Welcome to Transport in Mammals!
Hi there! This chapter is all about how your amazing body moves essential materials around. Think of your transport system as the world's most efficient courier service, delivering oxygen and nutrients, and picking up waste. Since mammals (like us!) are large, complex, and highly active, simple diffusion just won't cut it. We need a specialised system to keep trillions of cells happy and supplied.
We will cover the entire circulatory system, the incredible way blood transports gases, and how your heart keeps everything moving flawlessly.
8.1 The Mammalian Circulatory System
The mammalian transport system is an absolute necessity for survival. It ensures every cell gets the oxygen and glucose needed for respiration, and removes toxic waste like carbon dioxide and urea.
Key Features of the Mammalian Circulatory System
1. Closed Circulation: The blood is always enclosed within blood vessels (heart, arteries, capillaries, veins). It never comes into direct contact with the body cells, unlike in some invertebrates.
2. Double Circulation: The blood passes through the heart twice for every complete circuit of the body. This is crucial because it allows the blood pressure to be maintained after passing through the lungs, making transport much more efficient.
There are two main loops:
- Pulmonary Circulation: Carries deoxygenated blood from the heart to the lungs and oxygenated blood back to the heart.
- Systemic Circulation: Carries oxygenated blood from the heart to the rest of the body tissues and returns deoxygenated blood to the heart.
Main Blood Vessels and their Functions
The syllabus requires you to know the functions of the main vessels connecting the two loops:
- Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
- Pulmonary Vein: Carries oxygenated blood from the lungs back to the left atrium.
- Aorta: Carries oxygenated blood from the left ventricle to all parts of the systemic circulation (the whole body).
- Vena Cava (Superior and Inferior): Carries deoxygenated blood from the body tissues back to the right atrium.
Quick Tip: Remember that "Artery" means away from the heart, and "Vein" means towards the heart. The pulmonary vessels are the exception regarding oxygen content!
Structure and Function of Blood Vessels
You must be able to recognize and explain the structure-function relationship of arteries, arterioles, capillaries, venules, and veins, both from diagrams and micrographs (TS/LS).
Arteries (Including Elastic Arteries and Muscular Arteries)
Arteries carry blood away from the heart under high pressure.
Structure relates to function:
- Thick wall: To withstand high blood pressure without bursting.
- Narrow lumen: To help maintain high pressure.
- Thick layer of elastic fibres (Elastic Arteries, e.g., Aorta): These stretch when the heart contracts (systole) and recoil (spring back) when the heart relaxes (diastole). This smoothing out of blood flow prevents pressure surges and maintains continuous flow.
- Thick layer of smooth muscle (Muscular Arteries/Arterioles): Allows vasoconstriction and vasodilation to regulate blood flow distribution to specific organs.
Capillaries
Capillaries are the sites of exchange between blood and tissue fluid.
Structure relates to function:
- One cell thick walls (Endothelium): Provides a very short diffusion distance for fast exchange of oxygen, glucose, and waste products.
- Very narrow lumen (just wide enough for RBCs to pass in single file): Forces close contact between blood and capillary wall, speeding up exchange.
- Large total surface area: Due to extensive branching, maximising the area available for diffusion.
Veins (Including Venules)
Veins carry blood towards the heart under low pressure.
Structure relates to function:
- Large lumen compared to the wall thickness: Offers low resistance to flow.
- Thin walls (less muscle and elastic tissue): Since pressure is low, thick walls are unnecessary.
- Valves: Prevent the backflow of blood, as pressure is very low, ensuring blood only flows towards the heart. Blood is typically pushed by skeletal muscle contraction.
Quick Review: Vessel Structure
Artery = Away, Thick wall
Vein = Valves, Very large lumen
Capillary = Cell thickness of 1, Close contact for exchange
8.1 Blood, Tissue Fluid, and Water's Role
The Main Component: Water
Water is the main component of blood plasma and tissue fluid. Recall the properties of water relevant to transport (from Topic 2.4):
1. Solvent Action: Water is an excellent solvent due to its polarity, allowing it to dissolve and transport essential substances like glucose, amino acids, ions, and waste products (urea, CO₂) around the body.
2. High Specific Heat Capacity: This means water absorbs a lot of heat energy with only a small rise in temperature. This helps keep the internal body temperature (core temperature) stable, preventing rapid, damaging fluctuations.
Blood Cells (Limited to AS Syllabus)
You need to recognize and draw diagrams of these cells, often seen in blood smears:
- Red Blood Cells (Erythrocytes): Biconcave disc shape, no nucleus (in mammals), packed with haemoglobin. Function: Transport Oxygen.
- Neutrophils: Phagocytic white blood cell, multi-lobed nucleus. Function: Non-specific immunity (phagocytosis).
- Monocytes: Large white blood cell with a kidney-shaped nucleus. Function: Differentiate into macrophages (important in immunity).
- Lymphocytes: White blood cell with a large, round nucleus taking up most of the cell. Function: Specific immunity (B and T cells).
