Chapter 15 The Blood Vessels

Transcription

1 Chapter 15 The Blood Vessels

2 Circulatory System Heart: pumps blood Right side: pumps blood to lungs and back Pulmonary circuit Left side: pumps blood to rest of body Systemic circuit Aorta Arteries Arterioles Capillaries Venules Veins Major veins (venae cavae)

3 Figure 15-1 Copyright 2010 Pearson Education, Inc.

4 Figure 15-2 Copyright 2010 Pearson Education, Inc.

5 Blood Vessels (Systemic Circulation) Arteries Pressure reservoir The aorta and other large systemic arteries have very elastic walls The pressure produced by the force of left ventricle contraction is stored in the elastic walls and slowly released through elastic recoil During ventricular relaxation, elastic recoil of the artery walls maintains a continuous driving pressure for blood flow

6 Blood Vessels (Systemic Circulation) Arterioles Downstream from the arteries Arterioles have variable resistance Selectively constrict or dilate, directing blood flow to certain tissues Arteriole diameter is regulated by Local factors (tissue oxygen concentration, etc.) Autonomic nervous system Hormones

7 Figure 15-1 Copyright 2010 Pearson Education, Inc.

8 Capillaries Smallest diameter Has leaky epithelium Allows for exchange of materials between plasma, interstitial fluid, and cells Fresh blood coming from the heart brings oxygen and nutrients to the cells These materials leak out of the capillaries and move across to the cells (by diffusion) Cells send carbon dioxide and other waste products out into the IF and from there it enters the capillaries

9 Blood Vessels (Systemic Circulation) Venules Located at distal end of the capillary beds Venules start out small and progressively get bigger and bigger, eventually becoming veins Venules and veins collect blood to return to the heart

10 Blood Vessels (Systemic Circulation) Veins Return blood to the heart Act as a volume reservoir Hold more than half of the total amount of blood If BP falls too low, venous side of circulation can add more blood to the arterial side

11 Blood Vessels (Anatomy)

12 Blood Vessels (Anatomy) 3 layers: Tunica intima (inside layer) Made of endothelium (a type of epithelium) In larger arteries, an elastic tissue is also present Tunica media (middle layer) Made of smooth muscle Thickness of this layer varies Larger vessels, more muscle, smaller ones, less muscle Tunica adventitia or Tunica externa (outside layer) Made of fibrous connective tissue

13 Blood Vessels (Anatomy)

14 Figure 15-2 Copyright 2010 Pearson Education, Inc.

15 Endothelium Inner lining of blood vessels Capillaries consist only of this layer Once thought to be just a passive barrier Now known to be a very complex structure (not just a simple epithelium) Endothelium does the following: Secretes lots of different paracrines Helps regulate blood pressure, blood vessel growth, absorption of materials

16 Vascular Smooth Muscle Tunica media consists of this Can be arranged in either a circular or a spiral layer Vasoconstriction Narrows the diameter of the blood vessel lumen Vasodilation Increases the diameter of the blood vessel lumen Muscle tone: Smooth muscle cells are partially contracted at all times

17 Vascular Smooth Muscle Muscle tone in vascular smooth muscle can be influenced by: Neurotransmitters Hormones Paracrines Secreted by endothelium or by cells surrounding the blood vessels Contraction of vascular smooth muscle requires entry of Ca++ ions from the ECF (like in cardiac muscle)

18 More on Arteries and Arterioles (p.515) Aorta and major arteries have stiff and springy walls They have thick layers if smooth muscle and Large amounts of elastic and fibrous connective tissues Arteries and arterioles have a divergent branching pattern (one branch becoming many) Veins and venules have a convergent branching pattern (many branches becoming one)

19 More on Arteries and Arterioles (p.515) As major arteries branch into smaller and smaller arteries, the artery wall changes dramatically As each artery branches into a smaller one, the walls become less elastic and more muscular Arterioles (really small arteries) have several layers of smooth muscle, allowing them to contract or relax when influenced by chemical signals Microcirculation: arterioles, capillaries, venules

20 Metarterioles Even smaller arteries Some arterioles branch into these These have precapillary sphincters: Rings of smooth muscle that can be tightened to shut off blood flow to a capillary bed When the sphincter is constricted, blood is shunted around the capillary bed and into the venules When open, blood flows into the capillary bed

21 Figure 15-3 Copyright 2010 Pearson Education, Inc.

22 Capillaries Smallest of the blood vessels Lack smooth muscle, elastic tissue, fibrous tissue Consist only of one cell layer of endothelium supported by a basal lamina or basement membrane Pericytes Highly branched contractile cells which can surround the capillaries Can contribute to tightness of capillary Like blood-brain barrier

23 Capillaries Pericytes Secrete capillary growth factors Can differentiate into new endothelium or into new smooth muscle cells Diabetic retinopathy is characterized by loss of pericytes in the capillaries in the retina Cause: Unknown

24 Veins and Venules Smallest venules are similar to capillaries in structure As venules get bigger, smooth muscle begins to appear in their walls As veins progress towards the heart, they become bigger and bigger in diameter Largest veins are the superior and inferior vena cava Veins are more numerous than arteries and are usually larger in diameter Veins are closer to the surface than arteries

25 Angiogenesis The process by which new blood vessels develop In children, this is part of normal growth In adults (see next slide)

26 Angiogenesis In adults, angiogenesis occurs during wound healing during the monthly regrowing of the uterine lining (after menstruation) During endurance training Also occurs as part of the growth of a malignant tumor In coronary artery disease, new blood vessels can sometimes develop spontaneously and go around a blocked artery

