The amount of blood flow through the vessels in the body is controlled by the sympathetic and parasympathetic nervous system, as well as heart rate.

The sympathetic and parasympathetic systems act on arterioles by constricting or dilating them, respectively, and the heart rate affects the total amount of blood circulating over a period of time.

Of course, it goes much deeper than this: osmoreceptors, baroreceptors, and hydration controls via hormones and the kidneys all interplay in an interrelationship between blood pressure and perfusion. Baroreceptors in the aortic arch and carotid sinus mediate stimulation or inhibition of the medulla, adjusting the sympathetic vs parasympathetic dominance of output to the heart and blood vessels. 

Providers can stimulate these baroreceptors in the carotid sinus to decrease heart rate by increasing vagus nerve stimulation. This is often attempted in the setting of atrial tachycardias. Accidental stimulation can occur in some people when wearing collared shirts or ties, which may lead to syncope. 

Blood flow along these vessels is not constant but is controlled depending on need. For example, the musculoskeletal system will increase blood flow when challenged by physical exertion. Likewise, blood flow to other organs--the brain, digestive system, etc., each experience an increase in blood flow when they are active. Blood flow to the integumentary (skin) system is increased or decreased as needed to adjust the body's temperature (thermoregulation).

 

Vasoconstriction and Vasodilation

Blood vessel diameter throughout the body is controlled by the medulla in the brain and the autonomic nervous system through hormone release.

VASOCONSTRICTION: Arterioles are subject to changing their diameter, which gets smaller during vasoconstriction when acted upon by sympathetic nervous system agonists (norepinephrine and epinephrine) at their alpha-1 adrenergic receptors.

VASODILATION: In the same way, arterioles are subject to increasing their diameter via vasodilation when acted upon by parasympathetic nervous system beta-2 adrenergic agonists.

Generally, norepinephrine and epinephrine (released by the adrenal gland) are vasoconstrictive, acting on alpha-1 antagonists on the adrenergic receptors.

In the same fashion, alpha-1-antagonists block the uptake of these hormones and allow vasodilation.

As an exception to this rule, the arterioles of skeletal muscle, cardiac muscle, and the pulmonary circulation vasodilate in response to these hormones acting on beta-adrenergic receptors.

Again as a general rule, stretch (from high volume or forceful contraction) and high oxygen tension promote vasoconstriction, and carbon dioxide and low pH promote vasodilation.

The most significant adjustments of blood flow caused by vasoconstriction and vasodilation occur at the level of the arteriole, where these vessels transition to capillaries and reduce their pressure and velocity to allow for the exchange of gasses and nutrients between them (the capillaries) and tissue cells.

 

Hemodynamics

HEMODYNAMICS: refers to the flow of blood through the body and tissues and depends on certain factors.

BLOOD PRESSURE: 

Blood pressure depends upon adequate venous return to the heart, the pulse pressure or pressure of the contraction during systole (the squeeze of the heart), and peripheral resistance in the arteries and veins.

Factors that maintain systemic blood pressure to homeostatic levels include:

• heart rate,
• force of contraction,
• vessel elasticity,
• blood viscosity,
• hormones, and
• peripheral resistance.

OSMOSIS is the process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane.

Osmosis = Lower → Higher.

DIFFUSION is the opposite of osmosis and is defined as the process by which molecules of a solvent tend to pass through a semipermeable membrane from a more concentrated solution into a less concentrated one, thus equalizing the concentrations on each side of the membrane.

​​​​​​Diffusion = Higher → Lower.

Movement across a concentration gradient requires either energy from enzymes (active transport) or not (passive transport).

Facilitated diffusion is the process of spontaneous passive transport of molecules or ions across a cell’s membrane via the use of specific transmembrane integral proteins.

Passive transport means that the transport does not require chemical energy and instead relies on movement down concentration gradients.

Active transport, however, requires chemical energy from ATP to facilitate movement across the cell membrane.

