The Roles of the Blood

Mammalian Transport Systems

What is the cardiovascular system?

The mass transport system in mammals consisting of the heart, blood vessels and blood.

What is circulation?

The passage of blood through the blood vessels.

What is the role of the heart in the cardiovascular system?

It acts as a pump to move blood through the blood vessels.

What is the role of blood in the cardiovascular system?

It acts as the transport medium carrying substances around the body.

How do substances move between blood and body cells?

They move between the plasma or red blood cells and body cells by diffusion or active transport.

Why are capillary walls well suited for exchange?

They are only one cell thick, giving a short diffusion distance so substances can pass between the blood and tissues easily.

What are the main functions of blood?

Transporting substances, carrying hormones, helping defend the body against pathogens and distributing heat.

What are the main components of blood?

Plasma, erythrocytes (red blood cells), leucocytes (white blood cells) and platelets.

What is plasma?

The liquid component of blood in which blood cells and many dissolved substances are transported.

Approximately how much of blood volume is plasma?

Over 50%.

What digested food products are transported in plasma?

Substances such as glucose and amino acids.

Where are digested food products transported by plasma?

From the small intestine to the liver and then to parts of the body where they are used or stored.

How does plasma transport nutrients from storage areas?

It carries nutrient molecules from storage areas to cells that require them.

What excretory products are transported by plasma?

Carbon dioxide and urea.

Where does plasma transport excretory products?

From cells to organs such as the lungs and kidneys where they can be excreted.

What chemical messages are transported in plasma?

Hormones.

How does plasma help regulate body temperature?

It transfers heat from internal organs or active tissues to the skin, where heat can be lost to the surroundings.

How does plasma help regulate blood pH?

It acts as a buffer that resists changes in pH.

What is a buffer?

A substance or solution that resists changes in pH.

What are erythrocytes?

Red blood cells specialised for transporting oxygen.

Where are erythrocytes produced?

In the bone marrow.

Approximately how long does a mature erythrocyte live?

About 120 days.

What pigment is found in erythrocytes?

Haemoglobin.

What is the main function of haemoglobin?

To bind reversibly with oxygen and transport it from the lungs to body tissues.

What shape are erythrocytes?

Biconcave discs.

How does the biconcave shape of erythrocytes help oxygen transport?

It provides a large surface area to volume ratio, allowing oxygen to diffuse into and out of the cells rapidly.

Why do mature erythrocytes lack a nucleus?

This provides more internal space for haemoglobin, increasing the amount of oxygen they can transport.

Why does the absence of a nucleus contribute to the limited lifespan of erythrocytes?

Without a nucleus they cannot produce all the proteins needed for repair and maintenance.

Approximately how many haemoglobin molecules can one erythrocyte contain?

Around 250–300 million haemoglobin molecules.

What else can haemoglobin transport besides oxygen?

Some carbon dioxide produced during respiration.

What are leucocytes?

White blood cells involved in defending the body against infection.

How do leucocytes differ in size from erythrocytes?

They are much larger than erythrocytes.

How can leucocytes move through very small blood vessels?

They can change their shape and squeeze through them.

Where are leucocytes produced?

In the bone marrow, although some mature in the thymus gland.

What is the main function of leucocytes?

To defend the body against pathogens and infection.

What other process are leucocytes important in?

The inflammatory response when tissues are damaged.

Do leucocytes contain a nucleus?

Yes, all leucocytes contain a nucleus.

What are platelets?

Small cell fragments involved in blood clotting.

What cells produce platelets?

Large bone-marrow cells called megakaryocytes.

What are megakaryocytes?

Large cells in the bone marrow that produce platelets.

What is the main function of platelets?

To help form blood clots.

What is the structure of haemoglobin?

A large globular protein consisting of four polypeptide chains, each containing an iron-containing haem prosthetic group.

How many oxygen molecules can one haemoglobin molecule carry?

Four oxygen molecules.

