2402 : Anatomy/Physiology

Transcription

1 Dr. Chris Doumen Lecture : Anatomy/Physiology Hemo Dynamics and Blood Vessels I nt r oduc t i on TextBook Readings Pages 721 through 734. Make use of the figures in your textbook ; a picture is worth a thousand words! Work the Problems and Questions at the end of the Chapter Hemodynamics is the study of the forces that are involved in the circulation of blood throughout the body Arteries = vessels that carry blood away from the heart Veins = vessels that carry blood to the heart The complete systemic cardiovascular circulation can thus for example be summarized as follows Left Ventricle ---> Aorta ---> arteries ---> arterioles ---> Capillaries ---> venules -- -> veins ---> vena cava ---> Right atrium Blood Vessel Anatomy Outer layer : Tunica externa or Adventitia elastic fibers and collagen fibers in large arteries, they contain small blood vessels that feed this layer = vaso vasorum Middle layer : Tunica Media elastic fibers and smooth muscle Inner layer : Tunica interna single endothelial cell layer and a basement membrane Terminology Vasoconstriction The decrease in lumen of a blood vessel. It is due to due to contraction of smooth muscle layerand is mostly a sympathetic impulse Vasodilation The increase in lumen and due to relaxation of the smooth muscle Collin County Community College District

2 2402 : Anatomy/Physiology Page 2 of 8 T ypes of Blood Ve ss el s ARTERIES Large arteries Tunica media contains more elastic fibers, less muscle Thus called elastic arteries or conducting arteries due to their wide lumen Function as a pressure reservoir; blood ejected from the heart stretches the arteries after which they recoil They help to push blood through the body when heart is in diastole phase ( heart is at total rest 0.4 sec of each beat) Reduced elasticity reduces efficient blood flow Ex : aorta, carotid, subclavian Large walls help to withstand the continuous pressure on these vessels Medium Arteries Contain more smooth muscle in T. media. Called the muscular arteries Function to distribute the blood as they are capable of vasodilation and constriction, thus regulating blood flow Arterioles small arteries : they resemble muscular artery at beginning but end up into a capillary Capillaries Composed of a single layer of cells (endothelium layer) and a basement membrane No T. media or T. externa Function to permit exchange of nutrients and waste products between the blood and the tissues they serve. The more metabolic active a tissue is, the more extensive the capillary network

3 Page 3 of : Anatomy/Physiology Ty pes of Capillaries 1.Continuous endothelium cells provide an un-interrupted lining adjacent cells joined by tight junctions (like zippers) occasional gaps in the tight junctions allow for fluid and small solutes to pass common in skin, muscles brain has very snug fitting tight junctions with no gaps at all; forms the blood-brain barrier 2. Fenestrated same as above but endothelial cells have oval pores lined with thin membrane and allows for more permeability 3. Sinusoidal many fenestrations, fewer tight junctions which allows for passage of larger molecules Capillary beds are the interweaving networks formed by the capillaries. They consist of an incoming terminal arteriole and branches into a meta-arteriole meta-arteriole branches into a network of true capillaries true capillaries unite again with an extension of the meta-arteriole ( = thoroughfare channel ) thoroughfare channels ends up in the postcapillary venule precapillary sphincters between meta-arteriole and true capillaries can direct blood into the capillary bed or shunt it directly into the venule (= vascular shunt) VEINS Venules drain blood from the capillaries and guide the blood towards the veins initial venules are mostly T. interna, eventually developing a thin T. media and T. externa Veins Have all 3 Tunica present but very poorly develop;ed T. externa usually several times thicker than T. media Lumens are larger than corresponding arteries Many veins have valves that prevent backflow (low BP in veins) collapse of these valves results in varicose veins obesity, standing, pregnancy can cause such results

4 2401 : Anatomy/Physiology Page 4 of 8 Blood distribution Veins, venules 65 % Systemic arteries, arterioles 15 Pulmonary vessels 12 Heart 8 Syst. capillaries 5 Veins and venules are thus a blood reservoir (esp. liver and spleen). Protects against massive blood loss. HEMODYNAMICS Blood flow Blood velocity = volume of blood that flows per unit of time (ml/min) = distance blood travels per unit of time (cm/min) = blood flow per cross sectional area = ml/min/cm 2 (and since 1 ml = 1 cm 3, this becomes cm/min) The translation of this thus means that blood flows slower in a vessel with greater lumen Does this mean blood flows faster in a capillary versus an artery? In theory yes. However, each time an artery branches, the blood will flow in all these branches and one needs to take into account the total cross sectional area of all the branches. Although each capillary is extremely small, the total cross sectional area of all capillaries is much larger than the diamater of the original vessel. The C.S. area of the aorta is 3-5 cm 2 ; that of the capillaries is cm 2. Blood velocity thus drops from an original 40 cm/min to 0.1 cm/min. This slow blood flow is of extreme importance in that it allows for adequate time for efficient exchange of gases and nutrients between blood and surrounding tissues

