Lecture 7: Vascular Disorders and Thrombosis: Anatomy/Microanatomy

What is the primary function of the circulatory system in multicellular organisms?

To maintain homeostasis by delivering nutrients to cells and removing waste products.

What are the three major components of the circulatory system?

Blood - transport medium for nutrients, waste, gases, hormones.

Central pump (heart) - driving force that distributes blood through the system.

Vascular network - exchange of nutrients and waste; includes arteries and veins.

What is the functional difference between arteries and veins?

Arteries - high-pressure vessels carrying blood away from the heart.

Veins - lower-pressure vessels carrying blood towards the heart.

What is the role of the lymphatic system in circulation?

Drains extracellular (interstitial) fluid.

Lymphatic vessels parallel veins.

Lymph returns to the blood via the thoracic duct.

Do all multicellular organisms have a lymphatic system?

No - most, but not all, have a secondary network of lymphatic vessels.

What structural feature allows arteries to facilitate rapid blood flow?

Large-diameter lumen → minimal resistance to blood movement.

What structural features allow arteries to withstand high pressures near the heart?

Thick vessel walls with smooth muscle (tensile strength) and elastin fibers (elasticity for stretch and recoil).

What is the primary function of elastin fibers in arteries?

Enable stretch during systole and recoil during diastole to maintain continuous blood flow.

How do arterioles differ structurally from arteries?

Narrower lumens; thick smooth muscle layer; fewer elastin fibers; primary site of vascular resistance.

What is the primary functional role of arterioles?

Regulate blood distribution and systemic vascular resistance via vasoconstriction/vasodilation in response to sympathetic/parasympathetic innervation and local intrinsic stimuli.

Gross and histologic appearance of arteries/arterioles vs veins/venules?

Arteries/arterioles: thicker walls, more smooth muscle, more elastin fibers.

Veins/venules: thinner walls, mostly collagen, little smooth muscle/elastin.

What are the three layers of blood vessel walls?

Tunica intima - endothelium + connective tissue.

Tunica media - smooth muscle + elastin.

Tunica adventitia - connective tissue (collagen, nerves, vasa vasorum).

What is the predominant structural protein in veins and its functional implication?

Collagen → allows distention more than contraction.

How much of the total blood volume can veins hold?

Up to 65% (major capacitance vessels).

What anatomical features prevent backflow in veins?

Valves.

What mechanical factors aid venous return to the heart?

Skeletal muscle contraction ("muscle pump").

Valves preventing backflow.

Cardiac suction effect (increased pressure gradient during heart contraction).

Where does venous return begin and how does vessel structure change along the path?

Begins with post-capillary venules (capillary-like structure), gradually adding thin smooth muscle layers further from capillaries.

What is the primary function of capillaries in the circulatory system?

Site of nutrient and waste exchange between blood and surrounding tissues (extracellular/extravascular space).

How abundant are capillaries, and what proportion of total blood volume do they hold?

Most numerous vessels in the circulatory system; contain ~5% of total blood volume at any time.

Q: How does capillary structure facilitate exchange?

Very thin walls. Small lumen → RBCs move in single file.

Slow velocity of blood flow → increased exchange time. Typically ≤1 mm from any cell.

What are the three main types of capillaries and their naming basis?

Continuous, fenestrated, discontinuous - named for the type of endothelial lining and basement membrane.

Structure and permeability of continuous capillaries?

Continuous endothelium & continuous basement membrane.

Allows passage of small molecules and gases (O₂, CO₂) via microvesicles.

Restricts large molecule passage.

Tissue locations of continuous capillaries?

Brain, lung, muscle, bone.

Structure and permeability of fenestrated capillaries?

Discontinuous endothelium with pores (fenestrations).

Continuous basement membrane (negatively charged → repels negatively charged proteins like albumin).

Allows exchange of small and slightly larger molecules.

Tissue locations of fenestrated capillaries?

