Local Control of Blood Flow

Amount of blood flowing through a tissue in a given time (ml/min)

Tissues usually regulate their own blood flow according to their own metabolic & functional needs

To do this would require many times more blood flow than the heart can pump

tissue’s requirement

Blood flow to each tissue usually is

regulated at the minimal level that

will supply the tissue’s requirement

▪ No more , No less

Highest-kidneys=22 // liver=27

Lowest-adrenal glands=0.5

Acute/Long term

Achieved by rapid changes in local vasodilation or vasoconstriction

blood flow.of the

1-arterioles,

2-metarterioles,

3- precapillary sphincters,

occurring within seconds to minutes.

▪ To provide very rapid maintenance of appropriate local tissue

Intrinsic

Ex:

1- Myogenic regulation

2- Local chemical agents (Tissue metabolites – Endothelial products – Oxygen concentration)

3-Local temperature

Extrinsic

1-Neural (Autonomic)

▪2-Humoral (circulating hormones)

Due to ↑ and ↓ in the physical size and numbers of actual blood

vessels supplying the tissues.

Controlled changes in flow over a period of days, weeks, or even

months. ▪ Provide even better control of the flow in proportion to the needs

of the tissues.

Ex:

1-Angiogenesis(the development of new blood vessels.)

2–Vascular remodeling

Increase in tissue metabolism leads to increase in blood flow.

Decrease in oxygen availability to tissues increase tissue blood flow.

Two major theories

1. Vasodilator theory

2. Oxygen lack or(demand) theory

1- yes

2-Increased metabolite concentration leads to

local arteriolar smooth muscle relaxation

- Vasodilatation increases local blood flow ▪

-Metabolites washed away ▪

- Metabolite concentration decreases ▪

- Vasodilation reverses and blood flow returns to

normal

Response to hypoxia in systemic circulation is vasodilation

– Hypoxia leads to drop in ATP availability in smooth muscle –K

ATP

-K atp channels open ( K+ efflux leading to hyperpolarization)

– Smooth muscle relaxation

Endothelial factor increases smooth muscle sensitivity to Ca2+

(((vasoconstriction in respond to hypoxia Only in pulmonary not systemic))))

1- At high altitude

2-Carbon monoxide poisoning

• Poisons the ability of Hb to transport O 2

3-Pneumonia (dec O2 exch)

4- Cyanide poisoning

• Poisons ability of the tissues to use O2

1-There is formation of vasodilator substances in the tissue cells and act

on pre-capillary sphincters, metarterioles, and arterioles.

2-

i. Greater rate of metabolism

ii. Less availability of O 2

iii. Nutrients to tissue ↓

The number of precapillary sphincters that are open at any given time is roughly proportional to the requirements of thetissue for nutrition.

Increased local concentration of:

–1-Adenosine (cardiac muscle)

2– Carbon dioxide (skin, brain)

3– Potassium ions (skeletal muscle, brain)

4– Hydrogen ions (brain)

5– Histamine (injured tissue)

• Low local concentrations of Nutrients

1-glucose

2-thiamine

3-niacin

4- riboflavin

• Increased local temperature(direct vasodilator effect)

Oxygen required to cause vascular muscle contraction.

blood vessels simply relax and therefore naturally dilate.

Increased utilization of oxygen in the tissues as a result of increased metabolism could decrease the availability of oxygen to the smooth muscle fibers in the local blood vessels causing local vasodilation.

Increase in Blood Flow

Active Hyperemia:

▪ When any tissue becomes highly active, the rate of blood flow to the tissue increases.such as an

1-exercising muscle,

2-a GI gland during hypersecretory period

3- the brain during rapid mental activity

▪ 20-folds increase of blood flow to skeletal muscle during intense exercise

Reactive Hyperemia:

▪ When the blood supply to tissue is blocked for a few seconds to as long

an hour or more and than is unblocked, blood flow through the tissues

usually increases immediately to four to seven times.

▪ Lack of flow sets into motion all of those factors that cause vasodilation.

-When there is Lack of Glucose – local tissue vasodilation ▪ Deficiency of Amino acids and Fatty acid give similar effect

-Vitamin Deficiency

Beri Beri in which the patient has deficiencies of Vitamin B substances

(Thiamine, Niacin and Riboflavin)

▪ In their deficiency tissues become unable to take oxygen, decreased peripheral resistance causing vasodilatation.

