Physiology Exam 3 Lecture 9C [Regulation of The Vascular System and Blood Flow: Cabeza]

3 ways to control blood flow

Neurogenic control (nervous system) is fast

Local influences and hormones

Intrinsic smooth muscle controls (auto regulation)

Parasympathetic control

Heart- only SA and AV node to decrease heart rate

Vessels- Almost none except for facial skin and genitals

Sympathetic control

Heart- Whole heart- increases heart rate and myocardial contractility thus increasing cardiac output

Vessels- Primarily causes constriction of vessels (some dilation)

- Arteriole: affects almost all body arterioles

- Veins and venules: Causes constriction to increase blood return

Most of the blood is stored in

venous system

Sympathetic stimulation causes:

increased in vascular tone and thus increase in venous return

increased in cardiac output

Increase in blood pressure (due to increased CO and Vascular resistance)

Neurogenic control mainly affects

mean arterial pressure (MAP)

Neurogenic control can shift major blood flow away from specific systems to increase MAP

Decrease kidney perfusion

Decrease gut and spleen perfusion

Decrease skin perfusion

No sympathetic innervation of

precapillary sphincters

Has no control over local blood flow

What regulates sympathetic tone to blood vessels

Baroreceptors and the carotid sinus receptors

Baroreceptors Control

high and low blood pressure

Elevated blood pressure means that the aorta and carotid sinuses are going to stretch. The baroreceptors ...

as they get stretched they are going to increase the rate of firing

Increased firing inhibits the vasomotor center

Increased firing stimulates cardioinhibitory center

Inhibition of vasomotor center

Reduce sympathetic tone --> Reduce vasomotor tone --> Vasodilation --> Decrease BP

Stimulation of Cardioinhibitory Center

Increase Parasympathetic tone --> Reduce HR --> Decrease Blood pressure

SUMMARY:Blood flow to any given part of the body is regulated by

neuronal regulation, Local factors and hormones, and by intrinsic mechanisms

SUMMARY: Most blood vessels only respond to sympathetic tone

Most contract in response to sympathetic signaling

Some, like skeletal muscle, dilate in response to sympathetic signaling

SUMMARY: Sympathetic signaling increases

heart rate, CO, Total peripheral resistance, mean arterial pressure, and venous return to the heart

SUMMARY: Baroreceptors found on the carotid sinuses and the aortic arch, increase their firing with

increased blood pressure. This increase in firing causes a decrease in sympathetic firing and increase in parasympathetic firing of neurons controlling the heart and the blood vessels. The opposite happens when blood pressure is low

SUMMARY: Prolonged hypertension causes a

reset of the baroreceptors such that their new firing rate is lower at any given pressure. This is why most hypertension does not resolve on its own

Adrenaline, released by the adrenal medulla,l into the circulation has the same effect as

stimulating the sympathetic nerves

Antidiuretic Hormone increases

Mean arterial Pressure

- Primarily works on the kidneys

- Has a mild vasoconstrictive effect

Angiotensin II increases

MAP

- Primarily affects kidneys through the action of aldosterone

- Very potent vasoconstrictor (some hypertensive drugs work by antagonizing the angiotensin system)

Atrial Natriutetic Factor

Decreases MAP

- Is released by atria when stretched and it works primarily on the kidney to increase urine volume

Most hormones with the exception of adrenaline, decrease or increase MAP by affecting

blood volume through actions on the kidneys

Vascular beds control the amount of blood flow by regulating the

precapillary sphincters

Blood flow is maintained just enough to meet the

metabolic needs of the cells perfused by the capillary but not more

The local situation controls the

precapillary sphincters

Active cells begin to use O2 to make ATP and produce CO2 as a biproduct. The increase in CO2 will cause the

Acidification of the intracellular/extracellular space.