Renal glomeruli, intestinal villi, endocrine glands, choroid plexus, ciliary body of the eye.

Structure and permeability of discontinuous capillaries (sinusoids)?

Discontinuous endothelium and discontinuous basement membrane.

Allows maximum passage of molecules and even cells.

Tissue locations of discontinuous capillaries?

Filtration: liver. Removal/clearance: liver, spleen.

Surveillance: lymph nodes, spleen.

Hematopoiesis: bone marrow (cells pass into circulation on demand).

Why do different tissues have different types of capillaries?

Structural differences regulate what substances can pass, matching permeability to tissue function and needs.

What is the primary purpose of lymphatic vessels?

Drain excess interstitial fluid from tissues, return it to blood circulation, and allow transport of large particles (e.g., chylomicrons, cells).

Where do lymphatic vessels originate, and what is their structure at the start?

Begin as blind-ended lymphatic capillaries surrounding the microcirculation.

How does the endothelial structure of lymphatic capillaries differ from blood capillaries?

Overlapping endothelial cells.

Large interendothelial gaps → accommodate larger molecules/particles.

What prevents backflow of lymph once inside a lymphatic vessel?

Pressure within the lumen pushes overlapping endothelial cells closed.

Valves to prevent reverse flow.

How is lymph moved through the lymphatic system?

Contraction of surrounding skeletal muscles (muscle pump) and vessel distensibility.

Describe the flow pathway of lymph from tissues back to circulation.

Lymphatic capillaries → larger lymphatic vessels → lymph nodes → thoracic duct → vena cava → blood circulation.

How are lymphatic vessels similar to veins?

Both are low-pressure systems, distensible, and contain valves.

What is the basic structure of the endothelium?

A single layer of endothelial cells lining all components of the circulatory system.

What are the major functions of the endothelium?

- Fluid distribution

- Inflammation

- Immunity

- Angiogenesis

- Hemostasis

How does the endothelium act as a dynamic interface?

Regulates exchange between blood and tissues, participates in immune and inflammatory signaling, and modulates vascular tone and clotting.

What are the antithrombotic properties of normal endothelium?

Prevents clot formation by regulating hemostasis and is profibrinolytic (promotes breakdown of fibrin).

What happens to endothelium when injured?

Loses antithrombotic and profibrinolytic properties, often becoming pro-thrombotic and pro-inflammatory.

What is a rete mirabile?

A specialized vascular network of arterial blood vessels running through the center of large venous sinuses.

What is the primary function of the rete mirabile?

Countercurrent exchange for:

- Temperature regulation

- Ionic concentration gradients

- O₂/CO₂ exchange

- Equalizing blood pressure

What is the most well-known rete mirabile, and in which species is it found?

Rete mirabile cerebri in ruminants.

Where is the rete mirabile cerebri located?

Around the right and left internal carotid arteries as they pass the pituitary gland, and along the cranial floor.

What is the interstitium?

The extravascular compartment between parenchymal/stromal cells and the microcirculation.

What are the main functions of the interstitium?

Provide pathways for microvasculature (vascular adventitia), lymphatic vessels, nerves, and trafficking leukocytes.

Modulate systemic physiologic properties exerted by parenchymal cells.

Act as a general fluid pool/reservoir, cushioning organs and storing water/ions.

Provide structural framework for cell survival.

Why is the interstitium important clinically?

Plays a major role in influencing disease processes (e.g., edema, inflammation, metastasis).

What is the extracellular matrix (ECM)?

The structural, adhesive, and absorptive components within the interstitium.

What are the main components of the ECM?

Type I collagen - structural framework.

Glycoproteins - attachment sites for structural proteins and adhesion sites for transmigrating leukocytes.

Glycosaminoglycans - absorptive disaccharide complexes.

Proteoglycans - hydrophilic, bind large amounts of water and solute molecules.

Which cells produce ECM components?

Parenchymal cells, fibroblasts, glial cells (CNS only), macrophages, and trafficking leukocytes.