Ch 17: Local and Humoral Control of Tissue Blood Flow

As arterial saturation decreases, what must occur?

The flow through the region increases in order to almost make up for the decreased amount of oxygen in the blood

Vasodilator substances infiltrate the cells when:

There is a greater rate of metabolism or a lesser availability of oxygen or some other nutrients

Most vasodilators of vasoconstrictors have little effect on long-term blood flow unless:

They alter metabolic rate of the tissues

Vasodilator substances are believed to diffuse through:

Tissues to precapillary sphincters, metarterioles and arterioles to cause dilation

Best characterized vascular growth factors:

Vascular endothelial growth factor Fibroblasts growth factor Angigenin

Angiogenesis

Vascularity increase due to metabolism in a tissue being increased for a prolonged period of time

Increase in calcium ion concentration causes:

Vasoconstriction

Increase in potassium ion concentration causes:

Vasodilation

What happens in tissues when blood flow is reduced and cell metabolism continues at the same rate?

Vasodilator substances (Carbon dioxide, lactic acid, potassium ions) tend to increase

Two theories for regulation of local blood flow when either rate of tissue metabolism changes or the availability of oxygen changes:

Vasodilator theory Oxygen lack theory

Active Hyperemia

When any tissue becomes highly active, the rate of blood flow through the tissue increases

Metabolic Theory

When arterial pressure becomes too great, excess flow provides too much oxygen and too many other nutrients to tissues and washes out the vasodilators, which cause blood vessels to constrict and flow to return nearly to normal despite increased pressure

Reactive Hyperemia

When blood supply to a tissue is blocked for a few seconds to an hour or more and then is unblocked Blood flow through tissue is usually increased immediately 4-7x normal

Location of Macula densa

Where distal tubule lies adjacent to afferent and efferent arterioles at nephron juxtaglomerular apparatus (Kidney)

In what situation do metabolic factors override myogenic mechanisms?

Where metabolic demands of tissues are significantly increased - vigorous exercise - causes dramatic increases in skeletal muscle blood flow

Can blood vessels be made to disappear when not needed?

Yes

Vasopressin

antidiuretic hormone More powerful than Angiotensin II as a vasoconstrictor One of the body's most potent vascular constrictor substances Formed in hypothalamus Transported down by nerve axons to posterior pituitary gland

Increase in carbon dioxide concentration causes:

moderate vasodilation in most tissues but marked vasodilation in brain

6 specific needs for blood flow to tissues:

1) Delivery of oxygen 2) Delivery of other nutrients (glucose, amino acids, fatty acids) 3) Removal of carbon dioxide 4) Removal of hydrogen ions 5) Maintenance of proper concentrations of other ions 6) Transport of various hormones and other substances

Stages of creating new vascular channels:

1) dilation of small vascular loops that already connect vessel above blockage to the vessel below 2) Blood flow is still less than 1/4 of what is needed, so openings continue to occur within ensuing hours 3) vessels continue to grow for many months - forming multiple small collateral channels rather than one single large vessel

Dual system of control of norepinephrine and epinephrine:

1) direct nerve stimulation 2) indirect effects of norepinephrine/epinephrine in circulating blood

How long does it take for arterioles and capillary vessels to increase in number and size to meet the needs of the tissue?

A few weeks (unless pathological or too old to respond)

When a vein or artery is blocked, what happens?

A new vascular channel usually develops around blockage and allows at least partial resupply of blood to the affected tissue

How much vasopressin is released?

A very small, minute amount (plays little role in vascular control)

Anions that have significant effects on blood vessels are:

Acetate and citrate

Two phases of local blood flow:

Acute control Long-term control

Oxygen is important for:

Acute control of local blood flow and for long-term control

Decreased availability of oxygen causes which two substances to be released into the spaces between the tissue cells?

Adenosine and Lactic Acid (containing hydrogen ions)

What must be brought to the kidneys in order to allow the kidneys to regulate fluid volumes and electrolytes as well as to excrete waste products?

Adequate amounts of blood plasma

Adenosine is:

An important local vasodilator for controlling local blood flow

Endostatin

Antiangiogenic peptide derived from breakdown of collagen type XVII

Potential use of antiangiogenic substances

Arresting blood vessel growth in cancerous tumors and preventing large increases in blood flow needed to sustain nutrient supply of rapidly growing tumors

Slight decrease in hydrogen ion concentration causes:

Arteriolar constriction

Myogenic response is most pronounced in which type of vessel?

Arterioles

At what level is the blood flow to each tissue regulated?