HYDROSTATIC PRESSURE is the pressure exerted by a fluid on the walls of its container, as in the pressure of blood against the vessel walls.

ONCOTIC PRESSURE is the pressure required to prevent the flow of a liquid across a semipermeable membrane via osmosis.

Blood must overcome the oncotic pressure within the vessels with adequate hydrostatic pressure to allow for blood flow throughout the body.

 

Cardiac Output

Cardiac Output: the product of heart rate (heart contractions per minute) and blood volume ejected per beat (stroke volume).

CO = HR x SV.

Stroke volume is a product of the

• preload (blood volume in the chamber before contraction),
• myocardial contractility (strength of the heart's squeeze), and
• afterload (intra-aortic pressure.)

Impairment of cardiac output can occur with extremely high or low heart rates, low blood volume, a decrease in myocardial contractility, or high blood pressure.

Starling’s law says that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end-diastolic volume) when all other factors remain constant. Increases in volume will increase the contraction produced by each beat. The more you stretch ventricular muscle tissue, the greater the force of the next contraction. (Like a rubber band, the more tightly you pull it, the harder it snaps back.)

 

Autonomic Components

Heart muscle (myocardial) effectiveness relies upon cardiac output and also the influence of the autonomic nervous system on said cardiac output.

The autonomic nervous system (sympathetic and parasympathetic nervous systems) impacts cardiac output through interaction with neural receptors and the secretion of hormones like epinephrine.

Increases in heart rate and stroke volume are associated with stimulation of the sympathetic nervous system.

(Sympathetic nervous system = "fight-or-flight.")

Decreases in heart rate and stroke volume are associated with stimulation of the parasympathetic nervous system.

(Parasympathetic system = "rest-and-digest.")

 

Venous Return (Preload)

Venous return (preload) is affected by the skeletal muscle pump, thoracoabdominal pump, respiratory cycle, and gravity.

Increased Return of Venous Blood to the Heart:

The skeletal muscle pump replaces blood to tissues after muscular exertion, facilitates venous return of blood to the heart, and increases preload. 

The thoracoabdominal pump creates changes in pressure in the thoracic and abdominal cavities, creating a vacuum that also facilitates venous return to the heart.

Decreased Return of Venous Blood to the Heart:

Intermittent positive pressure breathing, positive end-expiratory pressure, continuous positive airway pressure, and bilevel positive airway pressure increase intrathoracic pressure and impede venous return to the heart.

Systemic vascular resistance and capacitance (afterload) are important factors/variables affecting the vascular system.

The Second Heart: The legs are considered the "second" heart of the human body because simple walking forces blood one-way (thanks to venous valves) up the legs as a result of the pressing and squeezing effects of the muscles on the leg veins. With up to 60% of the total blood volume in the musculoskeletal system, most of that in the legs, simple walking is a large component of the return of blood back to the heart.

 

Nitrogen: A Good Gas Gone Bad

Life at 1 Atmosphere: Nitrogen is 78% of the air, but at the alveoli, it is only oxygen that is absorbed into the blood. Nitrogen is an inert gas with equal pressures in the blood and in the ambient air. So, our nitrogen does not come from air/breathing. Instead, the nitrogen in blood is a waste product of amino acid and protein metabolism, resulting in urea, of which nitrogen is a major part. The kidneys excrete the urea. 

Henry's Law: At a constant temperature, the amount of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas. 

Life at Increasing Atmospheres: When divers rise to the surface (from >1 ATM to 1 ATM), gas can be released from tissues due to the tissue pressure of gases exceeding the outside pressure. This free gas forms bubbles that act as obstructing emboli to block blood flow of the blood vessels or even initiate the clotting cycle or the inflammatory process. The location of the bubbles will determine what symptoms occur, but generally...ischemia hurts! 

Life with the Bends: Decompression sickness requires mandatory transport to a hyperbaric chamber that can mimic for the diver a slow ascension to 1 ATM.