Blood Pressure Control: Neural, Hormonal, Local

• There are three levels to control Blood Pressure and Blood flow: Neural, Hormonal and Local.

Neural Control

• Uses cardiovascular reflexes to control blood pressure and blood flow.

Hormonal Control

• Catecholamines, Antidiuretic hormone, angiotensin II, and aldosterone hormones are central.
• ANP (Atrial natriuretic peptide) and histamine hormones are also critical.

Local Control

• Has its own regulatory system for blood pressure and blood flow.

Mechanisms of Homeostasis

• Receptors sense changes in the internal or external environment.
• The Control center processes information from receptors and sends a response to the effectors.
• The Effector adjusts the regulated parameter.

Negative Feedback Loops

• Changes in monitored parameters cause a control mechanism to counteract further changes of the parameter in the same direction.
• Results in a change that triggers a reversal of the detected change.

Positive Feedback Loops

• Changes in monitored parameters cause a control mechanism to cause/amplify changes of the parameter in the same direction.
• Results in a change that triggers amplification of the detected change.

The Cardiovascular (CV) Center

• Located in the medulla oblongata.
• The CV center includes a collection of gray matter regions ie. nuclei.
• There are 2 cardiac centers and the vasomotor center.
• Helps regulate Heart Rate, Stroke Volume, and blood vessel diameter.
• Receives input from higher brain centers (the cerebral cortex, the limbic system, and the hypothalamus).
• Receives input from peripheral afferent nerve fibers (baroreceptors, chemoreceptors, and proprioceptors).
• Increased frequency of sensory nerve impulses are sent to the CV center from higher brain centers, proprioceptors, baroreceptors, and chemoreceptors.
• Increased frequency of motor nerve impulses are sent from the CV center to effectors.
• The heart has a decreased rate, and blood vessels experience vasoconstriction.
• The cardioacceleratory center increases heart rate and contractility via sympathetic neurons.
• The cardioinhibitory center decreases heart rate via parasympathetic neurons.
• The vasomotor center regulates blood vessel diameter via sympathetic nerves that synapse on arteriolar smooth muscle and cause vasoconstriction.

Cardiovascular Reflexes

• Cardiovascular reflexes are produced by afferent signaling of baroreceptors, chemoreceptors and proprioceptors.
• Baroreceptors are the important receptors in cardiovascular regulation.

Chemoreceptors

• These are Located in the carotid sinus and in the walls of the ascending aorta.
• Those in the carotid sinus signal the medulla via cranial nerve IX.
• Those in the aortic bodies signal the medulla via cranial nerve X.
• They Respond to increased hydrogen ion content, increased CO2, and hypoxia.
• Chemoreceptors activate the sympathetic division, leading to increased Heart Rate, stroke volume, and Vasoconstriction.

Baroreceptors

• High pressure baroreceptors are located in the carotid sinus and aortic arch.
• They enter the CV center via cranial nerves IX & X, .
• Low pressure baroreceptors are located in the walls of the right atrium and vena cavae
• They enter the CV center via cranial nerve X.

Baroreceptor-Initiated Reflexes

• Some stimulus disrupts homeostasis by decreasing blood pressure.
• Baroreceptors in the arch of the aorta and the carotid sinus are stretched less sending decreased rate of nerve impulses.
• The CV center in the medulla oblongata and the adrenal medulla send out an increased sympathetic, decreased parasympathetic stimulation and increased secretion of epinephrine and norepinephrine.
• Effectors respond with Increased stroke volume and heart rate increase cardiac output.
• Blood vessels constrict increasing systemic vascular resistance which results in an increased blood pressure.
• Homeostasis is achieved when blood pressure is back to normal via increased cardiac output and increased vascular resistance.

Baroreceptor-Initiated Reflexes

• Arteries are stretched in response to elevated blood pressure resulting in baroreceptors increasing firing rate.
• Cardioinhibitory neurons are stimulated resulting in reduced heart rate and increased vagal tone.
• The vasomotor center is inhabited resulting in vasodilation, decreased vasomotor tone, and reduced sympathetic tone.

Catecholamines

• They circulate and bind directly to cardiac muscle fibers and blood vessel smooth muscle cells.
• This results in an increase in Heart Rate and Stroke Volume and constriction of veins and arterioles.
• Arteries supplying the brain and heart have little smooth muscle and are not subject to vasoconstriction from the sympathetic division.
• Vessels in the brain and heart autoregulate.

Anti-diuretic Hormone (ADH)

• It is produced by the hypothalamus and released from the posterior pituitary when blood loss is severe, and blood pressure is reduced.
• ADH Causes widespread vasoconstriction and decreased water loss (from skin & urine):
• Kidneys retain more water, which decreases urine output.
• Arterioles constrict, which increases blood pressure.
• Sudoriferous (sweat) glands decrease water loss by perspiration from skin.

Angiotensin II

• It is one of the powerful vasoconstrictor substances known, and usually acts on all arterioles simultaneously when released into the blood.
• This results in an increase in vascular resistance (when blood pressure is too low).
• Low blood pressure causes a renal cascade known as the RAA System.
• Low blood pressure causes the kidney to release renin.
• Angiotensinogen (released by the liver) converts renin into angiotensin I.
• The lungs convert Angiotension I into Angiotension II by angiotensin converting enzyme (ACE), which is located in the endothelial cells of the lung.
• Dehydration, sodium deficiency, or hemorrhage results in decreased blood volume, the Juxtaglomerular cells of kidneys respond by increasing renin.
• The liver increases Angiotensinogen.
• The lungs which includes angiotensin converting enzyme converts Angiotensin I into Angiotensin II which leads to Vasoconstrictions of arterioles increasing blood pressure.

Aldosterone

• Angiotensin also controls aldosterone secretion, when Angiotensin II levels increase, aldosterone is secreted by the adrenal cortex.
• Aldosterone increases salt and water reabsorption which raises blood volume, which raises blood pressure.
• Angiotensin II triggers the adrenal cortex to increase aldosterone which causes vasoconstriction of arterioles and cause the blood pressure to increase until it returns to normal.

Atrial Natriuretic Peptide (ANP) & Histamine

• ANP is released by cells of the atria when blood pressure is high triggering vasodilation
• ANP promotes loss of water and salt by the kidneys.
• Collectively, this reduces blood volume and vasoconstriction which acts to lower blood pressure.
• Histamine is released by mast cells, which causes vasodilation by relaxing blood vessel smooth muscle.
• Histamine is important in increasing the rate of blood flow to inflamed or damaged tissue.

Overall Regulation of "Organ Blood Flow"

• Neural regulation of flow refers to changes in flow due to vasoconstriction.
• Humoral flow control refers to changes in vessel diameter due to circulating hormones.
• Major regulatory factor is tissue metabolic activity triggering metabolic, or “local”regulation, is blood vessel dilation due to substances released by tissue cells.

Explore the three levels of blood pressure and blood flow control: neural, hormonal, and local mechanisms. Understand the cardiovascular reflexes, key hormones like catecholamines and ANP, and the local regulatory systems. Also learn about homeostasis, negative feedback loops, and positive feedback loops.