5 Page 5 of : Anatomy/Physiology Blood Pressure Blood flow in the complete circulatory system is equal to cardiac output (CO) CO = Heart rate x Stroke volume Besides HR and SV, two other factors influence CO : Blood pressure and Resistance It is a straightforward to see that the higher the pressure difference between two points, the higher the blood flow. Also, the more resistance the "pipes" offer, the lower the flow will be. Or putting it in mathematical form CO = Blood pressure or Blood pressure = CO x R Resistance Let us examine how each component comes into effect 1. Blood pressure 2. Resistance is the pressure exerted by the blood on the systemic artery walls increase (decrease) in CO results in increase (decrease) in BP since blood volume determines veous return and thus CO, it also determines BP is the opposition to blood flow (friction) several factors determine resistance increased blood viscosity (ratio of RBC to plasma) increases Resistance, increases BP at constant CO Resistance increases with vessel length (obese person needs more vessels to serve additional fat layers) Resistance is inversely proportional to the fourth power of the radius. So the smaller the radius, the higher the resistance. (If radius decreases by a factor 2, resistance goes up by a factor 16 ) Simply put, blood flow is directly proportional to the fourth power of the radius of a vessel. ( see relationship between CO and R ). Thus, by relaxing a vessel such that the rdius increases two fold, blood flow will increase 16 fold. It would appear from the other equation ( BP = CO x R ) and that due to the fact that the capillaries have an extremely small diameter, that BP would be high in the capillaries. However resistance of tubes placed in parallel add up reciprocally. The total resistance (Rt) of tubes with resistance R1, R2, R3,... put in parallel is such that 1 = Rt R1 R2 R3 When more than 16 of such tubes are arranged, the total Resistance (Rt) becomes much smaller that the resistance of one tube of similar total cross sectional area. That's why the BP in the capillaries is drastically smaller than for example aortic BP and the reason why there is a drop in BP going from aorta to capillaries.

6 2401 : Anatomy/Physiology Page 6 of 8 The resistance of the total vascular system is called the systemic vascular resistance (SVR). Most resistance occurs in the arterioles, capillaries and venules. The veins and arteries are too big to offer any major contribution to SVR. Ar t er i al Blood Pr ess ur e Pressure in a container, such as a ventricle or blood vessel, is determined by the volume in the container the stretchability of the container The easier the container can be stretched, the more volume can be added without increasing pressure. In a similar way, adding a small amount of water to a balloon that does not stretch well will increase the pressure more rapidly. This is expressed as Compliance. Compliance = Δ Volume / Δ pressure A high compliance indicates that a large increase in volume is correlated with small increase in pressure, or that small pressure increases will result in large volume expansions.

7 Page 7 of : Anatomy/Physiology When the heart ejects its blood, it creates a flow and a pressure. If the arteries were rigid and not expandable, pressure in the arteries would be high when the ventricles eject blood practically zero when the ventricles relaxed This would make the blood flow in spurts in our circuits and not a desired condition. The elastic properties and high compliancy of the elastic arteries results that, each time blood is ejected into the arteries, they will expand and stretch, and thus hold more blood. About 30 % of the heart stroke volume, exits the elastic arteries at each systole. The rest expands the arteries and builds up pressure. When the ventricles go into diastole, the elastic arteries recoil and expel the rest of the arterial blood into the circuits. The Blood pressure in the aorta and arteries is thus pulsating due to the elastic arteries that expand and recoil with each heart beat. General aspects of Arterial Blood Pressures The peak arterial pressure is called systolic pressure and is on average about 120 mm Hg The minimal arterial pressure (after recoil) is the diastolic pressure is about 80 mm Hg The difference between systolic and diastolic pressure is the pulse pressure that is being felt when palpitating for your heat rate. (also very visible when an aorta is cut; blood comes out in spurts). Reduced elasticity in the aorta and arteries increases the diastolic pressure ( less relaxing of arteries, pressure stays higher)

8 2401 : Anatomy/Physiology Page 8 of 8 MAP = Diastolic pressure + {Pulse pressure/3} Look at the figure above. You can see that Pulse pressure is highest in the aorta and elastic arteries and gradually decreases. It eventually disappears when reaching the muscular arteries and blood flow becomes steady. Blood pressure drops as well due to the accumulating effect of resistance. By the time blood reaches the capillaries, BP ~ = 40 mm Hg and at the venous end of the capillary bed BP ~= 20 mm Hg. The pressure difference between venules and atria is about 15 mm Hg. This and is too low to promote adequate venous return to the heart by simple pressure difference. Remember that blood flow is dependant on a Pressure differential. The pressure difference between venules and atria is about 15 mm Hg, and is too low to promote adequate venous return to the heart by simple pressure difference. Several factors help to bring the blood that resides in the veins side of the systemic circuit back to the heart ( = venous return ). Low resistance in the veins ; the low resistance conduit system allows low pressure differentials to provide adequate flow Veins run between muscles; contraction of muscles has a milking effect, squeezing blood upwards. Also, veins contain one-way valves that operate like elevator stages Respiratory pump action of inspiration causes a decrease in pressure in the chest cavity while an increase in the abdominal cavity by the downward movement of the diaphragm. Helps to promote blood flow into the thoracic area Sympathetic stimulation of smooth muscles in veins results in vasoconstriction and increases venous pressure, driving more blood back to the heart. Weak valves in the veins causes varicose veins ( due to too much pressure on them; obesity, pregnancy, standing). Also long immobilization with no leg movement may prevent adequate venous return as well and causes dizzy spells when standing up or when standing still for too long.