Lesson 6.7: Blood, Haemoglobin and Oxygen Transport

Introduction

Welcome to Lesson 6.7 of Foundation Biology! In this lesson, we will dive into the fascinating world of blood, focusing on its components, the critical role of haemoglobin, and how oxygen is transported throughout the body. By the end of this lesson, students, you will be able to:

Describe the components of blood and their functions.
Understand the structure and importance of haemoglobin and the oxygen dissociation curve.
Explain the concept of cooperative binding in haemoglobin.
Discuss the Bohr effect and its significance in physiology.
Outline how carbon dioxide is transported in the body.

Components of Blood

Blood is a vital fluid that circulates in our bodies, carrying oxygen, nutrients, and waste products. It is made up of several key components:

1. Red Blood Cells (Erythrocytes)

Red blood cells are responsible for transporting oxygen from the lungs to the tissues and taking carbon dioxide back to the lungs for exhalation.

Structure: They are biconcave discs, which increases their surface area for gas exchange. Each red blood cell contains about 270 million molecules of haemoglobin, a protein that binds to oxygen.
Function: The main function of red blood cells is to carry oxygen. They pick up oxygen in the lungs, where oxygen concentration is high, and release it where the concentration of oxygen is low such as in working muscles.

2. White Blood Cells (Leukocytes)

White blood cells play a crucial role in our immune system, defending against infections and foreign invaders.

Types: There are several types of white blood cells, including lymphocytes, neutrophils, and monocytes, each with specific roles in the immune response.
Function: They help in identifying and destroying pathogens, thus maintaining our health.

3. Platelets (Thrombocytes)

Platelets are small cell fragments that are essential for blood clotting.

Function: They adhere to the site of a blood vessel injury, gathering to form a plug that helps stop bleeding.

4. Plasma

Plasma is the liquid component of blood, making up about 55% of its volume.

Composition: It consists of water, electrolytes, proteins (such as antibodies and albumin), nutrients, hormones, and waste products.
Function: Plasma serves as a medium for transporting cells, nutrients, and waste through the circulatory system.

Haemoglobin and the Oxygen Dissociation Curve

Haemoglobin is a critical protein in red blood cells, enabling them to carry oxygen. Let's explore how it works!

Structure of Haemoglobin

Haemoglobin is composed of four polypeptide chains, each containing an iron atom capable of binding to oxygen.

The Oxygen Dissociation Curve

The relationship between the amount of oxygen bound to haemoglobin and the partial pressure of oxygen is represented by the oxygen dissociation curve.

Graph Explanation: The curve is S-shaped (sigmoidal), indicating that haemoglobin’s affinity for oxygen increases as more oxygen molecules bind. This is known as cooperative binding because the binding of the first oxygen molecule increases the likelihood of additional oxygen molecules binding.
Equation: The binding can be described using the classical Langmuir isotherm, but simply put, it shows the relationship between saturation (oxygen bound) and partial pressure ($pO_2$):

$$ \text{Saturation} = \frac{[O_2]^n}{K + [O_2]^n} $$

where $K$ is the dissociation constant and $n$ is the number of sites on haemoglobin.

The Bohr Effect

The Bohr effect describes how increased carbon dioxide concentration and decreased pH (due to carbon dioxide forming carbonic acid) decrease haemoglobin's affinity for oxygen.

Physiological Significance: This effect is crucial during intense physical activity. As muscles work harder, they produce more carbon dioxide, leading to a lower pH in the blood. This prompts haemoglobin to release more oxygen where it is most needed!

Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three forms:

Dissolved in Plasma: About 7% of carbon dioxide is carried dissolved in plasma.
Carbamino Compounds: Approximately 23% binds to haemoglobin, forming carbaminohaemoglobin.
Bicarbonate Ion: The majority (about 70%) is converted to bicarbonate ($HCO_3^-$) ions via the action of carbonic anhydrase in red blood cells, which helps maintain pH balance.

Conclusion

In summary, blood performs essential roles in our body, from transporting oxygen and carbon dioxide to providing immunity and clotting. Understanding the components of blood and the functions of haemoglobin offers valuable insights into human physiology.

Study Notes

Blood components: red blood cells, white blood cells, platelets, plasma.
Role of haemoglobin in oxygen transport and its cooperative binding properties.
Dynamics of the oxygen dissociation curve and the implication of the Bohr effect.
Mechanisms of carbon dioxide transport in the circulatory system.