Transport in Animals

Why do multicellular organisms need a circulatory system?

Why do multicellular organisms need a circulatory system?

• Need to exchange materials with their environments

• Small SA : Vol ratio, cannot rely on diffusion for exchange of essential substances as compared to unicellular organisms

• Carries fluids containing materials needed by organism, as well as waste materials needed to be removed

Heart

• Hollow, muscular organ located in chest cavity

• Pumps blood around the body

• Specialised cardiac muscle tissue for repeated involuntary contraction without rest

Artery

• Blood vessel which carries blood away from heart

• Walls contain lots of muscle and elastic tissue, narrow lumen to maintain and withstand high blood pressure

• Diameter: 0.4 - 2.5 cm

Arteriole

• Small arteries which branch from larger arteries that connect to capillaries

• Diameter:

Artery Structure

A: Outer Layer, contains collagen fibres, some elastic fibers

B: Middle Layer, contains elastic fibres, collagen fibres and smooth muscle

C: Inner Layer, the endothelium; smooth single-cell layer of squamous epithelium

D: Narrow lumen, relative to thickness of wall

Capillary Structure

• Wall made of endothelium, one cell thick

• Lumen is 7 µm in diameter, just big enough for RBC to fit through

Vein Structure

A: Outer layer, mostly collagen fibres

B: Middle layer, very thin, containing some smooth muscle and elastic fibres

C: Inner layer, single-celled layer of squamous epithelium

D: Large lumen relative to thickness of wall, irregularly shaped

Also contains valves to prevent backflow of blood (not shown in diagram)

Red Blood Cell (RBC) / Erythrocyte

• Approx. 7.5 µm in diameter

• Biconcave disc shape for increased surface area

• Contains haemoglobin (globular, conjugated protein containing 4 prosthetic haem groups)

• No nucleus to allow more room to carry haemoglobin

Tissue Fluid vs. Blood

Blood

• Cellular Components (RBCs, WBCs)

• Larger proteins

Tissue Fluid

• Similar to blood plasma as itself, however contains lower number of proteins

• Carries dissolved solutes and small proteins, as larger proteins cannot fit through small fenestrations

• Majority is water

Importance of water in blood

• Water has high specific heat capacity, which means it is highly thermally stable so the haemoglobin is not altered by changes in temperature (the haemoglobin is very important for transportation of gases)

• Is a universal solvent so important nutrients are transported in solute form in the blood

Tissue Fluid Formation

• Formed from plasma leaking out through small fenestrations in the capillary walls to surround the cells of the body

• Arterial end of capillary has higher hydrostatic pressure than venous end, so plamsa is pushed out of capillary

• At venous end, less fluid is pushed out of capillary as pressure within capillary is reduces

Binding of oxygen to haemoglobin

• Oxygen binds to haemoglobin to form oxyhaemoglobin

• 4O2 + Hb → HbO8

• Binding of O2 causes conformational change of Hb structure which makes the binding of the next 3 O2 molecules successively easier

• Dissociation process is reversed, as the last O2 molecule is hardest to dissociate

Carriage of Carbon Dioxide

CO2 moves from respiring tissue to RBC
Carbon Dioxide reacts with water to form carbonic acid (carbonic anhydrase catalyses this reversible reaction)

• CO2 + H2O ⇌ H2CO3

Carbonic acid dissociates into H+ and HCO3- ions

• H2CO3 → H+ + HCO3-

Chloride Shift

• HCO3- ions formed from H2CO3 dissociation is transported out of RBCs via a transport protein in the membrane

Oxygen Saturation Curves

Shift to right:

• Increased acidity (H+ ions or CO2)

• Increased temperature

• Causes lower affinity for O2
  (Low saturation of Hb even at higher partial pressures of O2)

• Also known as the Bohr Effect

Shift to left

• Decreased acidity (H+ ions or CO2)

• Decreased temperature

• Causes higher affinity for O2
  (High saturation of Hb even at lower partial pressures of O2)

Pathway of Blood (from Vena Cava)

Vena Cava → Right Atrium → Atrioventricular valve → Right ventricle → Semilunar valve → Pulmonary Artery → Lungs → Pulmonary Vein → Left Atrium → Atrioventricular valve → Left Ventricle → Semilunar Valve → Aorta → Body

Atrial Systole

• Atria walls contract

• Pressure inside atria increase which exceeds pressure inside ventricles

• Atrioventricular valves open and blood is pushed into ventricles

Ventricular Systole

• Ventricle walls contract

• Pressure inside ventricles increase which exceeds pressure inside the arteries (Pulmonary/Aorta)

• Semilunar valves open and blood is pushed through the arteries, either towards the lungs in the pulmonary artery, or the rest of the body through the aorta

Ventricular Diastole

Interpreting Cardiac Pressure Graphs

1. Sinoatrial node sends out wave of excitation.

2. Wave travels through the walls of atria, which contract.

3. Non conductive tissue stops the wave and directs it to atrioventricular node.
   ANV absorbs wave, causing a delay, before channeling it down the septum.

4. Purkyne tissue conducts the wave of excitation down the septum and up the ventricular walls.

5. Ventricles contract.