Peripheral circulation and Microcirculation

Aorta during early systole

Blood enters aorta faster than it exits causing arterial volume to increase

Fraction of blood ejected in early systole

About 70 percent of stroke volume is ejected in the first one third of systole

If arteries were rigid tubes

There would be no diastolic flow and pressure would fall to zero at end systole

Largest pressure drop in circulation

Occurs across arterioles and precapillary sphincters

Percent of total resistance in precapillary area

About 70 percent

Percent of total resistance in capillaries

About 10 percent

Percent of total resistance in postcapillary vessels

About 20 percent

Definition of systolic blood pressure

Highest arterial pressure during ventricular ejection

Definition of diastolic blood pressure

Lowest arterial pressure at end diastole

Factors determining diastolic pressure

End systolic pressure rate of outflow and duration of diastole

Effect of low arterial compliance on systolic pressure

Increases systolic pressure

Effect of high stroke volume on pulse pressure

Increases pulse pressure

Main determinant of acute arterial pressure changes

Stroke volume

Function of arteries regarding flow

Convert pulsatile flow into steady capillary flow

Smooth muscle key feature slow contraction

Smooth muscle maintains tension with low energy cost

Stretch activation in smooth muscle

Stretch leads to contraction known as the myogenic response

Intrinsic vascular tone

Basal contraction of arterioles even without external input

Local metabolic vasodilator mechanism

Low oxygen or high metabolites cause dilation

Examples of metabolic vasodilators

Include carbon dioxide hydrogen ions potassium ions and adenosine

Histamine source

Released by mast cells and basophils

Effect of histamine

Causes vasodilation and increases vascular permeability

Prostaglandins vascular roles

Some cause vasodilation others cause vasoconstriction

Trigger for myogenic response

Increase in transmural pressure

Purpose of myogenic response

Stabilizes blood flow despite pressure changes

Sympathetic activation effect on veins

Causes venoconstriction and increases venous return

Effect of arteriolar dilation on veins

Increases venous pressure and diameter

Effect of arteriolar constriction on veins

Decreases venous pressure and diameter

Skeletal muscle pump mechanism

Muscle contraction compresses veins and pushes blood toward the heart

Respiratory pump mechanism

Inhalation lowers thoracic pressure and increases venous return

Pericardial effusion risk

May cause cardiac tamponade by restricting filling

Cardiac tamponade effect on preload

Decreases preload and cardiac output

Main purpose of baroreceptor mechanism

Short term regulation of arterial pressure

Carotid sinus baroreceptors location

At the bifurcation of the common carotid artery

Aortic baroreceptors location

In the aortic arch

Baroreceptor adaptation

Reset during prolonged pressure changes making them ineffective long term

Efferent limb of baroreflex

Includes sympathetic and parasympathetic cardiovascular nerves

Increased baroreceptor firing effect

Reduces sympathetic tone and increases parasympathetic tone

Decreased baroreceptor firing effect

Increases sympathetic tone and decreases parasympathetic tone

Sympathetic effect on veins

Causes venoconstriction and increases central venous pressure

Sympathetic effect on arterioles

Causes vasoconstriction and increases total peripheral resistance

Sympathetic effect on heart

Increases heart rate and contractility

Parasympathetic effect on heart

Decreases heart rate and cardiac output

Medullary cardiovascular center location

In the medulla oblongata

Nucleus receiving baroreceptor input

Nucleus tractus solitarius

Arterioles branching pattern

Arteries branch six to eight times before becoming arterioles

Arteriole size

Diameter is about ten to fifteen micrometers

Metarterioles structure

Have intermittent smooth muscle rather than a continuous layer

Precapillary sphincter function

Regulates entry of blood into capillaries

Capillary wall thickness

About zero point five micrometers

Capillary diameter

About four to nine micrometers allowing only one red blood cell at a time

Intercellular cleft width

About six to seven nanometers

Albumin permeability in most tissues

Very low because clefts are smaller than albumin

Caveolae composition

Formed by caveolin with cholesterol and sphingolipids

Caveolae function

Transport macromolecules via endocytosis and transcytosis

Brain capillary pores

Tight junctions that prevent most molecules from passing

Liver capillary pores

Wide clefts allowing nearly all dissolved substances through

Kidney glomerular membrane

Fenestrated allowing very high filtration

Vasomotion

Intermittent opening and closing of metarterioles and precapillary sphincters

Primary regulator of vasomotion

Oxygen concentration in the tissues

Main transport mechanism across capillaries

Diffusion

Rate of water diffusion vs plasma flow

Water diffusion is about eighty times faster

Permeability vs molecular weight

Larger molecules have lower permeability

Interstitial space composition

Contains collagen fibers proteoglycan filaments and fluid

Proteoglycan filament composition

Mostly hyaluronic acid with small amounts of protein

Tissue gel function

Supports diffusion and limits bulk fluid flow

Main method of fluid movement in interstitium

Diffusion

Average capillary hydrostatic pressure

About seventeen millimeters of mercury

Plasma colloid osmotic pressure

About twenty eight millimeters of mercury

Interstitial colloid osmotic pressure

About eight millimeters of mercury

Interstitial hydrostatic pressure

Slightly negative creating outward pull

Net Starling pressure

Slightly positive favoring filtration

Daily lymph load from net filtration

About two to three liters per day

Fluid movement at arterial end

Net filtration outward

Fluid movement at venous end

Net reabsorption inward

Capillary filtration coefficient meaning

Measure of how easily fluid crosses the capillary

Lymphatic function in protein balance

Removes leaked proteins and returns them to blood

Thoracic duct drainage

Drains lower body and left upper body into the left subclavian vein

Right lymphatic duct drainage

Drains right head right arm and right chest into the right subclavian vein

Main driver of lymph flow

Interstitial fluid pressure

Effect of increased capillary permeability

More proteins leak out increasing filtration

Effect of inflammation on capillaries

Increases permeability causing edema

Intrinsic lymphatic pump

Contraction of lymphatic endothelial cells

External lymphatic pump

Caused by muscle contraction arterial pulsation and movement

Lymph flow during exercise

Increases ten to thirty fold

Lymphedema causes

Occurs due to lymphatic obstruction such as in cancer or filariasis

Filariasis effect

Parasitic blockage of lymphatics causing severe edema

Heart failure edema mechanism

Increased venous pressure raises capillary hydrostatic pressure

Liver disease edema mechanism

Reduced protein synthesis lowers plasma oncotic pressure

Kidney disease edema mechanism

Protein loss in urine lowers plasma oncotic pressure

Peripheral vascular system

Continuous system of conduits transporting blood from the left heart to the aorta to the vessels and back to the right heart

Major vessel classifications

Arteries arterioles capillaries venules and veins

Unifying characteristic of vessels

All vessels have a single continuous endothelial cell layer

Largest artery

The aorta with an internal diameter of about twenty five millimeters

Branching effect on cross sectional area

Cross sectional area increases exponentially with branching

Function of arteries

Distribute blood to capillaries and regulate peripheral distribution

Windkessel effect

Elastic recoil of the aorta provides continuous flow during systole and diastole

Arterial systolic function

The aorta expands and stores blood as potential energy during systole

Arterial diastolic function

The aorta recoils and propels blood forward during diastole

Arterioles function

Main resistance vessels that regulate distribution and pressure drop

Compliance definition

Change in volume divided by change in pressure