14: Gas Exchange + Circulation Pt. 2

• Circulatory Systems

  • Large animals move fluid through their bodies by bulk flow, or convective transport
  • Transport can occur over greater distance
  • Major functions:
  • Transport oxygen, CO2, nutrients, waste products, signaling molecules (hormones), immune system cells, and heat throughout the body
• Animals maximize the surface area available for diffusion of gases and other key solutes in a variety of ways:

  • Tiny animals have a small enough volume that diffusion over their body surface is adequate to keep them alive
  • Tapeworms and flatworms have a high ratio of surface are to volume
  • The highly folded gastrovascular cavity of jellyfish offers a large surface area for molecular exchange
• Larger animals require a circulatory system in order to achieve a large ratio of surface area to volume:

  • A circulatory system carries blood or hemolymph into close contact with every cell in the body
• There are 2 types of circulatory systems:

  1. Open
  2. Closed
• Open Circulatory System:

  • Hemolymph is pumped throughout the body in open vessels
  • The hemolymph is not confined exclusively to the vessels, but comes in direct contract with body tissues
  • As a result, the molecules being exchanged between hemolymph and the tissues do not have to diffuse across the wall of a vessel
• Closed Circulatory System:

  • Blood flows in a continuous circuit of closed vessels
  • Pressure provided by the pumping action of the heart
  • Blood flow can be directed in a precise way to respond to a tissues' needs
• Blood vessels are classified as arteries, capillaries, or veins
• Arteries:

  • Tough, thick-walled vessels
  • Take blood away from the heart under high pressure
  • Small arteries are called arterioles
• Capillaries:

  • Vessels whose walls are just one cell thick
  • Allows exchange of gases and other molecules between blood and tissues in networks called capillary beds
• Veins:

  • Thin-walled vessels that return blood to the heart
  • Small veins are called venules
• The structure of arteries, capillaries, and veins correlates closely with their function in a closed circulatory system
• The walls of the arteries are composed of smooth muscle fibres and elastic fibres
• The aorta:

  • A large artery that receives blood from the heart
  • Has elastic fibres in its wall, allowing it to expand when blood enters it under high pressure from the heart and subsequently propel blood forward through elastic recoil
• The walls of arterioles have muscle fibres that relax to allow the diameter to increase, reducing resistance and increasing blood flow

  • When the muscle fibres contract, they decrease the vessel diameter, thus increasing resistance and slowing blood flow
  • In this way, blood flow to specific tissues can be regulated by signals from the nervous system to the muscle fibres
• Capillaries:

  • Vessels where gases, nutrients, and wastes are exchanged between the blood and other tissues
  • There walls are only one cell layer thick
  • They form an extremely dense network throughout the body
  • Blood pressure drops dramatically as blood passes through the arterioles into the capillary beds
• Veins:

  • Carry the blood back to the heart
  • Because blood is under relatively low pressure as it exits the tissues, veins have thinner walls and larger interior diameters than arteries do
  • Blood flow in veins is sped up by skeletal muscle activity in the extremities, which compress large veins
• Larger veins contain one-way valves, which are thin flaps of tissue that prevent backflow of blood
• All veins contain some muscle fibres, which contract in response to signals from the nervous system
• Skeletal muscle contraction and negative thoracic pressure also assist in venous return
• Blood pressure in a closed circulatory system is regulated, in part, by active adjustment of the volume of blood in the veins
• The relatively high operating pressure of closed circulatory systems produces a small, but steady leakage of fluid from the blood vessels into the surrounding interstitial space
• Interstitial fluid builds up because of 2 forces:

  1. An outward-directed hydrostatic force in capillaries, created by the pressure on blood generated by the heart
  2. An inward-directed osmotic force in capillaries, created by the higher concentration of solutes in the blood plasma than in the interstitial space
• The mechanism to drain excess fluid is carried out by the lymphatic system

  • It is a collection of thin-walled, branching tubules called lymphatic ducts or vessels
  • Interstitial fluid that enters the lymphatic ducts is called lymph
  • Lymphatic vessels join together to form larger vessels
• The Lymphatic System:

  • The largest vessels return excess fluid to the veins entering the heart
  • If the lymphatic system is blocked, it leads to a buildup of fluid and swelling in the tissues
• Atria: receive blood returning from circulation
• Ventricles: generate force to propel the blood out of the heart and through the circulatory system
• Atria are separated from ventricles by atrioventricular valves
• Pulmonary Artery: carries blood to the lungs
• Pulmonary Veins: return freshly oxygenated blood to the heart
• Circulation is partially split into 2 circuits:

  1. The pulmonary circuit that takes blood to the lungs and gills
  2. The systemic circuit that takes blood to the body
• To overcome gravity, blood must be pumped at a high pressure

  • However, the capillaries and alveoli of the lungs are too thin to withstand high pressure
• Two separate circuits allow for a high-pressure systemic circuit and a low-pressure pulmonary circuit
• The human circulatory system returns blood that is low in oxygen from the body to the right atrium of the heart through two large veins called the inferior and superior venae cavae
• When the right atrium contracts, this deoxygenated blood is sent to the right ventricle, which then contracts, sending blood tot he lungs via the pulmonary artery
• The right ventricle powers movement of blood through the pulmonary circulation
• After blood has circulated through the capillary beds in the lungs' alveoli and becomes oxygenated, it returns to the heart through the pulmonary veins
• The oxygenated blood enters the left atrium
• When the left atrium contracts, it pushes blood into the left ventricle
• Blood flows from the atria to the ventricles to the arteries in only one direction because one-way valves separate the heart's chambers from each other and adjacent arteries
• The valves ensure one-way flow