Circulatory Systems in Animals

Transport Methods in Small Animals

• Small animals transport molecules through diffusion.
• Some, like flatworms, flatten their bodies to aid diffusion.
• Sponges use mass flow through pores for transport.

Need for Transport Systems in Larger Animals

• Larger animals require transport systems due to a small surface area to volume ratio.
• Diffusion becomes inefficient over large distances.
• Land animals often have impermeable surfaces, limiting water absorption.

Features of Transport/Circulatory Systems

• Circulatory Fluid (Blood): Transports molecules.
• Pump (Heart): Moves the fluid.
• Vessels: Fluid moves within these.
• Valves: Ensure one-way flow.
• Respiratory Pigment: Carries more oxygen (except in insects).

Open Circulatory System

• Blood is not confined to vessels, bathing tissues directly.
• Blood flows in large cavities called the hemocoel at low pressure.
• Insects: Blood enters a dorsal tube-shaped heart, is pumped towards the head, and enters the hemocoel for exchange between blood and body cells.
• Oxygen diffuses directly to tissues from tracheids, so blood does not transport oxygen.
• Insects do not use respiratory pigments

Closed Circulatory System

• Blood moves within blood vessels.
• The heart pumps blood under high pressure.
• Organs are bathed by tissue fluid, not blood.
• Respiratory pigments are present.

Single Circulation

• Blood passes through the heart once per circuit.
• Earthworms: Blood flows through dorsal vessels and returns via ventral vessels, with five "pseudo hearts" pumping blood.
• Exchange occurs in vessels surrounding the gut and body wall.
• Fish: The heart's ventricle pumps deoxygenated blood to the gills, where oxygen is picked up; oxygenated blood goes to tissues, then deoxygenated blood returns to the heart.

Double Circulation

• Blood flows through the heart twice per circuit.
• Pulmonary circulation: Blood flows to the lungs.
• Systemic circulation: Blood flows throughout the body.
• The heart has four chambers: two atria and two ventricles.
• High blood pressure is maintained.
• Hemoglobin carries oxygen.
• Blood pressure decreases in the lungs and must be increased again by returning to the heart.

Blood

• Connective tissue with cells working together.
• Large amount of extracellular matrix: Plasma (55%) and cells (45%).

Plasma

• Pale yellow liquid, 90% water, distributes heat.
• Solutes include albumins (create pressure), globulins (antibodies), and fibrinogen (blood clotting).
• Contains nutrients, mineral salts, waste (CO2), and hormones.

Blood Cells

• Three main types: Red blood cells (erythrocytes), platelets (thrombocytes), and white blood cells (phagocytes and lymphocytes).

Red Blood Cells (Erythrocytes)

• Most common type of blood cell, contains hemoglobin for transporting respiratory gases like oxygen.
• Produced in bone marrow, destroyed in the liver, lifespan of 120 days.
• Adaptations:
  • Small size (7.5 μm diameter) allows fitting in capillaries and close proximity to walls.
  • Flat, biconcave disc increases surface area for diffusion of gases.
  • Elastic membrane allows squeezing through capillaries.
  • Lack of organelles provides room for hemoglobin.

Hemoglobin (Hb)

• Transports a lot of oxygen.
• Transports more oxygen than if carried in plasma
• Each molecule carries four oxygen molecules.

Transport of Respiratory Gases

• Blood transports respiratory gases and other substances.
• Oxygen moves from gills/lungs to tissues.
• Carbon dioxide moves from tissues to gills/lungs.
• Hemoglobin binds oxygen in the lungs and releases it in respiring tissues.
• RBC contains 250 million Hb.
• Each Hb molecule comprises four polypeptide chains (two alpha, two beta), each with a heme group containing Fe2+.

Forming Hemoglobin

• In lungs, Hb combines with oxygen to form oxyhemoglobin.
• One Fe2+ in each heme group combines with one O2 molecule.
• One Hb molecule thus combines with four O2 molecules.
• In tissues, oxyhemoglobin dissociates and offloads oxygen.

Oxygen Dissociation Curves

• Measures uptake of O2 by Hb at different O2 concentrations.
• Shape: Sigmoid curve.

Cooperative Binding

• The first O2 molecule binds to a heme group with difficulty because it's less exposed.
• After the first O2 attaches, the molecule changes shape, exposing other heme groups, allowing subsequent O2 molecules to attach more readily.

Oxygen Dissociation Curve in Lungs

• The partial pressure (pp) of O2 is high (11 kPa).
• At these high pp of O2, Hb has a higher affinity for O2.
• Hb becomes saturated with O2.

Oxygen Dissociation Curve in Blood Vessels

• As blood flows down arteries towards tissues, the partial pressure of O2 falls.
• Hb doesn't offload significant O2 at this point.

Oxygen Dissociation Curve in Tissues

• In tissues, partial pressure of O2 is low (2-4 kPa).
• At these low pp of O2, Hb has a lower affinity for O2.
• The Hb offloads O2 to the tissues.

Bohr Shift

• If CO2 concentration increases, the Oxygen Dissociation Curve shifts down and to the right.

