Physiology

• Hemodynamics


  The study of movement of blood through the circulatory system
• Types of blood flow


  • Laminar flow
  • Turbulent flow
• Laminar flow


  A silent flow where the velocity increases from the vessel wall to the center of the stream
• Turbulent flow


  A noisy flow where the molecules move in different directions forming eddy currents
• Streamline flow occurs only at velocities up to a critical level
• Turbulent flow creates sounds (murmur or bruit)
• Factors determining volume of blood flow


  • Pressure gradient
  • Resistance to blood flow
  • Viscosity of blood
  • Diameter of blood vessels
  • Velocity of blood flow
• Pressure gradient


  The pressure difference between the two ends of the blood vessel
• Peripheral resistance

  The resistance in the peripheral vessels, particularly the arterioles
• Determinants of peripheral resistance


  • Radius of blood vessels
  • Vessel length
  • Viscosity of blood
• Viscosity of blood


  Determined mainly by RBC count and plasma protein, especially albumin
• In circulation, the diameter of the vessel is considered in relation to the cross-sectional area through which the blood flows
• The cross-sectional area is progressively increased as the arteries ramify and as the distance from the heart is increased
• The cross-sectional area of each branch is smaller, but the sum of the cross-sectional areas of all the branches is always greater than that of the parent vessel
• The diameter of the aorta depends upon the elasticity of the wall and its recoiling tendency helps in maintaining the flow and pressure
• The diameter of the arterioles depends upon the sympathetic tone
• Factors maintaining blood velocity


  • Cardiac output
  • Cross-sectional area of blood vessels
  • Viscosity of blood
• Velocity of blood flow is directly proportional to cardiac output
• Velocity of blood flow is inversely proportional to the total cross-sectional area of the vascular bed
• Blood velocity in the aorta is 0.5 m/sec, while in the capillaries it is 0.5 mm/sec
• Velocity of blood flow is inversely proportional to the viscosity of blood
• Autoregulation


  The intrinsic ability of an organ to regulate a constant blood flow, in spite of changes in the perfusion pressure
• Theories of autoregulation


  • Myogenic theory
  • Metabolic theory
• Myogenic theory

  The intrinsic contractile property of the smooth muscle fibers present in the blood vessels is responsible for autoregulation
• Metabolic theory


  The normal blood flow is maintained by the metabolic end products, which dilate the blood vessels when blood flow is reduced and vasoconstrict when blood flow increases
• Common vasodilators of metabolic origin


  • Adenosine
  • Carbon dioxide
  • Lactate and Hydrogen
• Light


  Refraction, refraction of the eye, accommodation and the pupil
• Retina


  Receptors: Rods and cones, phototransduction
• Central


  Visual pathway
• Refraction is the bending of light. Light bends whenever it travels from one transparent material to another.
• The reason light bends when it passes between two materials is because it changes speed.
• If the new material is more dense (for example air to glass) the light slows down and bends towards the normal.
• Light speeds up as it passes through a less dense material (for example, glass to air) and bends away from the normal.
• Refraction of light through a curved surface (e.g: cornea), obeys the rule of refraction.
• Light refraction through lenses also obeys rule of refraction, so the convex lens (e.g. human lens) is converging the light and has positive sign (plus lens +) while the concave lens is diverging the light and has negative sign (minus lens -).
• Light rays coming from a distant object are parallel (further than 6 m away).
• Light rays coming from a close object are divergent.
• Focusing light in the eye

  1. The cornea and the lens must converge the light by the correct amount

  2. The eyeball must be of the correct length (the distance between the cornea and the retina)

  3. The tear film, cornea, aqueous humour, crystalline lens, and vitreous must be clear so that the light can reach the retina without being interrupted
• Myopia


  The focus lies in front of the retina. This may be because the eye is abnormally long or the eye may be of normal length, but its power may be increased.
• Hyperopia


  The focus lies behind the retina. This may be because the eye is short or the refractive power of the eye is inadequate.