The Heart and Blood Vessels

• Arteries transport blood away from the heart, with larger arteries having a thick layer of muscle to withstand high pressures
• As blood moves farther from the heart, arteries branch into smaller diameters
• Large and medium-sized arteries are stiff yet somewhat elastic, stretching in response to high pressure and providing blood to capillaries even between heartbeats
• The elastic recoil of arteries maintains blood pressure between beats, similar to a city’s water system of branching pipes providing constant water pressure
• The structure of arteries includes three layers surrounding the lumen:

  • Endothelium: a slick surface of flattened, squamous epithelial cells
  • Layer of smooth muscle with elastic connective tissue
  • Outermost layer of tough connective tissue primarily collagen
• Arteries are at risk of injury due to constant high pressure, with damage potentially leading to aneurysms
• Aneurysms can cause severe chest pain, reduced blood flow, or be symptomless until rupture, leading to massive internal bleeding and often death
• Aneurysms of the aorta, the body’s largest artery, can be fatal and may develop over years, but can be detected and repaired surgically
• elastic recoil


  he force that maintains the blood pressure between beats
• endothelium


  The thin inner layer, flattened, squamous epithelial cells. It is a continuation of the lining of the heart. The flattened cells fit closely together, creating a slick surface that keeps friction to a minimum and promotes smooth blood flow.
• middle layer


  the thickest made mostly of smooth muscle and connective tissue. Can constrict to maintain blood pressure and withstand it.
• outermost layer

  supportive collagen that protects the vessels form injury.
• aneurysm

  blood may seep through the injured area and work its way between the two outer layers, splitting them apart

  ballooning of the artery wall. Some aneurysms cause the smooth muscle and endothelial layers to bulge inward as they develop, narrowing the lumen enough to reduce blood flow to an organ or region of the body
• arterioles


  smallest arteries that may not have their outermost layer. They also will have significantly less smooth muscle.

  They also have the ability to control the amount of blood in the capillary.
• precapillary sphincte


  serve as gates that control blood flow into individual capillaries
• vasoconstricted


  constriction of precapillary sphincte

• vasodilation


  precapillary sphincters increases their diameter
• capillaries


  Capillaries are thin-walled vessels that average only about one-hundredth of a millimeter in diameter
• capillary beds


  Extensive networks of capillaries
• Why does most of the fluid that was filtered out of the proximal (beginning) end of the capillary move back into the distal (far) end?

  there is a slightly higher concentration of dissolved molecules (especially proteins) in the capillary compared to the interstitial fluid and also because the blood pressure is lower at the distal end
• lymphatic system


  absorbing excess interstitial fluid, the capillaries of the lymphatic system also pick up substances in the interstitial fluid that are too large to diffuse into the blood capillaries, such as lipid droplets absorbed during digestion and invading microorganisms.
• veins


  blood flows back to the heart through venule

  far less pressure

  contains 2/3 of blood at a given time
• venule


  small veins
• varicose veins


  permanently swollen veins that look twisted and bumpy from pooled blood. Varicose veins can appear anywhere, but they are most common in the legs and feet
• three mechanisms assist the veins in returning blood to the heart

  1. contractions of skeletal muscles
  2. one way valves inside of veins
  3. movements associated with breathing
• How do muscles help blood return to the heart?

  Contraction. They press against the veins causing them to contract. Movment helps.
• skeletal muscle pump

  When the valves and skeletal muscle work together to pump blood back to the heart
• one way valves

  passive valves that close when blood begins flowing backwards
• how the lungs push blood back to the heart(respiratory pump)

  When we inhale, abdominal pressure increases and squeezes abdominal veins. At the same time, pressure within the thoracic cavity decreases, dilating thoracic veins.
• The human heart is a muscular, cone-shaped organ consisting mostly of cardiac muscle, which contracts spontaneously and is not connected to bone
• The heart contracts in a cyclic, coordinated, squeezing motion that propels blood through the blood vessels
• The heart is surrounded by a tough fibrous sac called the pericardium, which protects it and anchors it to surrounding structures
• The walls of the heart consist of three layers: the epicardium, myocardium, and endocardium
• The heart has four chambers: two atria and two ventricles, separated by a muscular partition called the septum
• Blood returning to the heart from the body enters the right atrium, then passes through a valve into the right ventricle, and is pumped to the lungs for oxygenation
• Four heart valves enforce the heart's one-way flow pattern and prevent blood from flowing backward
• The heart pumps blood through two circuits simultaneously: the pulmonary circuit (lungs) and the systemic circuit (the rest of the body)
• The heart has its own set of blood vessels called the coronary arteries that supply the heart muscle
• The cardiac cycle involves contraction of the atria, followed by contraction of the ventricles, and then relaxation of the entire heart
• The cardiac cycle consists of three steps:

  1. Atrial Systole: both atria contract, raising blood pressure in the atria and filling the ventricles to capacity

  2. Ventricular Systole: both ventricles contract, causing the AV valves to close and the semilunar valves to open, ejecting blood into the pulmonary trunk and aorta

  3. Diastole: both atria and ventricles relax, ventricular pressure falls, semilunar valves close, and AV valves open for blood to flow passively into the heart