Histology LC12 Capillaries and Veins PDF

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LC12 CAPILLARIES COLLEGE OF MEDICINE, BATCH 2026...

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LC12 CAPILLARIES COLLEGE OF MEDICINE, BATCH 2026 Transcribers: Gadaza, Gascon, Hierco, Lalap, Lang-ayan Dr. Peeble Agdamag | Dec. 12, 2022 Editors: Gadaza, Gascon, Hierco, Lalap, Lang-ayan CAPILLARIES I. Capillaries II. Capillary beds III. Metarterioles IV. Precapillary sphincters V. Types of Capillaries VI. Pericytes VII. Medical Application VIII. Venules IX. Veins X. Small or Medium Veins XI. Large Veins XII. Valves XIII. Test Yourself XIV. References I. Capillaries permit and regulate metabolic exchange between blood and surrounding tissues. Figure 2. TEM of a capillary cut transversely always function in groups called capillary beds whose size and overall E= endothelial cell nucleus L= lumen shape conforms to structure supplied BL = Basal lamina richness (density) of the capillary network is related to metabolic J= Junctional complexes activity of tissues P = pericyte composed of single layer of endothelial cells rolled up as a tube Tissues with high metabolic rates - have abundant capillaries surrounded by basement membrane -kidney make up > 90% of body’s vasculature -liver total length of > 100,000 km and total surface area of = 5000 m2. -cardiac and skeletal muscle cyclical opening and closing of sphincters, 🡪 essentially empty *If you had an accident, one of the sites where there would be a lot of bleeding is capillaries at any given time your kidney and liver, because they have lots of capillaries (this is where you with only about 5% (~300 mL in an adult) of the total blood volume need a lot of capillaries) moving through these structures. thin walls, extensive surface area, and slow, pulsatile blood flow optimize capillaries for exchange of water and solutes between blood and tissues. basal lamina helps determine which macromolecules interact with endothelial cells. Average thickness is only 0.25 μm distinctive feature is often nucleus curved to accommodate the very small tubular structure The cytoplasm contains mitochondria and most other organelles, as well as a large population of membranous vesicles typically Along with basal lamina, junctional complexes between the cells maintain the tubular structure, with variable numbers of tight junctions having an important role in capillary permeability. Figure 3. Branching network of capillaries in the myocardium Tissues such as cardiac muscle in the heart have high energy requirements so they have a dense, highly branched capillary network. Figure 1. Capillary with pericytes P = Pericytes (perivascular contractile cells) Figure 4. Capillaries in skeletal muscle in transverse section. Page 1 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag Cap = capillaries MF = muscle fibers Capillaries (Cap) are close to muscle fibers (MF) in skeletal muscle Tissues with low metabolic rates: -smooth muscle -dense connective tissue Examples: heart and the digestive tract (you have less capillaries there) Figure 6. Capillary beds Part a show a well-perfused capillary bed with all the sphincters relaxed and open; part b shows a capillary bed with the blood shunted away by contracted sphincters. At any given moment, most sphincters are at least partially closed, and blood enters the capillary bed in a pulsatile manner for maximally efficient exchange of nutrients, wastes, O2, and CO2 across the endothelium. Except in the pulmonary circulation, blood enters the microvasculature well oxygenated and leaves poorly oxygenated. Figure 5. Capillaries sectioned in transverse and longitudinal planes in a mesentery of the small intestine V. Types of capillaries 1. Continuous (Tight) Capillaries II. Capillary Beds tight, well-developed occluding junctions between slightly overlapping endothelial cells and continuity along endothelium supplied preferentially by one or more terminal arteriole branches well-regulated metabolic exchange across cells called metarterioles (encircled by scattered smooth cells) most common continuous with thoroughfare channels (lack muscle) connected with the post- capillary venules found in all muscle tissues and in areas with a blood-tissue barrier, such as the blood-brain barrier (central nervous system), blood-air barrier (lungs), and blood-thymus, blood-ocular, and blood-testis III. Metarterioles barriers. An overlying basal lamina (20-50 nm thick) encloses the endothelium terminal portions of arterioles and surrounds occasional pericytes, or Rouget cells. consist of a single layer of smooth muscle and, by vasoconstriction, E.g. Muscle, connective tissue, lungs, exocrine glands, and nervous control the amount of blood entering capillaries tissue Metarteriole muscle cells- act as precapillary sphincters that control blood flow Ultra-structurally numerous vesicles 🡪 transcytosis of macromolecules True capillaries in both directions branch from metarterioles Exchange is not via the holes. At beginning of each true capillary, muscle fibers act as precapillary Vesicles can be found in the tissue which is in charge of the exchange sphincters that contract or relax to control blood entry via dilution since there are no holes. That’s how they exchange gasses and electrolytes. IV. Precapillary sphincters Contract 5 to 10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner. When sphincters close, blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules. contract and relax so that blood can enter and exit you have a lot of sphincters: esophagus, stomach (pyloric sphincters) -doors to a specific organ or tissue Figure 7. Continuous Capillary Page 2 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag Figure 10. Ultrastructure of a fenestrated capillary sectioned in a transverse Figure 8. Ultrastructure of a continuous capillary sectioned in a plane in the choroid plexus of a CNS ventricle. transverse plane in the CNS. 2. Fenestrated Capillaries 3. Discontinuous Capillaries -have a sieve- like endothelial cells penetrated by numerous small circular openings or fenestrations ∼ 80 nm diameter A unique feature is the presence of minute, circular transcellular sinusoids openings—fenestrae—in endothelial cells. permit maximal exchange of macromolecules Fenestrations are small holes. There is better flow. The exchange of allow easier movement of cells between tissues and blood nutrients and oxygen are more extensive than that of continuous. Individual endothelial cells have large perforations without diaphragms -basal lamina is continuous and covers fenestrations. collectively form a discontinuous layer, with wide, irregular spaces -Endothelial cells, held together by tight junctions and gap junctions, between the cells usually rest on a thin basal lamina. - in organs with rapid interchange of substances between tissues and highly discontinuous basal laminae and much larger diameters, often 30 the blood to 40 μm, which slows blood flow kidneys, intestine, choroid plexus, and endocrine glands found in the liver, spleen, some endocrine organs, and bone marrow basal lamina is either absent or incomplete There are more holes than there are endothelium. It depends on the type of tissue whether it want a regulated or not regulated exchange of fluids Figure 9. Fenestrated Capillary Some fenestrations are covered by very thin diaphragms of proteoglycans; others may represent membrane invaginations during trans- cytosis that temporarily involve both sides of the very thin cells Figure 11. Sinusoid Page 3 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag Figure 12. Sinusoidal capillary (bone marrow) S= sinusoid A= adipocytes H= hematopoietic cells Figure 13. Sections through the three types of blood vessels VI. Pericytes There are 3 types of Venules: Supporting cells in the capillaries, they could become endothelium or 1. Small 2. Medium smooth muscle. 3. Large mesenchymal cells along continuous capillaries and postcapillary venules All arteries have corresponding veins beside it long cytoplasmic processes partly surrounding endothelial layer Secrete may ECM components and produce own basal lamina, which transition from capillaries to venules occurs gradually may fuse with endothelial cells immediate postcapillary venules similar to capillaries with pericytes, but With well-developed networks of myosin, actin, and tropomyosin range diameter from 15 to 20 μm contractile function to facilitate flow of blood cells. After tissue injuries, pericytes proliferate and differentiate to form primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage. smooth muscle and other cells in new vessels as the microvasculature is reestablished. Postcapillary venules converge into larger collecting venules that have -mesenchymal derived pluripotent stem cells that can give rise to more contractile cells. endothelial cells, fibroblasts, or smooth muscle cells in blood vessel With greater size, venules become surrounded by tunica media with two walls, depending on the type of vessel, especially in response to injury or three smooth muscle layers 🡪 muscular venules or stimulation by growth factor characteristic feature of all venules is large diameter of lumen compared to the overall thinness of the wall VII. Medical Application hyperglycemia in diabetes 🡪 diabetic microangiopathy (Very common disease that affects the capillaries, the reason for many complication) o diffuse thickening of capillary basal laminae of endothelium o decrease in metabolic exchange, particularly in kidneys, retina, skeletal muscle, and skin. o Impaired exchange of sugar, oxygen and nutrients o Capillaries are everywhere, i.e. kidney disease, change in skin color, increased stroke risk, increased risk for heart attack in diabetes, prone to cardiovascular diseases o COVID-19: Diabetes patients at risk for vascular diseases due to inflamed capillaries; sudden death in some patients 1- 3 months after COVID o Inflammation due to infection o Obesity is also inflammatory VIII. Venules Size varies from 10 microns (post-capillary venules) to 1 mm (muscular venules) larger diameter than capillaries; consist of endothelium surrounded by pericytes Functions o (1) Collect blood from capillaries o (2) Respond to vasoactive agents (e.g., histamine, serotonin) by altering permeability o (3) Also, a site of exchange of materials between tissue fluid and blood o (4) Site of exit of WBCs from blood into tissue Figure 14. Between capillaries and veins. Page 4 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag (a) Compared to arterioles (A), postcapillary venules (V) have large lumens and an intima of simple endothelial cells, with occasional pericytes (P). X400. Toluidine blue (TB). (b) Larger collecting venules (V) have much greater diameters than arterioles (A), but the wall is still very thin, consisting of an