic vascular resistance and cardiac output - raising BP

What effect does a ride in BP have on barareceptors?

• Baroreceptors = stretch more = send impulses to medulla at an increased rate

• CV centre increases sympathetic and decreases parasympathetic stimulation of the heart

• HR and contractility of the heart decreases

• vasodilation also occurs

OERALL = fall in vascular resistance and decreased cardiac output = BP falls

Where are chemoreceptors located?

Located in carotid and aortic bodies

What do chemoreceptors detect?

Changes in concentration of O₂, CO₂ , and H+

What conditions stimulate chemoreceptors to send impulses to the CV centre?

HYPOXIA - too little oxygen

HYPECAPNIA - to much carbon dioxide
ACIDOSIS - raised [H+]
ALKALOSIS - low [H+]

How does the CV centre respond to stimulation from chemoreceptors?

Increased sympathetic firing to arterioles and veins = vasoconstriction and raised BP

What does the respiratory centre of the brain change due to chemoreceptor activation?

Alters rate of breathing

What are the 3 endocrine controls of BP?

• RAA system

• Arginine Vasopressin - ADH system

• Atrial Naturitic peptides

What is the overall effect of the RAA system on BP?

Raises BP

What is the overall effect of the aginine vasopressin - ADH system on BP?

Raises BP

What is the overall effect of atrial natruitic peptide on BP?

decreases BP

Which cells in the kidney secreate renin in response to a fall in blood pressure?

Juxtaglomerular apparatus cells

What is the effect of renin on angiotensin?

Renin converts angiotensin into angiotensin I

What enzyme is converts angiotensin I into angiotensin II?

Angiotensin converting enzyme

How does angiotensin II raise BP?

Angiotensin II = potent vasoconstrictant = raises systemic vascular resistance

Angiotensin also stimulates aldosterone secretion

• aldosterone increases renal NaCl reabsorption = increasing retention of water

• this increases blood volume = increasing BP

Where is ADH produced and where is it released?

Produced = hypothalamus

Released = posterior pituitary gland

What stimulates the release of ADH from the posterior pituitary gland?

Dehydration/fall in blood volume

How does ADH raise BP?

• Causes vasoconstriction

• Promotes water reabsorption from bowman’s capsule = increased blood volume and increased BP

Where is ANP released?

Released from cells in the atria of the heart

How does ANP decrease BP?

It counteracts the effect of RAAS, sympathetic activation and ADH secretion

• promotes vasodilation

• promotes loss of NaCl and water in urine = fall in blood volume and pressure

Autoregulation of BP

The ability of tissues to automatically adjust its blood flow to match metabolic demands

• achieved by vasoconstriction/dilation of blood capillary network

What are examples of vasodilating chemicals?

• H+

• K+

• Lactic acid

• NO

• Kinins

• Histamine

What are some examples of vasoconstrictive chemicals?

• Thromboxane A2

• Superoxide radicles

• Serotonin

How do Beta blockers reduce blood pressure?

They inhibit sympathetic NS by binding to B-2 receptors

• cause vasodilation and decreased HR

Examples of beta blockers?

- olol drugs

• Propranolol

• Atenolol

• Bisoprolol

How do alpha blockers reduce blood pressure?

Inhibit a-1 adrenergic receptors

• much less effective and less commonly used

Example of alpha blockers?

Doxazosin

How do ACE inhibitors reduce blood pressure?

Inhibit angiotensin converting enzyme (ACE)

• preventing the conversion of angiotensin I to angiotensin II

• Preventing stimulation of aldosterone release

What are examples of ACE inhibitors

-pril drugs

• Ramipril

• enalapril

• lisinopril

How do diuretics decrease blood pressure?

Promote NaCl loss in urine = increasing water loss in urine

• this decreases blood volume = fall in BP

Examples of diuretics

• Bendroflumethiazide

• Furosemide

• Indapamide

How do Ca²⁺ channel blockers reduce BP?

They reduce smooth muscle contraction and cause vasodilation

• this reduces vascular resistance = fall in BP

Examples of calcium channel blockers?

-pine drugs

• Amlodipine

• felodipine