Mass transport in animals

Describe the structure of haemoglobin

• Globular protein made up 4 polypeptide chains with a prosthetic haem group attached per polypeptide chain

• The prosthetic haem group contains an Fe²⁺ ion which is able to reversibly combine with oxygen to form oxyhaemoglobin

• Each haemoglobin molecule can carry 4 oxygen molecules as a result

Why is haemoglobin soluble?

• Haemoglobin's hydrophobic non-polar R-groups are shielded or face inwards to the water

• Whereas the hydrophilic polar R-groups are positioned facing outwards on the external surface of the molecule

• The external hydrophilic R-groups can form hydrogen bonds with the water molecules in the surrounding cytoplasm or blood plasma, allowing the molecule to dissolve

• This makes haemoglobin soluble and good for transport because oxygen is insoluble

What is the formula for the reversible reaction between oxygen and haemoglobin

• Oxygen reacts with haemoglobin to form oxyhaemoglobin

• Oxygen + Haemoglobin ⇌ Oxyhaemoglobin

• 4O₂ + Hb ⇌ Hb(O₂)₄

How does oxygen bind to hameoglobin?

• Haemoglobin binds to oxygen by a process called co-operative binding

• The binding of the first oxygen molecule causes a conformational change in the tertiary structure of the haemoglobin molecule

• This increases its affinity for oxygen

• This makes it easier for the 2^nd and 3^rd molecule to bind so they bind quickly and easily

• After the 3^rd oxygen binds, haemoglobin is nearly saturated, so the fourth oxygen binds more slowly

Describe what is the loading of oxygen

Loading of oxygen:

• Occurs in the lungs, where oxygen concentration is high (high pO₂)

• Haemoglobin binds oxygen molecules at the haem groups

• The high oxygen concentration and higher pH in the lungs increase haemoglobin’s affinity for oxygen, so it picks up oxygen efficiently

Describe what is the unloading of oxygen

Unloading of oxygen:

• Occurs in respiring tissues, where oxygen concentration is lower and CO₂ concentration is higher (low pO₂)

• The low pH caused by CO₂ (Bohr effect) reduces haemoglobin’s affinity for oxygen, so oxygen is released.

• Oxygen diffuses from the red blood cells into the tissue cells, where it is used in respiration

Describe Bohr’s effect and how it looks on the dissociation curve

The Bohr effect is the decrease in haemoglobin’s affinity for oxygen at high carbon dioxide concentrations or low pH

In respiring tissues, CO₂ is produced, which reacts with water to form carbonic acid, lowering the pH of the blood

This causes haemoglobin to undergo a conformational change, reducing its affinity for oxygen and allowing more oxygen to be released to the tissues

On an oxygen dissociation curve, the Bohr effect causes a rightward shift, ensuring oxygen delivery is matched to the metabolic activity of tissues

Why is oxygen less rapidly unloaded at a high pO₂?

• Haemoglobin is highly saturated

• So most sites are full and affinity is high

• Therefore less oxygen is unloaded

Why is more oxygen unloaded at low pO₂?

• Haemoglobin’s tertiary structure changes to a structure with a lower affinity for oxygen

• So oxygen is unloaded rapidly

Why does maternal haemoglobin have a lower affinity than fetal haemoglobin?

• Maternal Hb has a lower affinity than fetal Hb

• This allows oxygen to be rapidly unloaded at the placenta

• Fetal Hb has a higher affinity than maternal Hb

• This allows the fetus’s Hb to bind to O₂ from the the mother’s blood even when the partial pressure of oxygen is low

Why do llamas Hb have a high affinity for O₂?

• Llama haemoglobin has a high affinity for oxygen because llamas live at high altitudes, where the partial pressure of oxygen (pO₂) is low

• A higher affinity allows haemoglobin to bind oxygen more efficiently even when oxygen levels are low

• This ensures enough oxygen is transported to the tissues for respiration

• So the oxygen dissociation curve shifts left

Define mass transport

Mass transport is the bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes

What are the features of the circulatory system in humans?

• Double circulation – Blood passes through the heart twice for each circuit: once to the lungs (pulmonary circulation) and once to the rest of the body (systemic circulation). This maintains high pressure and efficient oxygen delivery

• Closed system – Blood remains in vessels, allowing faster and more controlled transport of oxygen, nutrients, and waste

Describe the functions and adaptations of the arteries

Functions of arteries:

• Carry blood away from the heart (usually oxygenated)

• Transport blood under high pressure from the heart’s pumping

Adaptations of arteries:

• Thick muscular walls – withstand high pressure

• Elastic tissue – stretches when blood is pumped and recoils to maintain continuous flow

• Small lumen – helps maintain high pressure

• Smooth endothelium – reduces friction for efficient blood flow

• Thick layer of smooth muscle – strengthens arterial walls and can contract and relax to allow limited vasodilation and vasoconstriction to control and regulate blood flow