Why can one haemoglobin molecule carry four oxygen molecules?

It has four haem groups, each capable of binding one oxygen molecule.

What is oxyhaemoglobin?

The molecule formed when oxygen binds reversibly to haemoglobin.

What is the equation for the formation of oxyhaemoglobin?

Hb + 4O₂ ⇌ Hb(O₂)₄.

Is the binding of oxygen to haemoglobin reversible?

Yes. Oxygen can bind to haemoglobin and later dissociate from it.

What happens when the first oxygen molecule binds to haemoglobin?

It changes the shape of the haemoglobin molecule, making it easier for subsequent oxygen molecules to bind.

What is cooperative binding in haemoglobin?

Binding of one oxygen molecule changes haemoglobin's shape and increases its affinity for subsequent oxygen molecules.

Why does the final oxygen molecule bind much faster than the first?

Previous oxygen binding has changed haemoglobin's shape, increasing its affinity for oxygen.

What happens during oxygen dissociation from haemoglobin?

Oxygen molecules are released, with removal becoming progressively harder as fewer oxygen molecules remain bound.

Why does oxygen enter erythrocytes in the lungs?

The oxygen concentration is higher in the lungs than inside the erythrocytes, so oxygen diffuses down its concentration gradient.

What happens to haemoglobin in the lungs?

Oxygen diffuses into erythrocytes and binds to haemoglobin to form oxyhaemoglobin.

Why is oxygen loading efficient in the lungs?

The high partial pressure of oxygen causes haemoglobin to have a high affinity for oxygen and become highly saturated.

What happens to oxygen when blood reaches body tissues?

Oxygen dissociates from haemoglobin and diffuses from the erythrocytes into cells down its concentration gradient.

Why does haemoglobin release oxygen in respiring tissues?

The partial pressure of oxygen is lower in the tissues, reducing haemoglobin saturation and causing oxygen to dissociate.

What is partial pressure of oxygen?

A measure of the concentration of oxygen in a mixture of gases or a solution.

What does percentage saturation of haemoglobin mean?

The percentage of haemoglobin's oxygen-binding sites that are occupied by oxygen.

What is an oxygen dissociation curve?

A graph showing the relationship between partial pressure of oxygen and the percentage saturation of haemoglobin with oxygen.

What shape is the oxygen dissociation curve for human haemoglobin?

Sigmoid, or S-shaped.

Why is the oxygen dissociation curve sigmoid?

Cooperative binding means binding of the first oxygen increases haemoglobin's affinity for subsequent oxygen molecules.

What happens to haemoglobin at high partial pressures of oxygen?

Its percentage saturation is high because oxygen binds readily to haemoglobin.

What happens to haemoglobin at low partial pressures of oxygen?

Its percentage saturation decreases and oxygen is released.

Why is the steep part of the oxygen dissociation curve important?

A small decrease in oxygen partial pressure causes a large decrease in haemoglobin saturation, so large amounts of oxygen are released to tissues.

How does the oxygen dissociation curve benefit the lungs?

At the high partial pressure of oxygen in the lungs, haemoglobin loads oxygen readily and becomes highly saturated.

How does the oxygen dissociation curve benefit respiring tissues?

At lower oxygen partial pressures, haemoglobin rapidly releases oxygen for respiration.

Approximately how much oxygen may be released from haemoglobin when a person is resting or exercising gently?

About 25% of the oxygen being transported.

What happens to oxygen release during intense activity?

A much greater proportion of the transported oxygen can be released to active tissues.

How does carbon dioxide move from respiring cells into the blood?

It diffuses down its concentration gradient.

What are the three main ways carbon dioxide is transported in the blood?

Dissolved in plasma, bound to haemoglobin as carbaminohaemoglobin, and mainly as hydrogencarbonate ions.

Approximately what percentage of carbon dioxide is transported dissolved in plasma?

About 5%.

