Unit 4: Blood Vessels Lecture Study Guide PDF

**Unit 4: Lecture Study Guide** **Blood Vessels & Vascular System** **[Structure of Blood Vessel Walls:]** The walls of all blood vessels, except the very smallest (capillaries and venules), are composed of three distinct layers, or *tunics* (the **tunica intima**, **tunica media,** and **tunica...

**Unit 4: Lecture Study Guide** **Blood Vessels & Vascular System** **[Structure of Blood Vessel Walls:]** The walls of all blood vessels, except the very smallest (capillaries and venules), are composed of three distinct layers, or *tunics* (the **tunica intima**, **tunica media,** and **tunica externa**), that surround the central blood-filled space called the **lumen**. **Tunica Intima (interna)** This layer is in "intimate" contact with the blood in the lumen. This tunic contains *endothelium*, the simple squamous epithelium that lines the lumen of all vessels The flat endothelial cells form a smooth surface that minimizes the friction of blood moving across them In vessels larger than 1mm in diameter, a thin layer of loose connective tissue, the **subendothelial layer**, lies just external to the endothelium **Tunica Media** The middle tunic consists primarily of *circularly* arranged sheets of smooth muscle fibers, between which lie circular sheets of elastin and collagen fibrils. Contraction of the smooth muscle cells decreases the diameter of the vessel, a process called *vasoconstriction*. Relaxation of the smooth muscle cells, a process called *vasodilation*, increases the vessel's diameter. Both vasoconstriction and vasodilation are regulated by *vasomotor nerve fibers* of the sympathetic division of the autonomic nervous system. **Tunica Externa (adventitia)** The outermost layer of the vessel wall. This tunic is a layer of connective tissue that contains many collagen and elastic fibers. Its cells and fibers run longitudinally Functionally, the tunica externa protects the vessel, further strengthen its wall, and anchors the vessel to surrounding structures *\*\*Capillaries and venules do not have tunics; they're instead composed of endothelium with spare basal lamina* **[Three Major Types of Blood Vessels]** - In the order in which a drop of blood would encounter them as it passes through the systemic or pulmonary circuit. **Arteries** - - - - - - - - - - - - - - - - - - **Capillaries** - - - - - Continuous capillaries lack pores. They are the most common type of capillary, occurring in most organs of the body, such as skeletal muscles, skins and the central nervous system. - Fenestrated capillaries have pores spanning the endothelial cells These capillaries occur only where there are exceptionally high rates of exchange of small molecules between the blood and the surrounding tissue fluid (i.e. capillaries in the small intestine, which receive the digest nutrients from food, and those in the glomeruli of the kidneys, which filter blood, are fenestrated. So are the capillaries in the synovial membranes of joints, where many water molecules exit the blood to contribute to synovial fluid) - Some organs contain wide, leaky capillaries called sinusoids or *sinusoidal capillaries*. Each sinusoid follows a twisted path and has both expanded and narrowed regions. Sinusoids are usually fenestrated (have pores), and their endothelial cells have fewer cell junctions than do ordinary capillaries. Sinusoids occur wherever there is an extensive exchange of *large* materials, such as proteins or cells, between the blood and surrounding tissue. (i.e. bone marrow and spleen, where many blood cells move through their walls.) The large diameter and twisted course of sinusoids ensure that blood slows when flowing through these vessels, allowing time for the many exchanges that occur across their walls. - Capillary beds are a network of the body's smallest vessels Capillary beds run throughout almost all tissues, especially the loose connective tissues. A terminal arteriole leads to a **metarteriole**--a vessel that is structurally intermediate between an arteriole and a capillary--from which branch true capillaries. The metarteriole continues into a **thoroughfare channel**, a vessel structurally intermediate between a capillary and a venule. True capillaries merge into the thoroughfare channel, which then joins a venule. Smooth muscle cells called **precapillary sphincters** wrap around the root of each true capillary where it leaves the metarteriole.  **Precapillary sphincters** regulate the flow of blood to the tissue according to that tissue's needs for oxygen and nutrients. When the tissue is functionally active, the sphincters flow straight from the metarteriole into the thoroughfare channel and venule--thereby bypassing the true capillaries. In this way, capillary beds precisely control the amount of blood supplying a tissue at any time. **Veins** - - - - - - - - - - - - - - - - - - - - -  **[Circulatory Pathways:]** - - - - - - - - - - - - - - - - - - - - - - -