Lecture 22Outline of Last Lecture i. The bloodii. Plasmaiii. Blood cellsOutline of Current LectureI. Blood vesselsa. Arteries, veins, capillariesb. Structure of arteries and veinsc. Differences between large and small vesselsd. Differences between arteries and veinse. Vasa varosumf. Valves in veinsg. Capillaries h. Postcapillary venulesi. Sinusoids Current LectureI. Blood Vesselsa. 3 categories: arteries, veins, and capillaries b. Arteries carry blood away from the heart and toward the bodyc. Veins conduct blood from the capillaries toward the heartd. Capillaries are where nutrients, wastes, and respiratory gases are exchanged btwn the bloodstream and the surrounding tissuese. Simplified circuit i. Heart  elastic arteries  muscular arteries  arterioles  capillaries  venules  veins  heartf. Basic structure of arteries and veinsi. Arteries and veins share the same design, walls with 3 layers (or tunics)1. Tunica intima:a. Contains an endothelium (a simple squamous epithelium lining the lumen of the vessels)i. in direct contact with the bloodii. provides smooth, frictionless surface for blood to flow overb. in larger blood vessels (>1mm), there is 2nd part, the subendothelial layer (a think connective tissue just external to theendothelium)2. Tunica mediaa. Middle layerb. Contains concentric sheets of smooth muscle cellsi. Separated by sheets of elastic proteinc. Muscle cells are orient circumferentially i. Contracting/relaxing  changes the diameter of the vesselBIOLOGY 315 1st Edition3. Tunica externaa. Outer layer b. Mostly connective tissuec. Collagen fibers and fibroblasts oriented longitudinally d. Functions to strengthen the wall of the vesselg. Differences between large and small vesselsi. Larger blood vessels have thicker walls than smaller vessels1. Because fewer layersh. Differences between arteries and veinsi. Veins have thinner walls1. Artery walls are thicker because the blood pressure to be resisted is greater in arteries than in veins2. Vein often has a larger lumen than the arteryii. Arteries have thicker tunica media than veins 1. Arteries can constrict or dilate their diameters markedlyiii. Veins have thicker tunica externa 1. To strengthen their walls against pooling and backflow of the low-pressure blood they contain a. Less likely to bulge, tear, and become varicoseiv. Arterial walls contain more elastin 1. In the artery, the elastin helps to absorb the pulsatile beat of the heart and to snap back between heartbeats, smoothing the pulse along the artery tree and producing a constant pressure at the capillaries 2. Elastin sheets are especially thick and abundant in large, elastic arteries (such as the aorta)a. Near the heart and must absorb strong pressure from the heartbeats i. Vasa Varosumi. Supplies the blood vessels with blood-borne substancesii. tiny arteries, veins and capillariesiii. only large vessels have vasa varosum1. smaller do not need them because they are supplied by diffusion iv. only the outer part of the wall is supplies by these vessels1. the inner is supplied by the blood in the lumen j. Valves in veinsi. Blood in veins is under low pressure ii. Flows upward toward the heart, against gravity 1. Flaps of the valves point toward the heart2. Forced to shut when blood starts to flow the other wayk. Capillariesi. Such small molecules include water, ions, urea, simple sugars, amino acids, the smallest proteins, and respiratory gasesii. Most proteins are too large to pass across the capillary wall, as are red blood cellsiii. Walls of capillaries are very thiniv. Single layer endothelial cells1. Fried-egg shaped2. Just outside, is a basal laminav. Individual cells are held together by scattered desmosomes and by tight junctions 1. Separated by gaps intercellular clefts, through which substances can leave or enter the capillary vi. 2 classes of capillaries: continuous and fenestrated1. Fenestrateda. Scattered pores span the whole thickness of the endothelial cellsi. Thought to increase the permeability of the capillaryb. Occur in organs where exceptionally large numbers of small molecules pass in or out of capillariesi. Small intestine, joint capsules, endocrine glands2. Continuousa. No poresb. Occur in the majority of organs and tissues that do not have exceptional needsi. Skin, muscles, brainii. Most of the body’s capillaries are continuous vii. Most ions and molecules cannot diffuse through the lipid-based plasma membrane of endothelial cells, other routes are therefore important:1. Intercellular clefts: most small molecules pass through the intercellular clefts between the endothelial cells2. Pores: supplementing Route 1, additional small molecules pass through3. Direct diffusion through the endothelium4. Caveolae: the larger molecules that transverse the capillary walls, such as small proteins, do so by shuttling through endothelial cells within cytoplasmic vesicles called caveolaeviii. Impermeable capillaries and the blood-brain barrier1. The brain has impermeable capillaries that form the blood-brain barrier which protects our valuable neurons and brain tissue from toxins in the blood ix. White blood cells routinely migrate out of capillary blood into the surrounding connective tissue 1. Push their way out by squeezing through the clefts between endothelial cells= diapedesis x. The diameter of typical capillaries is nearly the same everywhere in the body 1. Just large enough to fit the erythrocytesxi. Scattered, spider-shaped cells called pericytes lie on the outside of capillaries xii. Capillaries are not straight, unforked vessels, instead they form highly branched networks that blanket the tissues they supply l. Postcapillary venulesi. Venules1. Smallest veinsii. Smallest venules are called postcapillary venules iii. Wider, postcapillary venules resemble capillaries in structure and functioniv. Wall is thin enough to allow exchange of molecules between blood and tissue fluid v. More migration of leukocytes out of the vascular system occurs at the postcapillary venules than at capillaries themselves vi. More of the fluid comes from the postcapillary venules than from the capillaries themselvesm. Sinusoidsi. Special capillaries that are very wide, twisty, and leakyii. Endothelial cells are separated by wide clefts1. At least have fewer junctions than ordinary capillaries iii. occur where there is an exceptionally large exchange of big substances between the blood and the surrounding