Describe the functions and adaptations of the veins

Functions of veins:

• Carry blood towards the heart (usually deoxygenated, except pulmonary vein)

• Transport blood under low pressure

Adaptations of veins:

• Thin walls – blood is under low pressure, so thick walls are unnecessary

• Wide lumen – allows blood to flow easily

• Valves – prevent backflow and ensure unidirectional blood flow

• Smooth endothelium – reduces friction and allows efficient flow

• Veins are often surrounded by skeletal muscles, which help push blood towards the heart when muscles contract

Describe the functions and adaptations of the arterioles

Functions of arterioles:

• Carry blood from arteries to capillaries

• Act as resistance vessels, controlling blood flow into capillary beds and the pressure of blood flow in tissues

Adaptations of arterioles:

• Thick muscular walls – can constrict or dilate to control blood flow to specific tissues

• Elastic tissue – allows them to withstand pressure changes from arteries

• Small lumen – creates a high resistance to blood flow which helps control flow and maintain blood pressure into capillaries

• Smooth endothelium – reduces friction for efficient blood movement

Describe the functions and adaptations of the venules

Functions of venules:

• Carry blood from capillaries to veins.

• Allow exchange of substances between blood and surrounding tissues in some areas.

Adaptations of venules:

• Thin walls – blood is under low pressure, so thick walls aren’t needed

• Wide lumen – reduces resistance to blood flow allowing it to flow easily

• Some valves – help prevent backflow in low-pressure areas

• Smooth endothelium – reduces friction and allows efficient blood flow

• Permeable walls – re-absorption of tissue fluid back into the blood

Describe the functions and adaptations of the capillaries

Functions of capillaries:

• Allow exchange of gases, nutrients, and waste between blood and tissues.

• Connect arterioles to venules.

• Ensure blood flow is slow enough for efficient exchange and to prevent damage of vessels

Adaptations of capillaries:

• One-cell-thick walls (endothelium) – provides a short diffusion pathway.

• Very narrow lumen – slows blood flow for more time for exchange.

• Large network / extensive branching – increases surface area for exchange.

• Permeable walls – allow substances like oxygen, glucose, and waste to pass through easily.

• Close association with tissues – ensures efficient delivery and removal of materials.

Define tissue fluid

• Tissue fluid is the fluid that surrounds cells in tissues made from small molecules that leave the plasma

• Acts as a medium for exchange between the blood and body cells

How is tissue fluid formed?

• At the arteriole end of a capillary, hydrostatic pressure due to blood pressure is higher than in the hydrostatic pressure in the tissue fluid

• This causes an overall outwards pressure

• This pressure forces plasma and small dissolved substances (e.g. glucose, oxygen, ions) out through gaps in the capillary wall

• This process is called ultrafiltration

• Large proteins and blood cells remain in the capillary because they are too large to pass through

• The fluid that leaves the capillary and surrounds the cells is called tissue fluid

• As blood moves towards the venule end, fluid leaves causing the hydrostatic pressure decreases

• The water potential at the venule end is lower than in the tissue fluid due to the presence of the large plasma proteins and blood cells

• So water moves out of the tissue fluid and back into the capillaries by osmosis

• Any excess tissue fluid is drained into the lymphatic system which eventually enters the circulatory system

Describe the heart

• Myogenic organ (contracts without nerve stimulation)

• Cardiac muscle is rich in mitochondria and are supplied by O₂ and nutrients by the coronary arteries to allow for continuous contractions

• 4 chambered structure (atrium are separated from ventricles by septum)

Describe the function of the atrioventricular valves

Atrioventricular (AV) valves:

• Located between the atria and ventricles (tricuspid on right, bicuspid/mitral on left).

• Function: Prevent backflow of blood from the ventricles into the atria.

• They open when atrial pressure is greater than ventricular pressure (during atrial systole).

• They close when ventricular pressure rises above atrial pressure (during ventricular systole).

• They operate passively due to pressure differences

Describe the function of the semi lunar valves

Semilunar valves:

• Located at the base of the aorta and pulmonary artery.

• Function: Prevent backflow of blood from the arteries into the ventricles.

• They open when ventricular pressure exceeds arterial pressure (during ventricular systole).

• They close when arterial pressure becomes greater than ventricular pressure (during diastole).

