1. Blood Plasma, Serum, Tissue Fluids and Lymph

2. Learning Objectives
e) describe the differences between plasma, serum, tissue fluid and lymph;
(f) describe the normal composition of blood plasma;

3. Boardworks AS Biology Circulation and Blood
What is blood?
Boardworks AS Biology
Circulation and Blood
Blood is a specialized transport medium that is also considered a special type of connective tissue. An average adult has 4–6 litres of blood.
Blood has a range of functions such as:
transport
defence
thermoregulation
maintaining pH of body fluids.
Photo credit: National Cancer Institute / Science Photo Library
Coloured scanning electron micrograph (SEM) of human blood showing red and white cells and platelets. Red blood cells (erythrocytes) have a characteristic biconcave- disc shape and are numerous. These large cells contain haemoglobin, a red pigment by which oxygen is transported around the body. They are more numerous than white blood cells (yellow). White blood cells (leucocytes) are rounded cells with microvilli projections from the cell surface. Leucocytes play an important role in the immune response of the body. Platelets are smaller cells (pink) that play a major role in blood clotting.
Teacher notes
Blood, along with bone and cartilage, is considered a type of connective tissue, even though it is quite different to other connective tissues, such as collagen. Embryologically, it has the same origin as other connective tissues, and could be said to connect other body tissues together through the transport of substances.

4. The composition of blood
Boardworks AS Biology
Circulation and Blood
Teacher notes
The amount of blood occupied by erythrocytes is called the haematocrit. An athlete’s haematocrit is used in drug testing - an elevated level may indicate the athlete has artificially boosted the number of erythrocytes by using the drug erythropoietin, or by ‘blood doping’ - transfusing erythrocytes, either their own (which have been previously harvested and stored) or from a compatible donor.
Only about 2% of all the oxygen in blood is transported in the plasma. The vast majority is bound to haemoglobin in erythrocytes. In contrast, most carbon dioxide is transported in the plasma.
See the ‘Gas Exchange’ presentation for more information about haemoglobin and the transport of oxygen.
See the ‘Immunology’ presentation for more information about leukocytes.

5. Features of erythrocytes
Boardworks AS Biology
Circulation and Blood
What are the specialized features of an erythrocyte?
flattened, biconcave disc shape: ensures large surface area to volume ratio for efficient gas exchange
large amount of haemoglobin: for transporting oxygen
no nucleus or organelles: maximises space for haemoglobin, so more oxygen can be transported
Teacher notes
Only the erythrocytes of mammals and a few other vertebrates have no nucleus. They are present in the erythrocytes of most other vertebrates. The lack of nucleus means erythrocytes cannot divide so most be continuously replaced by new cells produced in the bone marrow. Human erythrocytes live for about 120 days.
Haemoglobin makes up about 97% of the dry mass of an erythrocyte.
See the ‘Gas Exchange’ presentation for more information about haemoglobin.
diameter (6–8 µm) larger than capillary diameter: slows blood flow to enable diffusion of oxygen

6. Serum
When the blood clotting protein, fibrinogen, has been removed from the blood plasma, the solution which remains is called serum.

7. Plasma proteins and blood pressure
Boardworks AS Biology
Circulation and Blood
About 8% of blood plasma consists of plasma proteins, of which about half may be albumins.
These are a group of small proteins involved in the transport of other substances (e.g. fatty acids, hormones) and which help regulate the osmotic pressure of blood.
Photo credit: Borislav Mitev
Human serum albumin, to which six molecules of palmitic acid (grey) are bound.
Teacher notes
See the ‘Transport Across Membranes’ presentation for more information about osmosis.
The balance between the hydrostatic pressure of blood (‘blood pressure’) and the osmotic pressure of blood is important in the formation of tissue fluid.

8. Formation of tissue fluid
Boardworks AS Biology
Circulation and Blood
Formation of tissue fluid
Teacher notes
One sign of high blood pressure is oedema – swelling caused by the accumulation of tissue fluid. Students could be asked to explain how this occurs in terms of blood pressure and osmotic pressure.

9. Boardworks AS Biology Circulation and Blood
Lymph
Boardworks AS Biology
Circulation and Blood
Not all tissue fluid returns to the capillaries. The excess drains into the lymphatic system, where it forms lymph.
Lymph is a colourless/pale yellow fluid similar to tissue fluid but containing more lipids.
lymphatic capillaries
Teacher notes
Lymph contains fatty substances because digested lipids are transported from the digestive system via the lymphatic system before being returned to the circulatory system.
The lymphatic system drains into the circulatory system near the vena cava via the thoracic duct.

10. Page 17, answer questions 1,2 and 3.
Task
Page 17, answer questions 1,2 and 3.

11. Homework