Approximately what percentage of carbon dioxide combines with haemoglobin?

About 10–20%.

What is carbaminohaemoglobin?

A compound formed when carbon dioxide binds to haemoglobin.

In what form is most carbon dioxide transported in the blood?

As hydrogencarbonate ions in the plasma.

What happens when carbon dioxide enters an erythrocyte?

It reacts with water to form carbonic acid.

What enzyme catalyses the reaction between carbon dioxide and water?

Carbonic anhydrase.

What reaction is catalysed by carbonic anhydrase?

CO₂ + H₂O ⇌ H₂CO₃.

What happens to carbonic acid inside erythrocytes?

It dissociates into hydrogen ions and hydrogencarbonate ions.

What is the equation for carbonic acid dissociation?

H₂CO₃ ⇌ HCO₃⁻ + H⁺.

What happens to the hydrogen ions produced from carbonic acid?

They bind to haemoglobin, which acts as a buffer and helps prevent large changes in blood pH.

How does haemoglobin act as a buffer?

It accepts hydrogen ions, reducing changes in blood pH.

What happens to hydrogencarbonate ions formed inside erythrocytes?

They diffuse out of the erythrocytes into the plasma.

What is the chloride shift?

The movement of chloride ions into erythrocytes as hydrogencarbonate ions diffuse out.

Why does the chloride shift occur?

It helps maintain electrical balance as negatively charged hydrogencarbonate ions leave the erythrocytes.

What happens to carbon dioxide transport reactions in the lungs?

The reactions reverse, producing free carbon dioxide that diffuses from the blood into the lungs.

Why do the carbon dioxide reactions reverse in the lungs?

Carbon dioxide concentration is low in the lungs, so carbonic anhydrase catalyses the reverse reaction.

What is the Bohr effect?

The change in haemoglobin's oxygen dissociation curve caused by increased carbon dioxide levels, which reduces haemoglobin's affinity for oxygen.

What happens to haemoglobin's affinity for oxygen when carbon dioxide concentration increases?

Its affinity for oxygen decreases.

How does increased carbon dioxide affect the oxygen dissociation curve?

It shifts the curve to the right.

What does a rightward shift of the oxygen dissociation curve mean?

Haemoglobin has a lower affinity for oxygen and releases oxygen more readily.

Why is the Bohr effect useful in actively respiring tissues?

These tissues produce lots of carbon dioxide, lowering haemoglobin's oxygen affinity so more oxygen is released where it is needed.

What happens to haemoglobin in tissues with high carbon dioxide levels?

It requires a higher partial pressure of oxygen to become saturated and therefore releases oxygen more readily.

What happens to haemoglobin in the lungs where carbon dioxide levels are low?

Its affinity for oxygen is higher, allowing oxygen to bind readily.

Why does increased carbon dioxide alter haemoglobin's affinity for oxygen?

Carbon dioxide affects blood pH, altering haemoglobin's protein structure and therefore its oxygen affinity.

What is fetal haemoglobin?

A form of haemoglobin present in the developing fetus that has a higher affinity for oxygen than adult haemoglobin.

Why does fetal haemoglobin need a higher affinity for oxygen than adult haemoglobin?

It enables fetal blood to take oxygen from the mother's blood across the placenta.

How does the oxygen dissociation curve of fetal haemoglobin compare with adult haemoglobin?

It is shifted to the left.

What does the left-shifted fetal haemoglobin curve indicate?

Fetal haemoglobin has a higher affinity for oxygen and becomes saturated at lower oxygen partial pressures.

Why would oxygen transfer across the placenta be limited if fetal and maternal haemoglobin had equal oxygen affinities?

There would be less tendency for oxygen to dissociate from maternal haemoglobin and bind to fetal haemoglobin.

How does fetal haemoglobin maximise oxygen uptake from maternal blood?

Its higher oxygen affinity allows it to bind oxygen at the relatively low oxygen partial pressures found in the placenta.