▪ Peripheral Blood flow increases twofold to threefold.

In any tissue of the body, acute ↑ in B.P. causes

blood flow to ↑ .. BUT .. within less than 1 min

returns back to normal.

B/w B.P. 70-175 mmHg

1. Metabolic theory of Autoregulation

2. Myogenic theory of Autoregulation

when BP Increases, the excess blood flow provides more O2 and nutrients to the tissue.

▪ Excess O2 cause blood vessels to constrict and the flow return back to normal despite increase pressure.

▪ When high arterial pressure stretches the vessel, reactive vascular

constriction results, which reduces blood flow nearly back to normal.

▪ Stretch-induced depolarization of vascular myocytes

– Stretch-sensitive ion channels

▪ Leads to opening of L-type Ca2+ channels

– Increases in intracellular [Ca2+]

– Contraction of myocytes

When B.P. ↑ there is ↑ stretch of small blood vessels causes reactive

vascular constriction of the smooth muscle that reduces blood flow

nearly back to normal.

▪ When B.P. ↓ the degree of stretch of the vessel is less → smooth

muscle relaxes → reducing vascular resistance and helping to return

flow toward normal.

In Kidneys - Tubuloglomerular feedback mechanism

In brain tissue:

Skin blood flow

When microcirculation or microvascular B.F. increases endothelium derived

relaxing factor (nitric oxide) is released which causes dilation of larger B.V.

True

More important when the long-term metabolic demands of a tissue

changes.

the degree of vascularity of tissues (size and number of vessels) within a few weeks to match the need of the tissue.

Oxygen

by causing new vessels to sprout

from other small vessels.

.Vascular endothelial growth factor (VEGF)

▪ Fibroblast growth factor

▪ Platelet derived growth factor (PDGF)

▪ Angiogenin

Development of Collateral Circulation

When an artery or vein is blocked in any tissue of the body, a new vascular channel usually develops around the blockage and allows at least partial re supply of blood to affected tissue.

i. Norepinephrine and Epinephrine

ii. Angiotensin II

iii. Vasopressin (ADH)

iv. Endothelin

i. Bradykinin

ii. Histamine

iii. Serotonin

iv. Prostaglandins

v. Nitric oxide

Increase in sympathetic tone → vasoconstriction

Decrease in sympathetic tone → vasodilation

True

During sympathetic stimulation e.g. in stress or exercise, norepinephrine

and epinephrine are released at nerve endings.

Sympathetic nerves also stimulates adrenal medullae which secrete both

norepinephrine & epinephrine.

Effect is ↑ excitability of heart and vasoconstriction of arterioles and

veins

False

Angiotensin II is Powerful vasoconstrictor

Main effect is on arterioles

increases Total peripheral resistance so→ B.P. ↑

damaged blood vessels

present in endothelial cells of all blood vessels

when there is damage or crushing injury to blood vessels

True

Formed in the hypothalamus, transported and stored in the posterior

pituitary gland

hormone (ADH) is a hormone that helps your kidneys manage the amount of water in your body

Cause it responsible of Reabsorption of water from the renal tubules back into blood to control body fluid volume.

▪ Volume of urine is decreases hence called Antidiuretic hormone

Bradykinin causes both arteriolar

dilation and increases capillary

permeability → edema

▪ Bradykinin also helps to regulate

blood flow in the skin, in salivary

and GI glands.

Histamine is released in almost 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 the damaged tissues and basophils in the blood

Cause arteriolar dilation and increase capillary permeability=leaking of fluid =edema

↑ in Ca++ conc.→ stimulate smooth muscle contraction → Vasoconstriction

↑ in K+ ion → inhibit smooth muscle contraction → Vasodilation

. ↑ in Mg++ ion → inhibit smooth muscle contraction → Powerful Vasodilation

↑ in H+ (acidosis or ↓ in pH) → dilation of the arterioles

▪ ↓ in H+ (alkalosis or ↑ in pH) → arteriolar constriction

Causes moderate vasodilation in most tissues but marked vasodilation in the-brain.

▪ CO2 in the blood also has an extremely powerful indirect effect on vasomotor center → sympathetic stimulation → vasoconstriction throughout the body

True