Blood Flow and Velocity in animals

Blood Flow and Velocity in animals

Blood Flow

The blood flows at a steady rate through a long smooth vessel with each layer of blood remaining in the same distance from the wall i.e it flows in streamlines. The central portion of the blood stays in the centre of the vessel. This type of flow is called as laminar flow or streamline flow. If the blood flows in all direction in the vessel and continually mixing within the vessel it is called as turbulent flow. In turbulent flow, the blood flows cross wise in and along the vessel forming whirls in the blood called as eddy currents. Turbulent flow occurs when the rate of blood becomes too great or when it passes an obstruction in a vessel or when it passes over a rough surface.

• Blood flow through the blood vessel is determined entirely by two factors
  1. The pressure difference between the two ends of the vessel, which is the force that pushes the blood through the vessel
  2. The impediment to blood flow the vessel known as the vascular resistance

Blood flow through the vessel is calculated by Ohm’s Law

Q = ΔP/R

• Where,
  • Q = Blood flow
  • ΔP = Pressure difference between the two ends of the vessel (P₁-P₂)
  • P₁ = Pressure at the beginning of the vessel
  • P₂ = Pressure at the end of the vessel
  • R = Resistance

Blood velocity

It indicates the direction and the rate of blood flow in the blood vessels. It is directly proportional to the minute volume but inversely proportional to the cross sectional area of the blood vessel and circulation time. Velocity is high in the arteries and veins but slow in capillaries.

The arterial portion of the circulatory system shows progressive increase in the cross sectional area due to extensive branching of the arterial tree, thus causes progressive fall in the velocity towards capillary bed. On the contrary, the venous system shows decreasing cross sectional area towards heart that results in progressive increase in blood velocity.

In dogs, the cross-sectional area of aorta is 0.8 cm², large arteries is 3.0 cm² and in capillaries it is 600 cm².

The distance travelled by a column of blood in a given time through a specific blood vessel is known as linear blood velocity.

The peak velocity occurs during the period of maximal ventricular ejection followed by reduced velocity during the period of reduced ventricular ejection.