Increased acidity will cause

vascular smooth muscle to relax

Adenosine is also released from very active cells when sufficient

O2 is not available

Vessel smooth muscles relax because of

MLCK inhibition

- Vasodilation leads to decreased resistance and increase in blood flow

When Oxygen levels are high due to the low activity of tissues, the smooth muscle precapillary sphincters are better able to contract

this causes the diameter of the vessels to decrease, thus, causing an increase in resistance and a decrease in blood flow

When levels of activity markers (CO2, K+, Adenosine, H=) increase they tent to activate Katp channels and decrease L-type calcium channel activity:

This leads to hyperpolarization and little calcium entry so smooth muscles relax and vessels dilate

SUMMARY: Hormones can alter MAP

Adrenaline mimics sympathetic activity

Most hormones that affect MAP do so by acting on the kidneys

SUMMARY: Local factors that can be released when cells are metabolically active usually cause pre-capillary sphincter to relax and thus increase blood flow to the area

adenosine, K+, Low pH, cause vasodilation

Higher pH and oxygen tension cause vasoconstriction

SUMMARY: Adenosine increase the level of cAMP in vessel smooth muscles causing them to

relax (MLCK inhibition by PKA)

The vasculature is able to maintain a steady blood flow even with changes in___ this is due to an ________ of the smooth muscles of the arterioles

MAP, autoregulation

Autoregulation is very important for organs to maintain

even blood flow in spite of changes to cardiac output

Although most of the vasculature responds in ways already discussed, there are 2 cases where the flow of blood is regulated in special ways to accommodate the specific needs of the organs

Cerebral circulation is primarily regulated by autoregulation

Cardiac circulation can only take place during diastole

Cerebral circulation: Regulation of blood flow to specific capillary beds is determined by local factors like

CO2

During systole the heart tissue is so compressed that blood flow to the heart is

very small

SUMMARY: Vessel smooth muscle can react to stretch by

contracting against the stretch

SUMMARY: Vessel smooth muscle can hyperpolarize if not stretch thus

increasing blood flow

- Hyperemia is partially caused by this mechanism

SUMMARY: The major arteries supplying the head and brain have the ability to maintain blood flow constant over a large range of

pressures

SUMMARY: The heart is the only major ogan that is perfused during

diastole and not systole

- This occurs because during systole the muscle is compressed and the arterial diameters are small

- During diastole, the coronary arteries are supplied with blood because of the high aortic pressure

- The coronary arteries open to the aorta next of the aortic valve in coronary cusps of the valve

KEY TERMS: Neurogenic

Refers to control by the nervous system. In the case of the cardiovascular system it refers to control by the Autonomic nervous system

KEY TERMS: Baroreceptors

Sensory receptor cells that respond to stretch by increasing their firing rates. These cells are found in the carotid sinuses and on the aortic arch

KEY TERMS: Local control

Refers to factors taken in or released by the tissues of a given capillary bed. Factors that have important effects on the precapillary sphincters are oxygen levels, CO2 levels, adenosine levels, and lactic acid and H+ levels

KEY TERMS: Autoregulation

An intrinsic property of the arteriolar smooth muscle layer and the precapillary sphincters. When these smooth muscle cells are stretched they respond by contracting and thereby decrease the diameter of the blood vessels

KEY TERMS: Vascular tone

The level of smooth muscle contraction at any given time. Under normal circumstances, the sympathetic system fires at low levels of activity to keep the blood vessels partially contracted. Changes in sympathetic firing alter the level of contraction of the vessels

KEY TERMS: Vasomotor center

The area of the medulla, in the brain, that controls the firing rate of sympathetic neurons innervating the blood vessels. This center responds to the firing rate of Baroreceptors

KEY TERMS: Cardioinhibitory center

The area of the medulla which decreases the firing rate of sympathetic neurons innervating the heart and increases the firing rate of parasympathetic neurons innervating the SA and AV nodes of the heart

KEY TERMS: Carotid sinus

An area of widening on each carotid artery that contains blood pressure sensing baroreceptors

KEY TERMS: Aortic arch

an area of the aorta that contains blood pressure sensing baroreceptors

KEY TERMS: Antidiuretic hormone

A hormone produced by the posterior pituitary that increases water reabsorption and has mild vasoconstrictive effects

KEY TERMS: Angiotensin II

A hormone regulated by kidney function which has strong vasoconstrictive effects

KEY TERMS: Hyperemia

An increase in blood flow to an area without a substantial change in the need for blood

KEY TERMS: Systolic compression

The compression of the myocardium during systole which decreases blood flow to the heart. Most of the cardiac blood flow occurs during diastole when the myocardium is not compressed