Cardiovascular Control 1n23e

CARDIOVASCULAR CONTROL Marinette Rumusud-Jambaro,MD Diplomate in Internal Medicine

• Control of blood volume Aim: ensures that all the organs receive sufficient amount of blood Mechanisms: 1. Neural 2. Endocrine 3. Renal

Nervous Regulation of the Arterial Pressure • more global functions, such as; • redistributing blood flow to different areas of the body • increasing or decreasing pumping activity by the heart, • especially, providing very rapid control of systemic arterial pressure.

Nervous Regulation of the Circulation Autonomic Nervous System 1. Symphathetic NS - control circulation 2. Parasymphathetic NS - Regulation of heart function

Nervous Regulation of the Circulation • Transmit parasympa impulses through the vagus n. to the heart • Sympa impulses through the cord and peipheral sympa nerves to almost all blood vessels of the body

Nervous Regulation of the Circulation Symphathetic NS - Inc heart rate - Inc contractility of the heart

Parasymphathetic NS - Dec in heart rate - Dec pumping contractility of the heart

Nervous Regulation of the Circulation Autonomic Nervous System 1. Symphathetic NS - control circulation

Sympathetic Vasoconstrictor System and Its Control by the Central Nervous System • Tremendous vasoconstrictor nerve fibers • Few vasodilator fibers • Thus, vasoconstriction or vasodilation is produced by increasing or decreasing sympathetic activity

Vasomotor Centers of the Brain • Located bilaterally mainly in the reticular substance of the medulla and of the lower third of the pons

Vasomotor Centers of the Brain 1. A vasoconstrictor area - located bilaterally in the anterolateral portions of the upper medulla. - The neurons originating in this area distribute their fibers to all levels of the spinal cord, where they excite preganglionic vasoconstrictor neurons of the sympathetic nervous system. 2. A vasodilator area - located bilaterally in the anterolateral portions of the lower half of the medulla. - The fibers from these neurons project upward to the vasoconstrictor area - they inhibit the vasoconstrictor activity of this area, thus causing vasodilation.

Vasomotor Centers of the Brain . 3. A sensory area - located bilaterally in the tractus solitarius in the posterolateral portions of the medulla and lower pons. - The neurons of this area receive sensory nerve signals from the circulatory system mainly through the vagus and glossopharyngeal nerves, and output signals from this sensory area then help to control activities of both the vasoconstrictor and vasodilator areas - thus providing “reflex” control of many circulatory functions. - An example is the baroreceptor reflex for controlling arterial pressure.

Continuous Partial Constriction of the Blood Vessels • Is Normally Caused by Sympathetic Vasoconstrictor Tone. • Maintains a partial state of contraction of the blood vessel • “vasomotor tone”

Effect of total spinal anesthesia on the arterial pressure, showing marked decrease in pressure resulting from loss of “vasomotor tone”

Norepinephrine (NE) • Neurotransmitter of the sympathetic nervous system • Secreted at the endings of vasoconstrictor nerves • Acts directly on α-adrenergic receptor of vascular smooth muscle to cause vasoconstriction

NE and E • Released from adrenal medulla during sympathetic stimulation • α – adrenergic – vasoconstriction (NE,E) • β- adrenergic – vasodilation (E)

Vasomotor System influenced by Higher Nervous Centers 1. Reticular substance (pons,mesencephalon,diencephalon) - Cardioaccelerator /cardioinhibitor 2. Hypothalamus - Temperature changes - Vessel of skin constrict (heat conservation) - Dilate(heat dissipation) Emotional stress- influence HR & BP - Stimulate or inhibit medullary centers

Vasomotor System influenced by Higher Nervous Centers 3. Cerebral cortex - Stimulation causes various motor responses - Fight or flight response- complex response - inc HR, inc CO, inc BP in anticipation of flight or physical activity

Vasomotor System influenced by Higher Nervous Centers Emotional Fainting—Vasovagal Syncope. • In this case, the muscle vasodilator system becomes activated, and at the same time, the vagal cardioinhibitory center transmits strong signals to the heart to slow the heart rate markedly. • The arterial pressure falls rapidly, which reduces blood flow to the brain and causes the person to lose consciousnesS • Emotional fainting begins with disturbing thoughts in the cerebral cortex. • The pathway probably then goes to the vasodilatory center of the anterior hypothalamus next to the vagal centers of the medulla, to the heart through the vagus nerve.