Tissue Fluid: Formation and Function
Tissue fluid is the liquid that bathes all the cells of the body. It forms the medium through which substances (oxygen, nutrients, waste) are exchanged between the blood and the cells.
Step-by-Step Formation of Tissue Fluid in a Capillary Network
1. High Hydrostatic Pressure: At the arteriole end of the capillary, the blood is still under relatively high pressure from the heart's contraction. This is the hydrostatic pressure.
2. Outward Filtration: The hydrostatic pressure pushing fluid out of the capillary is greater than the water potential gradient pushing fluid in (created by the plasma proteins which are too large to leave the blood).
3. Fluid Squeezes Out: This pressure forces water, dissolved nutrients (glucose, amino acids), and oxygen out through the small gaps in the capillary walls to form tissue fluid.
4. Reabsorption: At the venule end, as fluid has left, the hydrostatic pressure is much lower. The water potential gradient (due to the presence of plasma proteins remaining in the capillary) becomes dominant, causing most of the fluid (and waste products) to move back into the capillary by osmosis.
5. Lymphatic System: Any excess tissue fluid that doesn't return to the blood capillaries drains into the lymphatic system, becoming lymph, which eventually rejoins the blood near the heart.
Key Takeaway for 8.1: The double circulation ensures high pressure delivery (systemic) and efficient gas exchange (pulmonary). Blood vessels are perfectly structured for high pressure containment (arteries), low pressure return (veins), and efficient exchange (capillaries). Tissue fluid is the necessary intermediary for cell survival.
8.2 Transport of Oxygen and Carbon Dioxide
This section explores how blood manages the massive task of picking up O₂ in the lungs and CO₂ in the tissues, and vice-versa.
The Role of Haemoglobin (Hb) in Oxygen Transport
Oxygen is largely insoluble in water, so 98.5% of it is transported bound to the globular protein haemoglobin inside red blood cells.
Haemoglobin structure (Recall Topic 2.3): It is a quaternary protein made of four polypeptide chains (two alpha ($\alpha$) and two beta ($\beta$)) and four prosthetic haem groups, each containing one iron ion (Fe²⁺). One Hb molecule can bind four O₂ molecules.
Reaction: \(Hb + 4O_2 \rightleftharpoons Hb(O_2)_4\) (Oxyhaemoglobin)
The Oxygen Dissociation Curve (ODC)
The ODC plots the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (pO₂). It is a characteristic S-shaped (sigmoidal) curve.
Explaining the S-Shape and Importance
1. Lungs (High pO₂): The curve is high and flat. Hb has a high affinity for oxygen. Even if the pO₂ drops slightly, Hb remains highly saturated. This ensures maximal loading of O₂ in the lungs.
2. Tissues (Low pO₂): The curve drops steeply. Hb has a low affinity for oxygen, meaning it readily releases O₂ to the respiring tissues where it is needed.
3. Cooperative Binding: The S-shape reflects how binding one O₂ molecule makes it easier for the next O₂ molecule to bind (and vice versa for release). This cooperative mechanism makes Hb extremely efficient at both loading and unloading O₂ under different conditions.
The Bohr Shift (The Importance of CO₂)
When tissues respire vigorously, they produce a lot of CO₂, which dissolves in water (blood plasma/RBC cytoplasm) to form carbonic acid, lowering the pH (making the blood more acidic).
The Bohr Shift is the phenomenon where a decrease in pH (or increase in pCO₂) causes the oxygen dissociation curve to shift to the right.
Importance of Bohr Shift: This shift means that at any given partial pressure of oxygen, Hb is less saturated. In other words, O₂ is released more easily. This is vital because the body needs oxygen most urgently in the very tissues that are actively producing CO₂ (i.e., respiring tissue).
Transport of Carbon Dioxide (CO₂)
CO₂ is transported in three ways:
1. Dissolved directly in plasma (about 5%).
2. Bound to haemoglobin (as Carbaminohaemoglobin) (about 10%).
3. As Bicarbonate ions (\(HCO_3^-\)) (about 85%). This is the most important method.
The Chloride Shift (Step-by-Step)
The majority of CO₂ transport happens inside the red blood cell, mediated by the enzyme Carbonic Anhydrase (CA), one of the fastest enzymes known.
Process in the Tissues:
1. CO₂ diffuses into the RBCs.
2. CA quickly catalyses the reaction: \(CO_2 + H_2O \rightleftharpoons H_2CO_3\) (Carbonic acid)
3. Carbonic acid dissociates: \(H_2CO_3 \rightleftharpoons H^+ + HCO_3^-\) (Bicarbonate ions)
4. The Bicarbonate ions (\(HCO_3^-\)) diffuse out of the RBC into the plasma.
5. To maintain electrical neutrality, chloride ions (\(Cl^-\)) move from the plasma into the red blood cell. This counter-movement is called the Chloride Shift.