• They also operate passively due to pressure differences

Describe the stages of the cardiac cycle (atrial systole)

• In atrial systole, the ventricles relax and the atria walls contract

• This decreases the volume of atrial chambers and increases atrial pressure

• This pushes blood through the open atrioventricular valves into the ventricles

• The semilunar valves remain closed, preventing blood from entering the arteries

• There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracted atria

Describe the stages of the cardiac cycle (ventricular systole)

• During ventricular systole, the ventricles contract

• This causes ventricular pressure to rise and ventricular volume to decrease

• When ventricular pressure exceeds atrial pressure, the atrioventricular valves close to prevent backflow into the atria

• As ventricular pressure continues to increase and becomes greater than the pressure in the aorta and pulmonary artery, the semilunar valves open

• This allows blood to be ejected into the aorta and pulmonary artery

Describe the stages of the cardiac cycle (ventricular diastole)

• In ventricular diastole, the ventricles and atria relax

• Pressure in the aorta and pulmonary artery becomes higher than in the ventricles

• This causes the semilunar valves to close to prevent backflow of blood

• Blood returns to the atria from the vena cava and pulmonary veins, causing atrial pressure to rise

• As ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open, allowing blood to flow passively into the ventricles

• This completes the cycle, and atrial systole begins again

Define cardiovascular diseases

Diseases of the heart muscle and blood vessels - resulting in restricted blood flow to the heart muscle and reduced oxygen delivery to tissues

What is an atheroma?

• A fatty deposit that forms within the artery wall

• This develops into a fiborous plaque that narrows the artery lumen

How is an atheroma formed?

• Damage occurs to the endothelium wall due to high blood pressure, smoking, or toxins

• White blood cells, mainly macrophages, move into the artery wall

• Cholesterol and other lipids move from the blood and into the artery wall

• Macrophages engulf the lipids, forming foam cells

• Fatty streaks become larger and more numerous as more lipids and white blood cells accumulate, causing the artery wall to thicken

• Connective tissue forms a fibrous cap over the fatty material deposited in the artery wall forming an atheroma

• The atheroma narrows the lumen of the artery and makes the artery less elastic, reducing blood flow and increasing blood pressure

• This further damages the artery wall and increases risk of thrombosis, aneurysm, and coronary heart disease

How do atheromas lead to CHD

• Atheromas narrow the lumen of the coronary arteries.

• Narrowed arteries reduce blood flow to heart muscle (myocardium).

• This decreases oxygen supply to the cardiac muscle.

• If the heart muscle receives insufficient oxygen, then cannot respire as normal so weaker muscular contractions

• This causes a reduced oxygen delivery to rest of the body

• If atheromas rupture, a blood clot (thrombus) may form.

• The thrombus can completely block the artery, causing a myocardial infarction (heart attack).

• Reduced blood flow also increases blood pressure, putting further strain on the heart

How does an aneurysm occur?

• An atheroma (fatty deposit) forms in the artery wall.

• The atheroma weakens and damages the artery wall, particularly the elastic and muscular layers.

• High blood pressure pushes against the weakened section of the artery wall.

• This causes a balloon-like bulge in the artery, called an aneurysm.

• The aneurysm may rupture, leading to internal bleeding, and possibly a haemorrhage which can be fatal

How does thrombosis occur?

• An atheroma (fatty deposit) forms in the artery wall, narrowing the lumen

• If the atheroma plaque ruptures, it can damaging the endothelium

• The endothelium becomes rough, which stimulates platelet adhesion.

• Platelets and fibrin accumulate at the site of damage leading to the formation of a blood clot (thrombus).

• The thrombus can partially or completely block the artery, restricting blood flow

• If the coronary arteries are blocked, this can cause a myocardial infarction

• Or the thrombus can break off and become dislodged, travelling as an embolus (hence can go and block another vessel)

How does myocardinal infarction occur?

• A coronary artery becomes partially or completely blocked, usually by a thrombus formed at the site of a ruptured atheroma.

• The blockage reduces or stops blood flow to the region of heart muscle supplied by that artery.

• Oxygen delivery to the cardiac muscle is insufficient for respiration.

• As a result, heart muscle cells are damaged or die, causing a myocardial infarction.

Describe how the cardiac cycle is controlled by the sinoatrial node (SAN) and the atrioventricular node (AVN)

SAN:

• Produces myogenic impulses automatically (pacemaker).

• Impulse spreads across atria, causing atrial systole.

• Ensures blood is pushed into ventricles.

• Impulse reaches AVN, which delays it before passing to ventricles.

AVN:

• Receives impulse from SAN.

• Delays impulse slightly so ventricles contract after atria.

• This ensures ventricles fill completely before ventricular systole

What factors cause CHD or increase the likelyhood and how

High blood pressure (hypertension):

• How it contributes: High pressure damages the endothelium (inner lining of arteries).

• Damaged endothelium makes it easier for LDL cholesterol to enter and form atheromas, increasing the risk of CHD.

Smoking:

• Chemicals in smoke (like nicotine and carbon monoxide) damage the endothelium.

• Increases LDL cholesterol and reduces HDL cholesterol.

• Nicotine also increases heart rate and blood pressure, adding strain on arteries.

High blood cholesterol / high saturated fat diet:

• Excess LDL cholesterol in blood can deposit in artery walls, forming atheromas.

• High cholesterol narrows the arteries, increasing resistance to blood flow

• This causes increasing blood pressure and chance of thrombosis