Bohr Shift in Lungs

• Partial pressure of CO2 is lower in the lungs, as CO2 diffuses out of blood into the alveoli.
• Bohr shift has no effect here.

Bohr Shift in Tissues

• In tissues, pp of O2 = 2-4KPa.
• PP of CO2 is high due to high levels of tissue respiration.
• Hb has a lower affinity for O2 when CO2 is present.
• Hb more readily offloads O2 to the tissues.

Fetal Hemoglobin

• Has a slightly different structure than adult hemoglobin.
• Fetal hemoglobin has a higher affinity for O2: able to saturate with O2 at lower partial pressures than adults.

Maternal and Fetal Hemoglobin in Placenta

• MOTHER: In the placenta, the partial pressure of O2 is low. Maternal Hb has a low affinity for O2 and offloads it.
• FETAL: At the same low pp O2, fetal Hb still has a high affinity for O2 and picks up the oxygen offloaded by the mother.

Adaptations to Low Oxygen Habitats

• Llamas at high altitudes and lugworms in sand have adaptations to low oxygen environments.
• Oxygen supply in llama myoglobin.
• At low partial pressure of O2 in muscle, myoglobin/llama has a high affinity for O2.

Carriage of Carbon Dioxide

• CO2 is produced in tissues and transported by blood to the lungs.
• Three ways CO2 is carried in the blood:
  • Dissolved in plasma (5%): CO2 diffuses into the plasma, dissolves in H2O, forming carbonic acid. Plasma buffers the pH.
  • As Carbamino-Hb (10%): CO2 diffuses into red blood cells and combines with amino groups in Hb, forming carbaminohemoglobin.
  • As Hydrogencarbonate ions (85%): CO2 forms carbonic acid (H2CO3) dissociates which dissociates into H+ ions diffuse out of RBC into plasma.
  • To maintain electrical balance, Cl- diffuses into RBC (chloride shift). H+ ions cause oxyhemoglobin to dissociate, releasing O2. H+ combines with Hb, forming haemoglobinic acid (HHb), which removes H+ so pH of RBC doesn't fall.

Blood Vessels

• Blood is contained within blood vessels with a central space called the lumen.
• Three types of vessels: arteries, capillaries, and veins.
• Blood vessel walls contain:
  • Outer layer: Tough elastic and collagen fibers resist overstretch.
  • Middle layer: Elastic fibers allow stretch and recoil to maintain blood pressure, and smooth muscle maintains blood pressure.
  • Inner endothelium: Single layer, thin, smooth squamous epithelium reduces friction.

Arteries

• Carry blood away from the heart.
• Thick, muscular walls withstand high blood pressure.
• Thick layer of muscle and elastic fibers allows stretch and recoil, maintaining pressure during heart relaxation.
• Branch into smaller arterioles that subdivide into capillaries.
• More smooth muscle in arterioles allows diameter changes.
• Less elastic tissue as blood loses fluctuating force.
  • Vasoconstriction: Smooth muscle in arteriole walls contracts, decreasing diameter, reducing blood flow.
  • Vasodilation: Smooth muscle in arteriole walls relaxes, increasing diameter, increasing blood flow.

Capillaries

• Form a network when arterioles divide upon reaching a target organ.
• Adaptations:
  • Thin walls for diffusion (one cell thick, squamous epithelium).
  • Diameter ~8 μm, allows passage of red blood cells.
  • Permeable walls with pores for exchange of water and solutes.
• Slowed rate of blood flow increases time for exchange.

Veins

• Venules are the start small veins capillaries join to form them when leaving organs.
• Blood has lost pressure, so walls are thinner with less muscle and elastic fibers.
• Venules join to form veins, which re bigger with the same three layers they form the venue
• Walls are thinner than arteries due to lower pressure.
• Blood flow through veins. -Blood lost most pressure :: 4 forces that keep blood flow: -Some pressure from capillaries. -Semi lunar valves prevent backflow (Only in veins) its muscle surrounding vein contracts. -Breathing creates -ve pressure in chest: A pressure in atria & draw blood into heart.

Tissue Fluid

• Capillary walls have gaps (fenestrations) through which liquid oozes out, bathing tissues forming this tissue fluid.
• Surrounds body cells, supplies oxygen and nutrients, removes CO2 and waste by diffusion.
  • Arteriole end: Blood arrives under high hydrostatic pressure (4 kPa) and forces water and nutrients out.
  • Plasma proteins remain (too big). Low π maintained by hydrostatic pressure forcing H2O out > π pressure for H2O moving in: net pressure forces fluids out.
  • Venule end: Hydrostatic pressure of blood decreases due to friction, while π within the capillary remains low because plasma proteins are retained. Hydrostatic pressure has less effect because pressure of blood t (l-6KPa) and pressure due to retaining plasma proteins remains =3.3 kPa. π has greater effect at drawing H20 in > Hydro-pressure forcing HaD out.

Lymphatic System

• Excess tissue fluid (10%) is removed by lymph vessels.
• Lymph vessels contain fluid called lymph.
• Oedema: Buildup of excess tissue fluid.