Circulatory System

Circulatory System Overview

• The circulatory system is a blood transportation network made up of the heart and blood vessels.
• It consists of 3 types of blood vessels: arteries, veins, and capillaries.

Blood Vessel Walls

• Blood vessel walls consist of three layers (tunica):
  • Externa (adventitia): outer connective tissue layer
  • Media: middle smooth muscle layer (contains varying amounts of elastic fibers)
  • Intima: inner endothelial lining

Tunica Intima

• Lined by endothelium consisting of simple squamous epithelial cells
• Basement membrane and subendothelial connective tissue support overlying cells

Tunica Media

• Intermediate, smooth muscular layer
• Contains alpha and beta adrenergic receptors, allowing for sympathetic regulation of blood pressure

Tunica Externa/Adventitia

• Outermost layer
• Anchors vessels to organs
• Comprised of type 1 collagen and elastic connective tissue (in arteries)

Capillaries

• Smallest vascular structures in the body
• Arise from terminal ends of arterioles and venules
• Primary function: deliver nutrient-rich blood to tissues/cells and take away waste from tissues/cells
• Classified into 3 types based on the arrangement of the endothelium along the vessel wall:
  • Continuous: most common type, endothelial cells are within close proximity and fitted with gap junctions
  • Fenestrated: capillary beds are perforated along the endothelial cells, most pores are covered with ultrathin diaphragm
  • Discontinuous (sinusoidal): tortuous, irregular vessels, widely spaced endothelial cells, and many large fenestrae with no pore diaphragm

Fetal Circulation

• Three vascular shunts exist in the fetal circulation to allow blood to bypass the liver and non-functioning lungs:
  • Foramen ovale: between the atria, blood bypasses the pulmonary circulation
  • Ductus arteriosus: links the distal arch of the aorta with the pulmonary trunk, blood bypasses the pulmonary circulation
  • Ductus venosus: shunts 30% of umbilical blood from the primitive liver to the IVC
• These shunts are open prenatally and close postnatally (after birth)

Transition to Adult Circulation

• A baby's first breath causes a decrease in pulmonary vasculature resistance, which increases the relative pressure in the LA > RA and causes the foramen ovale to close
• Increased O2 concentration within the blood leads to decreased prostaglandins, causing the walls of the ductus arteriosus and venosus to contract and close

Arteries

• Classified into two main types: elastic (conducting) and muscular (distributing)
• Elastic arteries: have substantial elastic tissue in the tunica media, accommodate high pulsatile forces
• Muscular arteries: have elastic fibers only at the intersection of the intima and media or media and adventitia, allow contraction (vasoconstriction) or relaxation (vasodilation)

Arterial System

• Arch of aorta: main blood supply to the head, neck, thorax, and arms
• Descending aorta: supplies all of the body apart from the superior aspect
• Carotid artery: carotid sinus (baroreceptors sensitive to blood pressure), carotid body (chemoreceptors sensitive to O2, CO2 levels, and H+ ion concentration)

Veins

• Veins structure is largely the same regardless of size, classified based on diameter and wall thickness (small, medium, large)
• Distinguishing features of veins vs. arteries:
  • Thinner walls
  • Larger lumina diameter
  • More abundant than arteries
• Superior vena cava (SVC): drains blood from head, neck, upper limbs, and thorax
• Inferior vena cava (IVC): drains abdomen and lower limbs

Venous System

• Cisterna chyli: dilated origin of the thoracic duct, receives fatty lymph from intestine
• Thoracic duct: collects lymph from left head, neck, thorax, and upper limb, and lower half of body
• Right lymphatic duct: collects lymph from right head, neck, thorax, and upper limb

Lymphatic System

• A series of organs, vessels, and nodes that collect and filter excess tissue fluid (lymph) before returning it to the venous circulation
• Functions:
  • To drain excess interstitial fluid from tissues into the venous system
  • To produce and transport immune cells (lymphocytes)
  • To mount an immune response against pathogens
  • To transport dietary lipids from the GI tract into the blood

This quiz covers the basics of the circulatory system, including its structure, arteries, veins, and lymphatics, as well as major vessels and the heart. It also touches on blood transportation and the three types of blood vessels.