6. The released hydrogen ions (\(H^+\)) are immediately picked up (buffered) by the haemoglobin, forming Haemoglobinic acid (HHb). This buffering action is key as it prevents the RBC cytoplasm from becoming too acidic, which would denature Hb.
Importance of Chloride Shift: It allows for the continued, high-volume conversion and transport of CO₂ as bicarbonate ions, while maintaining the electrochemical balance and pH of the cell.
Process in the Lungs: The reverse happens. Low pCO₂ in the alveoli causes bicarbonate and hydrogen ions to recombine (using CA) back into CO₂ and water. The CO₂ diffuses out into the alveoli, and the chloride ions diffuse back out of the RBC.
Key Takeaway for 8.2: Haemoglobin is highly specialised for gas transport. The ODC shows high affinity in the lungs and low affinity in the tissues. The Bohr shift ensures O₂ release is matched to demand. CO₂ is primarily transported as bicarbonate, facilitated by carbonic anhydrase and the chloride shift.
8.3 The Heart and the Cardiac Cycle
The heart acts as the central pump, driving the double circulatory system. You need to know its structure and how the intrinsic conduction system controls its rhythm.
Structure of the Mammalian Heart
The heart is a muscular organ divided into four chambers:
- Two Atria (receiving chambers): Receive blood from the body (Right Atrium) or lungs (Left Atrium).
- Two Ventricles (pumping chambers): Pump blood to the lungs (Right Ventricle) or body (Left Ventricle).
Valves: Ensure one-way flow, preventing backflow (regurgitation).
- Atrioventricular (AV) Valves: Between atria and ventricles (Tricuspid on right, Bicuspid/Mitral on left).
- Semi-lunar Valves: At the exit of the ventricles (Aortic and Pulmonary).
Differences in Heart Wall Thickness
The thickness of the muscle wall in each chamber reflects the pressure needed to pump the blood to its destination:
1. Atria Walls: Thinnest. They only need to pump blood a short distance into the adjacent ventricles (aided by gravity).
2. Right Ventricle Wall: Thicker than the atria. It pumps blood to the Pulmonary Circulation (the lungs). This requires relatively low pressure to avoid damaging the delicate capillaries in the lungs.
3. Left Ventricle Wall: Thickest. It generates the highest pressure to pump blood to the entire Systemic Circulation (the rest of the body). It requires a much greater force to overcome the resistance of the body's vast network of arteries and capillaries.
The Cardiac Cycle
The cardiac cycle is the sequence of events that takes place during one complete heartbeat. It is divided into periods of contraction (Systole) and relaxation (Diastole).
Stages and Pressure Changes
1. Atrial Systole (Contraction):
- Both atria contract, pushing remaining blood into the ventricles.
- Pressure in atria rises; pressure in ventricles is low.
2. Ventricular Systole (Contraction):
- Ventricles contract (starting at the base and moving upwards).
- Pressure rises sharply in the ventricles.
- When ventricular pressure exceeds atrial pressure, the AV valves slam shut (this is the first heart sound, "lub").
- When ventricular pressure exceeds the pressure in the aorta/pulmonary artery, the semi-lunar valves open, forcing blood out.
3. Diastole (Relaxation):
- Both atria and ventricles relax.
- Blood pressure drops in the ventricles.
- As blood starts to flow backwards from the arteries, the semi-lunar valves snap shut (this is the second heart sound, "dub").
- Blood flows from the vena cava/pulmonary vein directly into the relaxed atria and ventricles, ready for the next cycle.
Controlling the Rhythm: The Intrinsic Conduction System
The heart muscle is myogenic, meaning it can initiate its own contraction without external nervous input. This relies on specialised nodes and tissues:
1. Sinoatrial Node (SAN): Located in the wall of the right atrium. It acts as the natural pacemaker, initiating the wave of excitation (impulse).
2. Atrioventricular Node (AVN): Located in the septum between the atria. The impulse from the SAN reaches the AVN, which delays the impulse briefly (about 0.1s). This delay is essential to ensure the atria finish contracting before the ventricles start.
3. Purkyne Tissue (Bundle of His): The impulse travels quickly down the septum through the Bundle of His and then spreads rapidly throughout the ventricular walls via the Purkyne fibres. This causes the ventricles to contract almost simultaneously from the bottom up, pushing blood efficiently out into the arteries.
Remember: You are only required to know the roles of the SAN, AVN, and Purkyne tissue in initiating and conducting the impulse; you do NOT need to know about the nervous or hormonal control of the heart rate (e.g., adrenaline or vagus nerve).
Final Key Takeaways
1. Transport Medium: Blood/Tissue Fluid are mostly water, relying on its properties as a solvent and temperature stabilizer.
2. Gas Exchange: Haemoglobin's affinity for oxygen changes drastically with pO₂ (S-curve) and pH (Bohr shift), optimising delivery.
3. The Pump: The heart has varying wall thicknesses to handle the demands of the pulmonary (low pressure) and systemic (high pressure) circuits. Its rhythm is intrinsic, initiated by the SAN.