At the minimal level that will supply the tissue's requirements (no more, no less)

Precapillary sphincter

At the origin of the capillary

Bradykinin

Cause powerful vasodilation Small polypeptides that are split away by proteolytic enzymes fro alpha2-globulins in the plasma or tissue fluids

Skin blood flow is controlled largely by:

Central nervous system through the sympathetic nerves

Function of kidneys

Cleansing the blood of waste products

Angiotensin II

Constrict the small arterioles powerfully Normally acts on many of the arterioles of the body at the same time to increase total peripheral resistance (increasing arterial pressure) Integral role in regulation of arterial pressure

Vasomotion

Cyclical opening and closing of precapillary sphincters and metarterioles Open and close cyclically several times per minute

An increased utilization of oxygen in the heart is followed by:

Decreased oxygen concentration in the heart muscle cells Consequent degradation of ATP Increased release of adenosine

What leads to the formation of vascular growth factors?

Deficiency of tissue oxygen or other nutrients

In active Hyperemia, the increase in local metabolism causes the cells to:

Devour tissue fluid nutrients rapidly and also to release large quantities of vasodilator substances

Increase in either or booth of carbon dioxide and hydrogen in the brain causes:

Dilation of cerebral vessels and allows rapid washout of excess carbon dioxide or hydrogen ions from brain tissues

Increase in hydrogen ion concentration (decrease in pH) causes:

Dilation of the arterioles

Angiostatin

Fragment of protein plasminogen Inhibitor of angiogenesis

What two vasodilator agents have the ability to increase capillary porosity, allowing leakage of both fluid and plasma protein into the tissues?

Histamine and Bradykinin

Nitric oxide

Important endothelial-derived relaxing factor Lipophilic gas that is released from endothelial cells in response to a variety of chemical and physical stimuli

Tubuloglomerular Feedback

In kidneys. Composition of fluid in early distal tubule is detected by an epithelial structure of distal Dubuque itself called macula densa

Job of vasopressin

Increase greatly water reabsorption from renal tubules back into blood

Each tissue controls its own local blood flow in proportion to:

Its metabolic needs

Endothelin

Large 21 amino acid peptide that requires only nanogram quantities to cause powerful vasoconstriction Increased release is also believed to contribute to vasoconstriction when endothelium is damaged by hypertension

Epinephrine

Less powerful vasoconstrictor, and in some tissues even causes mild vasodilation

Lack of glucose in perfuming blood can cause:

Local tissue vasodilation

Which is better - acute or long-term control - at controlling the flow in proportion to the needs of the tissues?

Long-term control

Vascularity is measured by:

Maximum level of blood flow needed rather than by average need

The precise mechanisms by which changes in pressure cause opening or closing of vascular ion channels are uncertain but likely involve:

Mechanical effects of pressure on extracellular proteins that are tethered to cytoskeleton elements of the vascular wall or to the ion channels themselves

Angiotensin factors cause:

New vessels to sprout form other small vessels Step 1) dissolution of basement membrane of endothelial cells as point of sprouting Step 2) Rapid reproduction of new endothelial cells that stream outward through vessel walls Step 3) Cells continue to divide and fold over into a tube Step 4) Tube connects with another tube budding from another donor vessel and forms a capillary loop

During stress or exercise, sympathetic nerve endings in individual tissues release:

Norepinephrine

One of the most necessary of the metabolic nutrients is:

Oxygen

Smooth muscle requires ___________________ to remain contracted

Oxygen

Tissues almost never suffer from _____________ nutritional deficiency

Oxygen

Oxygen Lack/Nutrient lack theory

Oxygen and other nutrients are required as one of the metabolic nutrients to cause vascular muscle contraction. Without adequate oxygen, it is reasonable to believe that blood vessels would relax and dilate Could decrease availability of O2 to smooth muscle fibers

Bradykinin causes:

Powerful arteriolar dilation Increased capillary permeability

Norepinephrine

Powerful vasoconstrictor

Increase in magnesium ion concentration causes

Powerful vasodilation

Acute control is achieved by:

Rapid changes in local vasodilation or vasoconstriction of arterioles, met arterioles, and precapillary sphincters

Histamine

Released in every tissue of the body if the tissue becomes damaged or inflamed or is the subject of an allergic reaction Derived from mast cells in damaged tissues and from basophils in the blood

Retrolental fibroplasia

Retinal vessels grow out from retina in to the eye's vitreous humor and cause blindness

Autoregulation

Return of blood flow toward the normal level

Angina Pectoris

Severe chest pain caused by ischemia of the heart muscle

What type of stress occurs when blood flows through arteries and arterioles?

Shear stress Because of viscous drag of blood against vascular walls

Long-term control is:

Slow, controlled changes in flow over a period of days, weeks or months

Angiotensin II

Stimulates synthesis of nitric oxide and binds to specific receptors on endothelial cells

Myogenic Theory

Sudden stretch of small blood vessels causes smooth muscle of vessel walls to contract

Nitric oxide synthase enzymes

Synthesize nitric oxide from arginine and oxygen and by reduction of inorganic nitrate