Circulatory systems

Question 1

Circulatory systems basic information

Answer 1

• Every organism must exchange materials
  with its environment…but diffusion is very slow
• Natural selection has led to two solutions to this problem:
  ▪ Body shape and size that keeps cells in direct contact with the environment
  ▪ A circulatory system that moves fluid between cell’s immediate surroundings and the tissues where gas exchange occurs
• Some Animals Lack Circulatory Systems
  ▪ A flat body enhances diffusion by increasing the surface area and minimizing diffusion distance
  ▪ Some animals use cilia, flagella, gastrovascular cavities, etc. to circulate substances

Question 2

Circulatory systems are comprised of

Answer 2

• A circulatory fluid
• A set of interconnecting vessels
• A muscular pump, the heart
• Exchange gases, absorb nutrients, transport hormones, immune cells, etc. and dispose of wastes
• Can be open or closed and vary in number of circuits

Question 3

Open circulatory systems

Answer 3

Blood or hemolymph bathes organs directly in an open circulatory system
▪ Found in arthropods, most molluscs, some
annelids, and some invertebrate chordates

Question 4

More information

Answer 4

• Single or double circuits
  ▪ Single: blood passes through the heart once in each complete circuit
• Double: blood passes through the heart twice
  ▪ Blood moving from the heart to the rest of the body (systemic circuit)
  ▪ Blood moving from the heart to respiratory organ (pulmonary circuit)
• There is a shift from single to double circulation with evolution of the vertebrates

Question 4

Closed circulatory systems

Answer 4

• Blood is transported within closed vessels in a closed circulatory system
  ▪ Found in some annelids, cephalopods, and vertebrates
• Advantages:
  ▪ More efficient at transporting circulatory fluids to tissues and cells (higher pressure)
  ▪ Distribution of blood flow can be regulated
• Disadvantages:
  ▪ Energetically expensive
• Heart: two or more muscular chambers
  ▪ Atria receive blood
  ▪ Ventricles pump blood out of the heart
• Blood vessels:
  ▪ Arteries: from heart to capillaries
  ▪ Veins: from capillaries to heart

Question 5

Fish

Answer 5

• Single circulation
• Blood passes through the heart once
  in a circuit
• Blood passes through two capillary beds before returning to the heart
• Benefits:
  ▪ Not energetically expensive
• Limitations:
  ▪ Heart needs to pump with enough force to pump blood through two capillary beds
  ▪ Heart receives oxygen-poor blood
• Four chambered heart system
• Sinus venosus
  ▪ Pacemaker; weakly contractile
• Atrium
  ▪ Weakly contractile; used to fill ventricle
• Ventricle
  ▪ Main propulsive force
  ▪ Bulbus arteriosus
  ▪ Extension of ventricle

Question 6

Amphibians

Answer 6

• Incomplete Separation of Pulmonary/Systemic Circuits
• Blood passes through the heart twice:
  1. Pulmonary circuit (heart → respiratory organs → heart)
  2. Systemic circuit (heart → rest of body → heart)
• Benefits:
  ▪ Allows for shunting of blood depending on if on water or land
• Limitations:
  ▪ Still some mixing of blood in ventricle
• Three-chambered heart
• Two separate atria (left receives blood from respiratory organs, right receives blood from rest of body)
• One ventricle (with partial separation of
  oxygen-rich and oxygen - poor blood)

Question 7

Reptiles

Answer 7

• Almost Complete Separation of Pulmonary/Systemic Circuits
• Blood passes through the heart twice but there is more division of the ventricle (some reptiles have completely divided ventricles)
• Benefits:
  ▪ More separation of oxygen-rich and oxygen-poor blood
  ▪ Still opportunities for shunting
• Limitations:
  ▪ Still some mixing of blood
• Left and right atria
• One ventricle divided into three chambers
• Left atrium → cavum arteriosum (in ventricle) → body
  ▪ Right atrium → cavum venosum → cavum pulmonale (both in ventricle) → lungs
  *Crocodiles have completely separated ventricles, but they don’t have complete double circulation because of a link between the right and left systemic aortae

Answer 8

• Complete Separation of Pulmonary/Systemic Circuits: double circulation
• Blood passes through the heart twice with complete double circulation
  Benefits:
  ▪ Allows for more oxygenation of tissue (important in endotherms that use a lot of oxygen to generate heat)
  Limitations:
  ▪ Energetically expensive
• Birds and mammals are endotherms and require more O2 than ectotherms
• Increased O2 consumption requires more blood flow to tissues
• More blood flow = higher systemic pressure
• If systemic and pulmonary circuits were not divided, then the pressure would also rise in the pulmonary circuit = pulmonary edema = fluid in lungs = drowning
• Four-chambered hearts with two atria and two ventricles
  ▪ Left atria and ventricle receive oxygen-rich
  blood and right atria and ventricle receive oxygen poor blood

Question 9

Blood flow through the mammalian
circulatory system

Answer 9

• Lungs → L. atrium (via pulmonary veins) → L. ventricle (via bicuspid valve) → body (via systemic aorta)
• Body → R. atrium (via superior and inferior
  venae cavae) → R. ventricle (via tricuspid
  valve) → lungs (via pulmonary arteries)

Question 10

Cardiac cycle

Answer 10

• From the beginning of one heartbeat to the beginning of the next heartbeat
• One contraction (systole) and one relaxation (diastole) of cardiac muscle
• Atria contract first, together – then ventricles
• The volume of blood pumped by the left ventricle in one heartbeat = stroke volume
• Stroke volume (ml/beat) x heart rate (beats/min) = cardiac output (ml/min)