endothelium with more numerous pericytes or smooth muscle cells. X200. Toluidine blue. Figure 15. Veins usually travel as companions to arteries and are classified as small, medium, or large based on size and development of the tunics. Micrograph of small vein (V) shows a relatively large lumen compared to the small muscular artery (A) with its thick media (M) and adventitia (Ad). The wall of a small vein is very thin, containing only two or three layers of smooth muscle. X200. H&E. Figure 15. LM of a venule and arteriole in transverse section. The venule has XI. Large Veins a thin wall and a relatively larger lumen than the arteriole. The lumen of each vessel holds many erythrocytes, but the venule lumen also has many white blood big venous trunks, paired with elastic arteries close to heart, cells, a feature often seen in sections of venules. Venules have thin walls and are Tunica intima – well developed thus the main site of migration of leukocytes from the bloodstream to tissues. Via contraction, smooth muscle in arterioles regulates pressure in the arterial Tunica media- thin with alternating layers of smooth muscle and connective tissue. system. 385×. H&E. Tunica adventitia thicker than media in large veins and frequently contains IX Veins longitudinal bundles of smooth muscle. B On the media and adventitia contain elastic fibers, but internal and carry blood back to heart from microvasculature all over the body external elastic laminae like those of arteries are not present Blood under very low pressure Function - to collect blood from medium sized veins and return it to heart moves toward heart by contraction of smooth muscle fibers in media and by external compressions from surrounding muscles and other organs Valves project from the tunica intima to prevent backflow of blood Veins have special structures -veins have thinner muscular layer Not unlike artery that have muscle that can contract and propel blood wherever they should go -have valves, they are like doors, two doors that open and close and they bring back blood to your heart They are small,medium and large veins Large vein - Superior/ Inferior Vena Cava Medium and small - if they are together with your muscular artery X. Small or Medium Veins diameters of 10 mm or less located close and parallel to corresponding muscular arteries Figure 16. Large Veins A. tunica intima – thin subendothelial layer B. tunica media - small bundles of smooth muscle cells + reticular fibers + XII. Valves delicate network of elastic fibers. C. tunica adventitia well developed collagenous layer paired folds of the intima projecting across the lumen in medium and Function - to collect blood from smaller venous vessels large veins rich in elastic fibers lined on both sides by endothelium numerous in veins of the legs o help keep the flow of venous blood directed toward the heart Page 5 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag XIII. Test Yourself Answer key 1. Permit and regulate metabolic exchange between blood and surrounding 1C.2A.3D.4A.5B.6A.7B.8C.9E.10A tissues. A. Veins B. Venules XIV. References C. Capillaries D. Metarterioles Mescher, A. L., & Uchôa, J. L. C. (2018). Junqueira's basic histology: Text and 2. Capillary beds are supplied preferentially by one or more terminal Atlas. McGraw Hill Education. arteriole branches called _________ continuous with _________. A. Metarterioles, Thoroughfare channels B. Venules, Thoroughfare channels Netter, F. H., Ovalle, W. K., Nahirney, P. C., & Chovan, J. (2021). Netter's C. Throughfare channels, Metarterioles essential histology: With correlated histopathology. Elsevier Heath D. Thoroughfare channels, Venules Sciences. 3. Capillaries are composed of how many layers of endothelial cells? A. 4 B. 3 C. 2 D. 1 4. These paired folds of the intima projecting across the lumen in medium and large veins A. Valves B. Arterioles C. Throughfare channels D. Endothelial cells 5. Which description is true of continuous capillaries? A. Unusually wide lumens B. Most common in both brain and muscle C. Abundant fenestrations D. Lack a complete basement membrane E. Phagocytic cells often seen inserted in the intercellular clefts 6. Which of the following is true of pericytes? A.. Are associated with the basal lamina of capillary endothelial cells B. Have similar histological features as contractile cells of the myocardium C. Form a layer of cells joined by gap junctions D. Are terminally differentiated E. Capable of forming multinucleated muscle fibers 7. During light microscopic examination of a tissue, you note a vessel that has no smooth muscle but a large amount of connective tissue at its periphery. Which of the following vessels are you examining? A. Arteriole B. Venule C. Elastic artery D. Capillary E. Large vein 8. These are big venous trunks, paired with elastic arteries close to heart. A. Arteriole B. Venule C. Large Vein D. Small and Medium Vein E. Pericytes 9. Supporting cells in the capillaries, they could become endothelium or smooth muscle. A. Arteriole B. Venule C. Large Vein D. Small and Medium Vein E. Pericytes 10. Tight, well-developed occluding junctions between slightly overlapping endothelial cells and continuity along endothelium A. Continuous B. Discontinuous C. Fenestrated D. Thoroughfare E. Pericytes Page 6 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag XV. Appendices Page 7 of 8 [HISTOLOGY] 1.12 CARTILAGE – Dr. Peeble Agdamag Page 8 of 8