Transport in Humans Notes

Need for Transport Systems in Living Organisms

• Simple Unicellular Organisms (e.g., Amoeba):     - These organisms do not require a specialized transport system.     - Gas Exchange: Oxygen and nutrients diffuse directly through the cell membrane to reach all parts of the cell.     - Waste Removal: Metabolic waste products are removed from the cell through simple diffusion.     - Factors for Efficiency: Their small size ensures a short distance between the cells and the external environment, and a large surface area to volume ratio ($SA/V$) allows for a faster rate of diffusion.

• Simple Multicellular Organisms (e.g., Hydra):     - These organisms are typically only two cell layers thick.     - This structure allows direct exchange with the environment as most cells remain in close proximity to the external medium, facilitating uptake of nutrients and oxygen and removal of waste.

Limitations of Diffusion and the Surface Area to Volume ($SA/V$) Ratio

• Theoretical Constraints on Cell Size:     - As a cell increases in size, its metabolic requirements for nutrients grow, yet the distance for these nutrients to reach the cell's center also increases.     - Large cells produce more waste products; however, the time required to remove them increases significantly.     - Consequence: The accumulation of waste products and the lack of incoming nutrients would eventually kill a cell that grows too large.

• Surface Area to Volume Ratio ($SA/V$) Relationship:     - $SA/V$ ratio refers to the amount of surface area an object has compared to its volume.     - As an organism increases in size, its volume increases much faster than its surface area, leading to a decreasing $SA/V$ ratio.     - This decrease negatively affects the rate of exchange for heat, gases, nutrients, and wastes.

• Mathematical Comparison of Different Organism Sizes:     - Organism 1 (Length $1\,mm$ or $10^{-6}\,m$): Surface Area = $6 \times 10^{-12}\,m^2$; Volume = $10^{-18}\,m^3$; $SA/V$ Ratio = $6,000,000\,m^{-1}$.     - Organism 2 (Length $100\,mm$ or $10^{-4}\,m$): Surface Area = $6 \times 10^{-8}\,m^2$; Volume = $10^{-12}\,m^3$; $SA/V$ Ratio = $60,000\,m^{-1}$.     - Organism 3 (Length $10\,mm$ or $10^{-2}\,m$): Surface Area = $6 \times 10^{-4}\,m^2$; Volume = $10^{-6}\,m^3$; $SA/V$ Ratio = $600\,m^{-1}$.     - Organism 4 (Length $1\,m$ or $10^0\,m$): Surface Area = $6 \times 10^0\,m^2$; Volume = $10^0\,m^3$; $SA/V$ Ratio = $6\,m^{-1}$.     - Organism 5 (Length $100\,m$ or $10^2\,m$): Surface Area = $6 \times 10^4\,m^2$; Volume = $10^6\,m^3$; $SA/V$ Ratio = $0.06\,m^{-1}$.     - Conclusion: As organisms become larger, it becomes increasingly difficult for them to exchange materials with surroundings via surface diffusion alone.

The Need for a Specialized Circulatory System in Humans

• Challenges of Multi-cellularity:     - Increased Metabolism: Larger body size leads to increased metabolic requirements, raising the amount of materials that must move in and out.     - Distance and Immersion: In complex organisms like humans, most cells are not in contact with the external environment. Direct exchange is impossible because the diffusion distance is too great.     - Reduced Efficiency: Because the $SA/V$ ratio is low, diffusion alone is too slow and inefficient to sustain life.

• The Biological Solution:     - Specialized Organs: Humans developed organs specialized for exchange (e.g., the small intestine for nutrient absorption; lungs for gas exchange).     - Transport Vessels: To overcome the diffusion distance, a circulatory system transports materials from these specialized organs to all other parts of the body.

Components of the Human Circulatory System

• The human circulatory system consists of three primary components:     - Circulating Fluid: Blood.     - Vascular System (Transport Vessels): Blood vessels.     - Central Muscular Pump: The heart.

• Objective: To transport gases, nutrients, and waste products between cells and specialized exchange organs (e.g., lungs, intestines).

Composition and Functions of Blood

• Blood Classification: Blood is characterized as a specialized connective tissue.

• Components by Volume (via Centrifugation):     - Plasma ($55\%$ of blood): A clear yellowish liquid.     - Cellular Elements ($45\%$ of blood): Includes Red Blood Cells ($44\%$) and a very small layer (<$1\%$) called the "Buffy Coat" consisting of White Blood Cells and Platelets.

• Hematology Standards (Reference Ranges):     - Haemoglobin: $13.5 - 18.0\,g/dL$.     - Red cell count: $4.5 - 6.5 \times 10^{12}/L$.     - Total White cell count: $4.0 - 11.0 \times 10^9/L$.     - Platelets: $140 - 440 \times 10^9/L$.     - Haematocrit (PCV): $0.40 - 0.54$.     - MCV (Red cell volume): $78 - 98\,fL$.

Plasma: The Liquid Matrix

• Definition: Clear yellowish part of blood after cellular components are removed.

• Composition: $90\%$ water.

• Dissolved Solutes:     - Proteins: Antibodies, Fibrinogen (clotting), Prothrombin (clotting), and Albumin (regulates water potential).     - Nutrients: Glucose, amino acids, fats, vitamins.     - Waste Products: Urea, $CO_2$, creatinine.     - Hormones: e.g., Insulin.     - Ions: $Na^+$, $K^+$, $Mg^{2+}$, $Ca^{2+}$, $Cl^-$, $HCO_3^-$. These maintain osmotic balance and blood pH ($7.4$) and regulate membrane permeability.

