Blood Vessels and Flow Through the circulatory system

Blood Vessels and Flow Through the circulatory system • Heart pumps blood by increasing local pressure • As blood travels through circulatory system, it encounters friction

Physics of Blood Flow

Physics of Blood Flow What sets the resistance of a tube? Think of drinking through a straw • Is it easier to drink through a very long straw or a shorter straw? • What is the difference between drinking through a skinny straw and a fat straw? • What is the difference between drinking a milkshake or water through a straw?

Poiseuille’s Equation Q = P* r 4 /8 L where: Q = flow P = pressure gradient L = length of the tube = viscosity of the fluid r = radius of the tube

Physics of Blood Flow

Circulatory systems and electrical circuits • Circulatory systems can be modeled as electrical circuits • Heart = Power source • Capillaries = Resistance RT = R 1 + R 2 1/RT = 1/R 1 + 1/R 2

Blood flow distribution Arteriolar & capillary branching reduces overall resistance

Physics of Blood Flow - velocity • Flow – volume of fluid transferred per unit time • Velocity – distance per unit time • Blood velocity = Q/A • A: cross-section area of the channels • Velocity of flow is inversely related to total cross-sectional area • Total cross-sectional area of capillaries is very large velocity is slow allows time for diffusion

Physics of Blood Flow - Gravity Hydrostatic pressure – pressure of a vertical column of fluid due to gravity P = pg h • p: density of the fluid • g: acceleration due to gravity • h: height of the fluid column Blood pressure varies in different parts of the body and when standing or prone Blood must move with and against the forces of gravity

Physics of Blood Flow - Gravity • Measured blood pressure when standing includes a hydrostatic (gravity) component

Vertebrate blood vessels 3 layers • Tunica intima Endothelium Basement membrane • Tunica media Smooth muscle Elastin • Tunica externa (Tunica adventitia) Collagen

Vertebrate blood vessels

Capillaries Lack tunica media and tunica externa Walls vary among capillaries • Continuous: cells are held together by tight junctions; found in skin and muscle • Fenestrated: cells contain pores; specialized for exchange; found in the kidneys, endocrine organs, and the intestine • Sinusoidal: cells are held together by fewer tight junctions; most porous for exchange of large proteins; found in liver and bone marrow

Capillary Beds

Regulation of blood flow • Pressure, resistance, and flow vary throughout the circulatory system and in response to changing conditions

Capillary considerations Role of capillaries Facilitate exchanges between the blood & tissues Capillary & rbc diameters are matched Capillary blood velocity is the slowest

Capillary blood flow Pressure pulsatility: - Reduced by compliant arterial vessels (aorta, bulbus, conus) - Pulsatility is minimal in capillaries Capillaries: - Total vascular cross-sectional area is vastly increased: reduced diffusion distance Blood velocity: - Proportional to total cross-sectional area - Slowest in capillaries to aid exchange Mean capillary blood pressure is appreciable

Blood Pressure

Relationship between velocity and cross-sectional area Velocity = Q/A

Blood Pressure • Pressure fluctuations are largest at the heart • Degree of fluctuation decreases through the circulation

Arteries- pressure reservoirs Arteries dampen the pressure oscillations caused by the pumping action of the heart

Mean Arterial Pressure (MAP) MAP = 2/3 diastolic Pressure + 1/3 systolic Pressure

Distribution of blood to tissues Vasoconstriction and Vasodilation

Distribution of blood to tissues • • Tissue metabolic need sets blood distribution Arterioles arranged in parallel Blood can be redistributed as needed Controlled by arterioles

Control of arteriolar diameter • • • Myogenic autoregulation Tissue metabolic activity Paracrine signaling molecules Endocrine system Nervous system

Myogenic autoregulation Stretch activated Ca 2+ channels

Effects of tissue metabolic activity

Metabolic Activity of Tissues

Paracrines (Nitric Oxide)

Nervous/Endocrine Regulation

Exchange at capillary governed by “Starling forces” • Capillary hydrostatic pressure • Interstitial hydrostatic pressure • Capillary colloid osmotic pressure • Interstitial colloid osmotic pressure

Capillary blood pressure & filtration Capillary endothelial lining • the barrier between blood & tissues • varies according to tissue function Circulatory Systems Zool 364

Net Filtration Pressure (NFP) Blood pressure forces fluid out of capillaries Starling principle: NFP = (Pcap – Pif) – ( cap - if) • Terms: hydrostatic pressure in the capillary (Pcap) and interstitial fluid (Pif); osmotic pressure in the capillary ( cap) and interstitial fluid ( if) Figure 9. 36