Blood Vessels

1. Blood Vessels Mr. Neuberger

2. Blood vessels • There are 3 principle categories of blood vessels- Arteries, Veins, and Capillaries • Arteries- The efferent vessels, they carry blood away from the heart • Veins- The afferent vessels, they carry blood to the heart • Capillaries- microscopic, thin-walled vessels that connect the smallest arteries to the smallest veins

3. The vessel wall • The walls of arteries and veins are composed of three layers called tunics • 1. The Tunica Interna (tunica intima)- • lines the inside of the vessel and is exposed to the blood. Composed of a simple squamous epithelium called the endothelium, overlying a basement membrane and a sparse layer of loose connective tissue • 2. The Tunica Media- • Middle layer, usually thickest. It consists of smooth muscle, collagen, and sometimes elastic tissue. Ratio of muscle to elastic tissue varies greatly. Strengthens the vessels and prevents blood pressure from rupturing them. Provides vasomotion (changes in vessel diameter) • 3. The Tunica Externa (tunica adventitia)- • Outermost layer. Consists of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs. Small vessels called the vasa vasorum supply blood to at least the outer half of the vessel wall

4. arteries • Considered to be the resistance vessels because they have a strong, resilient tissue structure that resist high BP from within • The heart causes a surge of pressure into the arteries with each beat, the arteries have to be strong enough to withstand these pressure surges • Arteries are more muscular than veins and they retain their round shape even when empty • Divided into three categories by size

5. Arteries

6. Arteries • Conducting (elastic or large) Arteries- • These are the biggest arteries in the body, think aorta or pulmonary trunk as an example • Expand during ventricular systole to receive blood, and recoil during diastole. • This expansion takes some stress off of the smaller arteries further down the body. Their recoil between beats prevents blood pressure from dropping too low in the heart • Distributing (muscular or medium) Arteries- • Small branches that distribute to specific organs • Brachial, femoral, and renal arteries are examples • Typically have 40 layers of smooth muscle , ¾ of the wall thickness • Resistance (small) Arteries- • 25 layers of smooth muscle, relatively little elastic tissue • Arterioles- smallest resistance arteries, sometimes only one RBC can pass through at a time

8. Arterial sense organs • Certain major arteries have sensory receptors in their walls that monitor blood pressure and chemistry, serves to regulate heartbeat, vasomotion, and respiration • Carotid Sinuses- baroreceptors (pressure sensors) that respond to changes in blood pressure. • Located in the wall of the internal carotid artery just above the branch point • Contain nerves that sense these changes • Carotid Bodies- Chemoreceptors, monitor changes in blood composition • Primarily transmit signals to the brain to stabilize blood pH, oxygen, and carbon dioxide levels by regulating breathing • Aortic Bodies- Chemoreceptors, similar structure to carotid bodies, and same function

9. Capillaries • The “business end” of the cardiovascular system are the capillaries and venules • The rest of the cardiovascular system exists to serve the exchange process that occurs here • Called the exchange vessels, outnumber and are more important than the venules • Composed only of an endothelium and basal lamina • Number of capillaries has been estimated at a billion, and their total surface area at 6,300 m² • Scarce in tendons and ligaments, occasionally in cartilage, and absent from epithelia and the cornea and lens of the eye

10. Types of capillaries • Distinguished by the ease with which they allow substances to pass through their walls, permeability • Continuous Capillaries- Occur in most tissues such as skeletal muscle • Their endothelial cells, held together by tight junctions, form a continuous tube • A thin protein-carbohydrate layer, the basal lamina, surrounds the endothelium and separates it from the adjacent connective tissues. Endothelial cells separated by intercellular clefts, glucose can pass through • Contain pericytes- elongated tendrils that wrap around the capillaries, contain the same contractile protein as muscle • Fenestrated Capillaries- have endothelial cells riddled with holes called filtration pores • Allow for the rapid passage of small molecules but still retain most proteins and larger particles in the bloodstream • Sinusoids (discontinuous capillaries)- irregular blood-filled spaces in the liver, bone marrow, spleen, and some other organs • Several types of proteins can pass through

12. Capillary permeability • Structure is closely related to permeability • There are 3 the materials can travel through: • (1) the intercellular clefts between endothelial cells • (2) the filtration pores in fenestrated capillaries • (3) through the endothelial cell plasma membrane and cytoplasm • Nonpolar molecules such as oxygen, carbon dioxide, lipids, and thyroid hormone diffuse easily through the enothelial cells

13. Capillary beds • Capillaries are organized into groups called capillary beds- 10-100 capillaries supplied by 1 metarteriole. • Beyond the origins of the capillaries, the metarteriole continues as a thoroughfare channel leading directly to a venule • Capillaries empty into the distal end of the thoroughfare channel or directly into the venule • When capillary openings (circular muscle) are closed blood bypasses the capillaries • There is not enough blood in the body to fill the entire vascular system at one, so ¾ of the body’s capillaries are shut down at any given time • About 90% of skeletal muscles have little to no blood flow during periods of rest, when exercise occurs, capillary beds elsewhere (ex. Skin, Intestines) shut down to compensate for it

15. Veins • Regarded as the capacitance vessels of the cardiovascular system because they are relatively thin-walled and flaccid, and expand easily to accommodate an increased volume of blood. • They have a greater capacity for blood than arteries do • At rest, about 54% of the blood is found in the systemic veins, 11% in the systemic arteries • Since they are subject to low blood pressure, their walls are thin • Blood flow is steady, not pulsating like arteries • Collapse when empty, no need for thick, pressure-resistant walls • They merge to form larger veins as they travel back to the heart, opposite of the arteries

16. Types of veins • PostcapillaryVenules are the smallest of the veins • Receive blood from capillaries directly or by way of the distal ends of the thoroughfare channels • Tunica interna with only a few fibroblasts around it and no muscle, surrounded by pericytes, more porous than capillaries • Muscular Veins receive blood from the postcapillaryvenules, 1-2 layers of smooth muscle • Medium Veins are the veins with specific names, such as radial and ulnar vein. • Have venous valves directed towards the heart. The pressure in the veins is not high enough to push the blood back to the heart against gravity. The work is done by the massaging action of the skeletal muscles, skeletal muscle pump. • Venous Sinuses are veins with esp. thin walls, large lumens, and no smooth muscle, not capable of vasomotion • Large Veins have some smooth muscle in all three tunics

18. Circulatory routes • Heart -> arteries -> capillaries -> veins -> heart • Blood usually only passes through one network of capillaries from the time it leaves the heart until the time it returns • Portal System blood flows through two consecutive capillary networks before returning to the heart, ex. Kidneys • Anastomosis is a point where two veins or arteries merge with each other without intervening capillaries • Arteriovenous Anastomosis (Shunt) blood flows from an artery directly into a vein, ex. Fingers, ears • Arterial Anastomosis two arteries merge, providing collateral routes of blood supply to a tissue • Venous Anastomoses where one vein empties directly into another, provide alternate drainage