Role of the Nervous System in the Rapid Control of Arterial Pressure 1. Most arterioles throughout the body are constricted – inc TPR---inc BP 2. The veins and larger vessels of the circulation are constricted – displacing blood from the peripheral vessels towards the heart----inc pumping force of the heart – inc arterial P 3. The heart is directly stimulated by autonomic nervous system– enhanced cardiac pumping

Autonomic Nervous System Increased arterial Pressure 1. During exercise - muscle require inc blood flow - motor areas of NS activated to cause exercise most of the RAS in the brain is also activated----cardio accelerator 2. “alarm reaction” inc arterial P to immediately supply blood to muscles that might need to respond instantly to flee from perceived danger

Reflex Mechanism for Maintaining Arterial Pressure • there are multiple subconscious special nervous control mechanisms that operate all the time • to maintain the arterial pressure at or near normal. • Almost all of these are negative reflex mechanisms

Reflex Mechanism for Maintaining Arterial Pressure • A rise in arterial pressure stretches the baroreceptors and causes them to transmit signals into the central nervous system. • “” signals are then sent back through the autonomic nervous system to the circulation to reduce arterial pressure downward toward the normal level.

Reflex Mechanism for Maintaining Arterial Pressure Arterial Baroreceptor Reflex Control System - Initiated by stretch receptors (baroreceptors) - Walls of large systemic arteries (carotid sinus & aortic arch)

Reflex Mechanism for Maintaining Arterial Pressure Baroreceptor is located in adventitia of carotid sinus and aortic arch Characteristics: a. Response to stretch, not to pressure itself b. Activity is directly related to BP level c. More sensitive to pulsatile pressure than nonpulsatile pressure.

Reflex Mechanism for Maintaining Arterial Pressure

inc

BP

Carotid &aortic (inc. impulse firing) receptors

NTS vagal center

cardiac vagus

HR

cardiac symp CVLM

TPR

RVLM

inh vasoconstrictor

CO

PR dec BP

Inhibition of symphathetic and stimulation of parasympathetic activity 1. Vasodilation of veins and arterioles 2. Dec. HR 3. Dec. strength of contraction --- dec in arterial P due to dec in peripheral resistance, dec CO----

Baroreceptors as a “Buffer” Maintains arterial Pressure during: 1. Changes in body posture - supine- upright = P in head and upper parts fall --- reduction in P dec. signals sent to vasomotor centers --- strong sympa act --non- functional baroreceptors – dec cerebral blood flow ---LOC

Baroreceptors as a “Buffer” Maintains arterial Pressure during: 2. Daily activities (eating, excitement,defacation) --inc P primary purpose: To reduce daily variation in arterial Pressure.

Baroreceptors “buffer”

Control of Arterial Pressure: Chemoreceptor • O2 lack • CO2 excess • H+ excess • •

-----carotid bodies ----- aortic bodies

Significances of chemoreceptor reflex Under normal condition chemoreflex plays a litte role in control of cardiovascular activity, but in an emergency (asphyxia, hypoxia, acidosis, severe hypotension) blood pressure is maintained by this reflex.

Kidneys in the Long-term control of Arterial Pressure - Closely linked to control of body fluid volume by the kidneys - “ renal-body fluid system” - renal sodium excretion/reabsorption - Water excretion/reabsorption - Slow mechanism but most potent for long term BP regulation

Renal output of salt and water -

Effect of arterial P on urine output Pressure diuresis( inc output volume) Pressure natriuresis (Na) Balance bet renal output and intake of salt and water

Renal output of salt and water 2 factors determining long-term arterial P 1. Renal output curve for salt and water 2. The level of salt and water intake --so long as these 2 factors remains constant, arterial P also remains exactly at normal level of 100mmHg.

Renin-Angiotensin-Aldosterone System

JG cells • Renin is synthesized and stored in juxtaglomerular cells (JG cells) of the kidneys. • JG cells are modified smooth muscle cells located in the walls of the afferent arteriole

RAA syatem Angiotensin II 1. Constricts efferent arterioles

2.salt and water retention ---inc bld volume ---inc BP

RAA syatem Angiotensin II 3.Stimulates adrenal glands - Secrete aldosterone - inc salt and water reabsorption by the epithelial cells of renal tubules. --- inc blood volume--- inc BP

Volume loading and hypertension • Artificial kidneys • Tumor of adrenal glands --excessive aldosterone

Causes of hypertension Impaired renal circulation 1. Constriction of the afferent arterioles 2. Stenosis of renal arteries 3. Dec. GFR

Causes of hypertension 4. one-kidney Goldblatt hypertension 5. two-kidney Goldblatt hypertension 6. Coarctation of the aorta 7. Pathcy ischemia of one or both kidneys

Causes of hypertension 8. Toxemia of pregnancy -- impaired renal natriuresis - Thickening of glomerular membranes 9. primary/essential hypertension -90%

Thank you…..

Regulators of the Heart: Factors Influencing Stroke Volume

Figure 14-31: Factors that affect cardiac output