• Functions:     - Transport of soluble materials throughout the body.     - Distribution of heat to maintain uniform body temperature.

Red Blood Cells (Erythrocytes)

• Characteristics:     - Constitute $99\%$ of all blood cells.     - Produced in the bone marrow (spongy tissue inside bones like hips and thighs).     - Destroyed in the spleen and liver.     - Lifespan of approximately $120$ days.

• Structural Adaptations for Function:     - Lack of Nucleus/Organelles: Allows more space for haemoglobin to be packed, increasing oxygen-carrying capacity.     - Biconcave Shape: Thinner in the middle to increase flow rate and optimize the $SA/V$ ratio for faster oxygen diffusion.     - Flexible and Deformable Membrane: Enables the cell to change into a "bell-shape" to squeeze through tiny capillaries.

• Haemoglobin Structure:     - Consists of 4 globular protein subunits, each with a heme group.     - Each heme group has an Iron ion ($Fe^{2+}$) at the center which binds reversibly to oxygen.     - Note: In plants, chlorophyll has a similar structure but with Magnesium ($Mg^{2+}$) at the center instead of Iron.

Transport of Gases in the Blood

• Oxygen Transport:     - Over $99\%$ of oxygen is bound to haemoglobin.     - Oxyhaemoglobin: The oxygen-loaded form (bright red).     - Deoxyhaemoglobin: The oxygen-unloaded form (purple-blue).

• Carbon Dioxide Transport:     - Dissolved in Plasma ($5\%$): $CO_2$ is $20$ times more soluble in water than oxygen.     - Bound to Haemoglobin ($5\%$): Forms carbaminohaemoglobin.     - As Bicarbonate Ions ($90\%$): This is the primary method of transport.

White Blood Cells (Leukocytes)

• General Features:     - Larger than red blood cells but fewer in number ($5,000 - 10,000$ per $\mu L$).     - Produced in bone marrow; lifespan of only a few days.     - Possess a nucleus; lack haemoglobin (colourless).     - Mobility: Irregular shape and ability to squeeze through thinnest capillary walls.

• Phagocytes:     - Features: Lobed nucleus and granular cytoplasm.     - Mechanism: Engulf and ingest foreign particles, bacteria, and dead cells via phagocytosis.     - Types:         - Neutrophils ($60\%$): Most abundant; first responders to bacterial infection.         - Monocytes: Largest WBCs; differentiate into macrophages or dendritic cells.         - Macrophages: Migrated monocytes that detect and ingest bacteria/dead cells in tissues.

• Lymphocytes:     - Features: Large rounded nucleus and non-granular cytoplasm ($25-35\%$ of WBCs).     - B Cells: Produce antibodies that bind to antigens on pathogens, causing them to clump for phagocytes to destroy.     - T Cells: Directly destroy infected or cancerous cells and signal B cells.

• Antigen Definition: A substance that induces antibody formation because it is recognized as foreign.

Tissue Rejection and Platelets

• Tissue Rejection: The immune system recognizes antigens on transplanted organs as foreign and destroys them.     - Prevention: Using donors of close relations (tissue match), taking immunosuppressant drugs, or bone marrow transplants.

• Platelets (Thrombocytes):     - Not true cells; they are membrane-bound fragments of cytoplasm pinched off from bone marrow cells.

• Blood Clotting Process:     - Trigger: Damage to the endothelium (inner lining of vessels).     - Pathway: Platelets trigger a cascade that converts soluble fibrinogen into insoluble fibrin threads via the enzyme thrombin.     - Result: Fibrin threads form a network to entangle cells, preventing blood loss and pathogen entry.

ABO Blood Groups and Transfusion

• Blood Type Identification:     - Type A: Antigen A on RBC; Antibody b in plasma.     - Type B: Antigen B on RBC; Antibody a in plasma.     - Type AB: Both Antigens A & B on RBC; No antibodies in plasma (Universal Acceptor).     - Type O: No antigens on RBC; Both Antibodies a & b in plasma (Universal Donor).

• Agglutination:     - Occurs if a donor's antigens react with a recipient's antibodies (e.g., Type B donor to Type A recipient).     - Process: Recipient's Antibody b binds to donor's Antigen B, causing red blood cells to clump (agglutinate).     - Danger: Clumps can block blood vessels in vital organs, leading to death.

Questions & Discussion

• Q: Which statement does not explain the need for a circulatory system?     - A: "Diffusion occurs quickly across a cell membrane." This is a general property of membranes and does not explain why large organisms specifically need a transport system compared to small ones.

• Q: Which statement about haemoglobin is incorrect?     - A: "Haemoglobin binds to carbon dioxide to form deoxyhaemoglobin." (Deoxyhaemoglobin is simply haemoglobin without oxygen, while the $CO_2$ bound form is carbaminohaemoglobin).

• Q: What is the function of WBCs?     - A: Antibody formation, killing cancer cells, and phagocytosis. (Clotting is a function of platelets).

• Q: Will blood group O affect a recipient that is blood group B?     - A: No, because Group O red blood cells have no antigens to be attacked by the recipient's antibodies.