Unit IV- Blood Vessels II

Question 1

General features of capillaries

Answer 1

-smallest vascular channels with an average diameter of 5 to 10 microns and length that varies from 0.05 to 1 mm depending on the organ (there are approximately 60,000 miles of capillaries in the human body)

Question 2

Endothelial cell

Answer 2

• cytoplasm as a Golgi complex, a few mitochondira, and some RER and free ribosomes
• some structural support is provided by intermediate filaments desmin and vimentin
• some endothelial cells have one, some the other, and some both
• large numbers of pinocytotic vesicles are associated with the plasma membrane
• endothelial cells secrete a basal lamina
• have life span of 150-180 days so they are relatively slowly renewing
• an intact endothelium is nonthrombogenic and synthesizes a variety of bioactive substances including types I, III, IV and V collagens, fibronectin, laminin, coagulants, and anticoagulants

Question 3

Pericytes

Answer 3

• stellate cells that surround capillaries and some post-capillary venules
• have extensive branching processes and possess large nuclei rich in heterochromatin, small Golgi, mitochondria, RER, and filaments extending into the processes
• share the basal lamina of the endothelial cell
• some pericytes are capable of contraction to regulate blood flow
• can serve as stem cells for endothelial and smooth muscle cells after injury

Question 4

Continuous Capillaries

Answer 4

• location: most common of three types, they are found in muscle, nervous and connective tissues, and the exocrine pancreas
• structure and function: the endothelial lining and basal lamina are complete. The endothelial cells are sealed by occluding (tight) junctions that permit passage of small molecules. The cytoplasm has many vesicles that transport large molecules

Question 5

Fenestrated capillaries

Answer 5

• Location: These capillaries are found in the pancreas, intestines, and endocrine glands. A special type with no diaphragms is found only in the renal glomerulus
• Structure and Function: they have continuous but thin endothelium and basal lamina. The endothelial cells have small pores or fenestrae in their walls 60-80 nm in diameter covered by a diaphragm except in the renal glomerulus
• fenestraed capillaries appear to be specialized for rapid interchange of substances between blood and tissue

Question 6

Sinusoids

Answer 6

• Location: three types- sinusoidal capillaries found in endocrine glands, discontinuous sinusoids found in the liver and bone marrow and venous sinusoids in the spleen
• Structure and Function: irregular blood channels that conform to the shape of the structure they line. Sinusoidal capillaries have a continuous basal lamina and fenestae with diaphragms that are much large than in fenestrated capillaries
• discontinuous sinusoids have a discontinuous or absent basal lamina and have large gaps between endothelial cells that permits enhanced exchange between blood and tissue

Question 7

Capillary Permeability

Answer 7

• two distinct pore systems in endothelial cells: small (9-11 nm) and large (50-70 nm)
• the small system is thought to be the gaps between cell junctions that allow water and small hydrophilic molecules to pass (gap passes more during inflammation)
• the large pore system involves transcytosis by pinocytoic or trans-Golgi derived vesicles

Question 8

Veins versus Arteries

Answer 8

• veins show greater variation in size and structure than arteries
• veins are of larger caliber and more numerous in the body (contains 70% of total blood volume)
• veins have more extensive vasa vasorum and lymphatics
• veins contain more collagen and less elastic substance and smooth muscle
• veins owe most of their thickness to adventitia
• small and medium veins have valves that are formed from an infolding of the intima with an elastic connective tissue core

Question 9

Large Veins

Answer 9

• intima- contains a continuous endothelium and basal lamina, but no internal elastic lamina
• media- is frequently incomplete or absent
• adventitia- is the largest, best developed tunic and forms 95% of the thickness of the vessel wall. It contains thick bundles of longitudinally oriented smooth muscle and collagen and elastic fibers. The adventitia has well-developed vasa vasorum and lymphatics
• superior and inferior vena cava and the portal vein

Question 10

Small and Medium Veins

Answer 10

• intima- has a continuous endothelium and basal lamina. Complete internal elastic laminae are usually only found in veins of legs. Some valves
• media- poorly developed except in the limbs. Smooth muscle is circumferential
• adventitia is the thickest tunic and usually has no smooth muscle. It does contain collagen and elastic fibers, fibroblasts, lymphatics, and vasa vasorum

Question 11

Venules

Answer 11

• two classes: pericytic and muscular
• functions are to continue the exchange of materials that was started in the capillaries. They are the preferred location for leukocyte emigration. They become leaky in reponse to histamine and serotonin
• pericytic venules- post-capillary venules that are 10-50 microns in diameter and resemble large capillaries (have a pericyte)
• muscular venules- receive the blood from the pericytic venules and accompany arterioles. The media has 1 or 2 circular layers of smooth muscle cells. They have a thin adventitia of loose connective tissue

Question 12

Classification of Veins

Answer 12

Large vein- intima: endothelium, subendothelium CT; Media: CT, SM; Adventitia- longitudinal bundles of smooth muscle
Medium and small vein- intima: endothelium,CT, some valves; media- CT fibers, SM circumferential; adventitia- collagen layers with fibroblasts
Venule- intima: endothelium; media- sparse CT, few SM cells; adventitia- some collagen

Question 13

Portal systems

Answer 13

• carry blood from one capillary bed to another
• examples include the hepatic portal vein between the intestines and the liver, the hypophyseal portal veins of the pituitary, and the efferent arterioles of the renal cortex

Question 14

Arteriovenous anastomoses

Answer 14

• direct cross connections between arterioles and venules

- smooth muscle cells form a sphinchter that regulates blood flow and allows a bypass of a capillary bed

Question 15

Lymphatic Vessels

Answer 15

• Characteristics- large lumens, thin walls, irregular shape, extensive branching that runs parallel to blood capillaries and veins
• Functions- collect lymph from extracellular spaces and return it to the blood vascular system, aid in circulation of lymphocytes
• Structure- lymphatic collecting vessels are similar to veins, but more irregular in size and shape
• lymphatic capillaries have a discontinuous or absent basal lamina and are held open by elastic anchoring filaments