27 Blood Pressure Highest in the arteries, lowest in the veins By the time it gets to the capillary beds, the pressure is mostly dissipated Driving force: contraction of left ventricle Creates a pressure wave (pulse) which travels through the blood vessels Pressure decreases as blood moves through the circulatory system Due to resistance to flow And to friction between the blood cells

28 Figure 15-4, overview Copyright 2010 Pearson Education, Inc.

29 Figure 15-5 Copyright 2010 Pearson Education, Inc.

30 Blood Pressure Systolic pressure During ventricular systole (contraction) Pressure in aorta averages 120 mm Hg during this period Diastolic pressure During ventricular diastole (relaxation) Pressure in aorta falls to a low of 80 mm Hg during this period Pressure units: mm Hg Millimeters of mercury (Hg)

31 Blood Pressure Pulse or pressure wave Caused by L. ventricle contraction Moves 10 times faster than the blood is moving A pulse felt in the arm is the pressure wave slightly after ventricular contraction (takes a short time to move over to the arm)

32 Pulse Pressure A measure of the strength of the pressure wave Systolic pressure Diastolic pressure = Pulse pressure In the aorta: 120 mm Hg 80 mm Hg = 40 mm Hg (pulse pressure)

33 Figure 15-5 Copyright 2010 Pearson Education, Inc.

34 Veins and Blood Flow Back to the Heart By the time blood reaches the veins, no pressure wave left Venous blood flow is steady not pulsatile (like arteries) Venous blood flow is pushed along by the continuous movement of blood out of the capillaries and into the venules

35 Veins and Blood Flow Back to the Heart Blood in veins above the heart use gravity to flow downhill to the heart Blood in veins below the heart must flow uphill to get back to the heart Venous return to the heart is aided by One-way valves in the larger veins (fig. 15-6) Skeletal muscle pump (p. 501) Respiratory pump (p. 501)

36 Figure 15-6 Leg veins with one-way valves Copyright 2010 Pearson Education, Inc.

37 Skeletal Muscle Pump Skeletal muscle contractions that squeeze veins, compressing them and pushing blood closer towards the heart In the larger leg veins with one-way valves, once the blood has been squeezed upward by muscle contraction, then the valve closes, keeping the blood from flowing backward due to gravity During walking, running, etc. this helps move blood back to the heart When sitting or standing still, the pump isn't working

38 Respiratory Pump Another mechanism for moving blood through the veins Respiratory pump is created by breathing in During inspiration, the chest expands and the diaphragm moves downward towards the abdomen This expands the chest (thoracic) cavity and also creates a subatmospheric pressure within the cavity

39 Respiratory Pump This lower pressure in the chest cavity decreases pressure in the inferior vena cava as it passes through the thorax The lower pressure helps draw more blood into the vena cava from the abdominal veins During this time period (inspiration), the abdominal veins are experiencing higher pressure around them Summary: During inspiration, higher pressure on abdominal veins and lower pressure on thoracic veins

40 Arterial Blood Pressure Since ventricular pressure is difficult to measure, it is customary to assume that arterial pressure reflects ventricular pressure Arterial pressure is usually measured in the brachial artery Mean Arterial Pressure (MAP) (p. 517) Diastolic pressure plus 1/3 of pulse pressure Gives a single value for the driving (pulsatile) arterial pressure

41 Mean Arterial Pressure (MAP) Diastolic pressure plus 1/3 of pulse pressure MAP = diastolic P + 1/3 (systolic P diastolic P) Example: systolic =120 and diastolic = 80 MAP = 80 mm Hg + 1/3 ( mm Hg) MAP = 93 mm Hg MAP is always closer to diastolic P since diastole lasts twice as long as systole

42 Hypotension BP is too low The driving force for blood flow isn't dtrong enough to overcome gravity Blood and O2 supply to brain may become impaired, can cause dizziness Hypertension BP is too high, can become chronically elevated

43 Hypertension BP is too high, can become chronically elevated Continued high pressure on the walls of the blood vessels can cause weakened areas to rupture and bleed into the surrounding tissue Cerebral hemorrhage A rupture in the brain May cause a stroke Rupture in a major artery (e.g. aorta) Can cause rapid loss of BP, may quickly be fatal

44 Measuring BP Usually measured in the brachial artery Sphygmomanometer Used to measure BP Consists of an inflatable cuff and a pressure gauge. A stethoscope is used to hear the sounds The cuff is placed around the upper arm and is then inflated until it exerts a pressure higher than the patient's systolic pressure and blood flow is temporarily cut off

45 Figure 15-7 Copyright 2010 Pearson Education, Inc.

46 Measuring BP Next, the pressure in the cuff is gradually released When cuff pressure falls below systolic pressure, then blood begins to flow again Korotkoff Sound The thumping noise, heard through the stethoscope The noise is caused by the blood squeezing through the still partially compressed artery The sound diasppears once the artery is fully open again

47 Figure 15-7 Copyright 2010 Pearson Education, Inc.

48 Measuring BP Systolic Pressure Highest pressure in the artery The pressure at which the first Korotkoff sound is heard Diastolic Pressure Lowest pressure in the artery The pressure at which the Korotkoff sounds disappear

49 Hypertension (High BP) Average value for BP: 120/80 Subject to wide variability Within a single individual, moment to moment Varies from person to person Hypertension At rest, a systolic pressure consistently over 140 At rest, a diastolic pressure consistently over 90 Current guidelines (as of 2003) recommend that individuals maintain their BP below 120/80

50 Prehypertension Individuals are considered to be prehypertensive if: Systolic pressure Is consistently And/or Diastolic pressure Is consistently Prehypertension can be treated by providing counseling on lifestyle modifications and also by certain medications (diuretics, etc.)