14 - Blood Flow

CONTROL OF BLOOD FLOW IN TISSUES

Tissue perfusion is involved in delivery of oxygen and nutrients to, and removal of wastes from:
• Tissue cells.
• Gas exchange in the lungs.
• Absorption of nutrients from the digestive tract.
• Urine formation in the kidneys.

•Extrinsic control of blood flow involves the
sympathetic nervous system and hormones to
control blood flow through the whole body

CONTROL OF BLOOD FLOW

Autoregulation is an example of intrinsic control of blood flow in which automatic adjustments of blood flow to each tissue are made in proportion to its needs by modifying the diameter of local arterioles

REGULATION OF LOCAL BLOOD FLOW

• Metabolic controls of autoregulation are most strongly stimulated by a shortage of oxygen at the tissues.

• Cells lacking oxygen can release nitric oxide, a powerful vasodilator, to increase local blood flow.

Nitric oxide is normally balanced by endothelins, vasoconstrictors released by endothelium that are also present in the bloodstream. If nitric oxide levels rise, they overcome the effects of the endothelins and blood vessels dilate

•Myogenic control involves the localized response of vascular smooth muscle to passive stretch. Increased arterial pressure stretches vessel walls more than normal, so smooth muscle responds by constricting, causing decreased blood flow to the tissue to avoid damage from elevated pressure.

• Long-term autoregulation develops over weeks or months and involves an increase in the diameter of blood vessels and an increase in the number of vessels in a specific area, called angiogenesis

BLOOD FLOW TO MUSCLES

• Blood flow to skeletal muscles varies with level of activity and fibre type. Muscular autoregulation occurs almost entirely in response to decreased oxygen concentrations.

• In active or exercise hyperemia (excess blood in blood vessels), blood flow increases in direct proportion to muscle or metabolic activity. Local controls override sympathetic vasoconstriction and blood flow can increase tenfold compared to when at rest

BLOOD FLOW TO THE BRAIN AND TO THE HEART

• Cerebral blood flow is tightly regulated because neurons can be damaged by ischemia (inadequate blood supply). The brain is also sensitive to extreme blood pressures; mean arterial pressure (MAP) below 60 mmHg can cause syncope (fainting) while MAP above 160 mmHg can cause cerebral edema.

• Movement of blood through coronary blood vessels on the outside of the heart is affected by the pumping of the ventricles. During systole, coronary vessels are compressed, so blood is only supplied to cardiac muscle during diastole

LOCAL VS SYSTEMIC VASOCONSTRICTION

•Local vasoconstriction is like construction on one road; traffic (blood pressure) on that road (artery) goes down because cars (blood) take other roads which aren’t under construction. There are enough roads that decreasing cars on one doesn’t affect citywide traffic (systemic blood pressure) overall.

• Systemic vasoconstriction is like having construction on every road. The roads feel much more crowded (higher blood pressure) because there isn’t as much room to fit all of the traffic

BLOOD FLOW IN CAPILLARIES

• Slow blood flow through capillaries promotes diffusion of nutrients and gases, and bulk flow of fluids.

• The speed of blood flow changes as it passes through the systemic circulation; it is fastest in the aorta and slows down as blood vessel diameters decrease.

• Capillary exchange of nutrients, gases, and metabolic wastes occurs between the blood and interstitial space through diffusion

BULK FLOW

• Fluid is forced out through intercellular clefts in the capillaries at the arterial end, and most of the fluid returns to the blood at venous

end.

• Bulk flow of fluids across capillary walls causes continuous mixing between plasma and interstitial or extracellular fluid. This process helps to maintain the interstitial environment.

• The direction and amount of fluid flow depend on the two opposing forces of hydrostatic pressures and colloid osmotic pressures

BULK FLOW: hydrostatic pressure

• Hydrostatic pressure is the force of a fluid against a membrane.

• Capillary hydrostatic pressure is the capillary blood pressure that tends to force fluids through capillary walls. This pressure is greater at arterial end of the capillary bed than at the venule end.

• Interstitial fluid hydrostatic pressure is the pressure pushing fluid back into the blood vessel.
It is usually assumed to be zero because lymphatic vessels drain interstitial fluid from our tissues

BULK FLOW: colloid osmotic pressure

• Colloid osmotic pressure, the force opposing hydrostatic pressure, is created by the presence of large, non-diffusible molecules that are too big to move through the capillary wall.

• Capillary colloid osmotic pressure is the “sucking” pressure from plasma proteins, such as albumin, pulling fluids back into the capillary.

• Interstitial fluid colloid osmotic pressure is inconsequential because interstitial fluid has a very low protein content

BULK FLOW

Fluids will leave the capillaries (filtration) if net hydrostatic exceeds net colloid osmotic pressure. This occurs at the arterial end of the capillary bed. Fluids will enter the capillaries (reabsorption) if net colloid osmotic pressure exceeds net hydrostatic pressure. This occurs at the venous end of the capillary bed

EDEMA

• Edema is an abnormal increase in the amount of interstitial fluid. It is caused by either an increase in outward pressure (driving fluid out of capillaries) or a decrease in pressure (failure to return fluid back to the capillaries or to drain interstitial fluid into the lymphatic system).

• An increase in capillary hydrostatic pressure can be caused by anything that increases blood pressure in the capillary, such as incompetent venous valves, blood vessel blockage, congestive heart failure, or high blood volume

• An inflammatory response can cause an increase in interstitial fluid colloid osmotic pressure. Inflammation increases capillary permeability and allows proteins to leak into interstitial fluid, which causes fluid to be pulled into the interstitial space.

• A decrease in capillary colloid osmotic pressure hinders fluid return to blood. This can be caused by hypoproteinemia (low levels of plasma proteins like albumin) due to malnutrition, liver disease, or kidney inflammation causing plasma proteins to be lost in urine