Consistent patterns of blood circulation in arteries and

Consistent patterns of blood circulation in arteries and veins. Bases of hemodynamics.

Blood vessels

The Arteries • Arteries and arterioles take blood away from the heart. • The largest artery is the aorta. • The middle layer of an artery wall consists of smooth muscle that can constrict to regulate blood flow and blood pressure. • Arterioles can constrict or dilate, changing blood pressure.

The Capillaries • Capillaries have walls only one cell thick to allow exchange of gases and nutrients with tissue fluid. • Capillary beds are present in all regions of the body but not all capillary beds are open at the same time. • Contraction of a sphincter muscle closes off a bed and blood can flow through an arteriovenous shunt that bypasses the capillary bed.

Anatomy of a capillary bed

The Veins • Venules drain blood from capillaries, then join to form veins that take blood to the heart. • Veins have much less smooth muscle and connective tissue than arteries. • Veins often have valves that prevent the backward flow of blood when closed. • Veins carry about 70% of the body’s blood and act as a reservoir during hemorrhage.

Hemodynamics • Velocity of blood flow is related to the XS area • Fastest flow in the arteries with the slowest in the capillaries • Most blood flow is laminar (silent) in arteries there is a pulsatile flow of forward and then back due to the elasticity of the arterial walls • Turbulence (noisy) occurs only at the maximal flow rates just distal to the semilunars during ejection and as the semilunar valves close

Physical laws governing blood flow and blood pressure • Flow of blood through out body = pressure gradient within vessels X resistance to flow - Pressure gradient: aortic pressure – central venous pressure - Resistance: -- vessel radius -- vessel length -- blood viscosity

Factors promoting total peripheral resistance (TPR) • Total peripheral resistance = TPR -- combined resistance of all vessels -- vasodilation resistance decreases -- vasoconstriction resistance increases

Arteries and blood pressure • Pressure reservoir • Arterial walls are able to expand recoil because of the pressure of elastic fibers in the arterial wall • Systolic pressure: maximum pressure occurring during systole • Diastolic pressure: pressure during diastole

Arterial blood pressure

Blood pressure values: what do they mean? • Pulse pressure: PP = SP-DP • Mean arterial blood pressure = MABP • MABP = SBP + (2 XDBP) 3 CO = MABP = SV x HR TPR

Capillaries • Allow exchange of gases, nutrients and wastes between blood and tissues • Overall large surface area and low blood flow • Two main types: - continuous capillaries: narrow space between cells permeable to small or lipid soluble molecules - fenestrated capillaries: large pores between cells large molecules can pass

Local control of blood flow in capillaries • Presence of precapillary sphincters on the arteriole and beginning of capillaries • Metarteriole: no sphincter continuous blood flow controls the amount of blood going to neighboring vessels

Movement of materials across capillary walls • Small molecules and lipid soluble molecules move by diffusion through the cell membrane • Larger molecules, charged molecules must pass through membrane channels, exocytosis or in between 2 cells • Water movement is controlled by the capillary hydrostatic and osmotic pressures

Forces controlling water movement • Arterial side of the capillary: ▫ High capillary hydrostatic pressure (BHP), lower capillary osmotic pressure (BOP, due to proteins and other molecules in the blood) Net filtration pressure pushes fluid from the blood toward the tissue (but the proteins remain in the capillary • Venous side of the capillary: - Lower hydrostatic pressure (due to resistance) and higher capillary osmotic pressure Net filtration pressure moves fluid back toward the capillary • Interstitial fluid hydrostatic (IFHP) and osmotic pressures (IFOP) remain overall identical

Fluid movement in the capillary • Arteriole side: fluid moves toward the tissues • Venous side: fluid reenters the capillary • Overall: for every 1 liter of fluid entering the tissues, only 0. 85 l reenter the capillary • The remaining 0. 15 l is reabsorbed as lymph by lymphatic capillaries and eventually returned back to blood circulation • When this system fails: Edema

Causes of edema • Increased hydrostatic blood pressure - heart failure (left or right), - excess fluid in the blood • Decreased blood osmotic pressure ▫ Liver, kidney diseases, malnutrition (kwashiorkor), burn injuries • Increased interstitial hydrostatic pressure (lymphatic capillary blockage) - breast cancer surgery, elephantiasis • Leaking capillary wall - histamine release during allergic reaction

Veins • Veins are blood volume reservoir • Due to thinness of vessel wall less resistance to stretch = more compliance

Factors influencing venous return • 1 - Skeletal muscle pump and valves • 2 - Respiratory pump • 3 - Blood volume • 4 - Venomotor tone Compression of veins causes blood to move forward past a valve that then prevents it from returning backward.

• Changes in thoracic and abdominal pressure that occur with breathing also assist in the return of blood. • Varicose veins develop when the valves of veins become weak. • Hemorrhoids (piles) are due to varicose veins in the rectum. • Phlebitis is inflammation of a vein and can lead to a blood clot and possible death if the clot is dislodged and is carried to a pulmonary vessel.

Lymphatic circulation • Driven by factors similar to venous circulation: - muscle activity - valves - respiration • Lymph = plasma-proteins • Lymphatic circulation collects fluid not reabsorbed by the capillaries • Lymph is filtered in nodes before return to blood circulation