Blood FlowBlood Flow

Why blood flow occur?• Due to the pressure difference of two vessel ends

Vascular resistance to blood flow• The rate of blood flow to an organ is related to the

resistance to flow in the small arteries and arterioles that serve the organ

• Vasodilatation decreases resistance and increases the flow, where as vasoconstriction increases the resistance and decreases the flow

Factors affecting the resistance• Length of the blood vessel – When the length

is more resistance also increase• Viscosity of blood – when the viscosity

increases resistance also increases• Radius of the blood vessel – when the radius is

more resistance is less

What is Total Peripheral resistance?• The sum of all the vascular resistances within the

systemic circulation is called the Total Peripheral Resistance

•By endothelium secretary factors

Vasodilators• Nitric oxide• Bradykinin• Prostacyclin

Vasoconstrictors• Endothelin 1

•Myogenic control

When the vascular blood pressure is high the vessels constrict and reduce the blood flow to the tissues

When the vascular blood pressure is low the vessels dilate and increase the blood flow to the tissues

•Metabolic control

• When the Oxygen concentration decreases due to increased metabolic rate and when the Carbon Dioxide concentration increases local vasodilatation occurs and increase the blood flow to the tissues

Regulation of Blood Flow and Pressure by Kidneys• By Anti Diuretic Hormone ( ADH )• By Aldosterone• By Atrial Natriuretic Factor ( ANF )• By Renin – Angiotensin system

By Anti Diuretic Hormone ( ADH )

• When blood plasma osmolality ( osmotic pressure / thickness ) increases; it detects by osmoreceptors in hypothalamus and stimulate the posterior pituitary to secrete ADH

• ADH stimulate water reabsorption from the filtrate during the urine formation and thereby increase the fluid volume in the plasma as well as the pressure

By Aldosterone• Aldosterone secreted by Adrenal Cortex stimulates

the reabsorption of salt• Retention of salt indirectly promotes retention of

water• Which will lead to increase the pressure and

volume of blood

By Atrial Natriuretic Factor ( ANF )• ANF is produced by the atria of the heart and

promoting salt and water excretion in the urine• By that it reduces the blood volume as well as the

pressure

By Renin – Angiotensin system• When blood flow and pressure reduce in renal

artery a group of cells in kidney secretes an enzyme called Renin in to blood

• This renin leads to activation of Angiotensin which can increase vasoconstriction of arteries and thereby increase the pressure

• It also leads to secretion of aldosterone which leads to salt and water retention by kidneys and increase the volume of blood