Chapter 2.3: Circulatory Systems in Mammals
Welcome to your study notes for Section 2.3: Circulatory Systems in Mammals (CCEA AS Biology Unit AS 2: Organisms and Biodiversity). Transport systems are the body’s delivery network. In this chapter, we will explore why large animals cannot rely on simple diffusion, how the mammalian heart pumps blood under precise pressure control, how blood vessels adapt to their specific roles, how cells receive nutrients via tissue fluid, and how haemoglobin efficiently picks up and drops off oxygen.
Don’t worry if this chapter feels detailed at first! We have broken down each process into clear, logical steps with simple analogies and memory tips to help you succeed in your exams.
---1. Principles of Circulatory Systems and Mass Flow
Why Do Multicellular Mammals Need a Mass Transport System?
Small, unicellular organisms (like amoebae) have a large surface area to volume ratio (\(SA:V\)) and short diffusion distances. They can absorb oxygen and nutrients directly across their surface by simple diffusion.
In contrast, mammals have:
• A very small surface area to volume ratio (\(SA:V\)).
• Large diffusion distances between the outside environment and their internal cells.
• A high metabolic rate requiring rapid, continuous supplies of oxygen and glucose, and rapid removal of toxic wastes like carbon dioxide.
Because simple diffusion across the body surface is far too slow, mammals rely on a mass transport system (the circulatory system) where liquids move in bulk in one direction due to pressure differences.
Open vs. Closed Circulatory Systems
• Open Circulatory System: Found in insects and arthropods. Blood-like fluid called haemolymph is pumped by a heart into an open body cavity (the haemocoel), directly bathing the internal organs at low pressure.
• Closed Circulatory System: Found in vertebrates (including mammals). Blood remains completely enclosed inside continuous blood vessels. This allows blood to be pumped at higher pressures, travelling much faster and more efficiently to targeted tissues.
Single vs. Double Circulatory Systems
• Single Circulation (e.g., Fish): Blood passes through the heart once in a complete circuit of the body (\(\text{Heart} \rightarrow \text{Gills} \rightarrow \text{Body Tissues} \rightarrow \text{Heart}\)). As blood passes through the tiny capillaries of the gills, hydrostatic pressure drops significantly. This means blood flows to the rest of the body under very low pressure, resulting in slow delivery to active tissues.
• Double Circulation (e.g., Mammals): Blood passes through the heart twice per complete circuit. It consists of two distinct loops:
1. Pulmonary Circulation: Deoxygenated blood is pumped from the right side of the heart to the lungs, picks up oxygen, and returns to the left side of the heart (\(\text{Heart} \rightarrow \text{Lungs} \rightarrow \text{Heart}\)).
2. Systemic Circulation: Oxygenated blood is pumped from the left side of the heart to the rest of the body and returns deoxygenated to the right side (\(\text{Heart} \rightarrow \text{Body Tissues} \rightarrow \text{Heart}\)).
Key Advantages of Double Circulation:
• Blood pressure is stepped up after passing through the pulmonary circuit. The left side of the heart pumps systemic blood under high pressure and velocity to body tissues.
• The pulmonary circuit operates under lower pressure, which prevents damage to the delicate capillary beds in the lungs and avoids fluid accumulation (pulmonary oedema).
Key Takeaway: Mammals require a closed, double circulatory system because a small \(SA:V\) ratio and high metabolic demands require fast, high-pressure mass transport to body tissues without damaging the fragile gas exchange surfaces in the lungs.
---2. Anatomy of the Mammalian Heart
The heart is a muscular organ made of myogenic cardiac muscle (myocardium) that contracts rhythmically on its own without external nervous stimulation.
The Four Chambers
• Right Atrium: Thin-walled chamber receiving deoxygenated blood from the body via the vena cava.
• Right Ventricle: Muscular chamber that pumps deoxygenated blood to the lungs via the pulmonary artery.
• Left Atrium: Thin-walled chamber receiving oxygenated blood from the lungs via the pulmonary vein.
• Left Ventricle: Very thick muscular chamber that pumps oxygenated blood to the entire body via the aorta.
The Heart Valves
Valves ensure unidirectional flow of blood, preventing backflow when pressures change.
• Atrioventricular (AV) Valves: Located between the atria and ventricles.
— Tricuspid Valve: Located on the right side (between right atrium and right ventricle).
— Bicuspid (Mitral) Valve: Located on the left side (between left atrium and left ventricle).
— Both AV valves are tethered by strong, non-elastic chordae tendineae (tendinous cords) attached to papillary muscles projecting from the ventricle walls. These prevent the valve flaps from inverting (turning inside out) into the atria during the intense pressure of ventricular contraction.
• Semilunar Valves: Located at the base of the two major outflow arteries.
— Pulmonary Valve: At the base of the pulmonary artery.
— Aortic Valve: At the base of the aorta.
Memory Trick for AV Valves: TRI before you BI — you encounter the Tricuspid on the right side first, and the Bicuspid on the left!
Major Associated Blood Vessels
• Vena Cava (Superior and Inferior): Carries deoxygenated blood from systemic tissues back to the right atrium.
• Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
• Pulmonary Vein: Carries oxygenated blood from the lungs to the left atrium.
• Aorta: Carries oxygenated blood under high pressure from the left ventricle to systemic tissues.
• Coronary Arteries & Veins: Branch off the base of the aorta to supply the heart muscle itself with oxygen and glucose, and remove metabolic wastes.
Why is the Left Ventricle Wall Thicker than the Right?
A classic exam question! The left ventricular wall is significantly thicker and more muscular than the right ventricular wall because it must generate a much higher hydrostatic pressure to overcome the high resistance of the entire systemic body circuit. The right ventricle only pumps blood through the short, low-resistance pulmonary circuit to the lungs nearby.
Exam Warning: Both ventricles pump the exact same volume of blood per beat (stroke volume)! Never state that the left ventricle is thicker because it holds or pumps "more blood". It is thicker purely to generate higher pressure.
---3. The Cardiac Cycle and Electrical Coordination
The cardiac cycle is the sequence of events in a single heartbeat, lasting approximately \(0.8\text{ s}\) at rest.
Phases of the Cardiac Cycle
1. Atrial Systole:
• The atria contract; the ventricles are relaxed.
• Pressure in the atria rises above pressure in the ventricles.
• The atrioventricular (AV) valves open, and blood is pushed into the ventricles.
• The semilunar valves remain closed.
2. Ventricular Systole:
• The ventricles contract strongly from the apex (base) upwards; the atria relax.
• Pressure inside the ventricles rapidly exceeds atrial pressure, forcing the AV valves to snap shut to prevent backflow (this produces the first heart sound, "lub").
• When ventricular pressure rises above arterial pressure in the aorta and pulmonary artery, the semilunar valves open.
• Blood is forcefully ejected into the aorta and pulmonary artery.
3. Diastole (Complete Relaxation):
• Both atria and ventricles relax.
• Ventricular pressure falls below the pressure in the aorta and pulmonary artery, causing the semilunar valves to snap shut (producing the second heart sound, "dup").
• Blood returns via the vena cava and pulmonary vein, passively filling the atria. As atrial pressure slightly exceeds ventricular pressure, the AV valves open passively, beginning gentle ventricular filling.
Electrical Coordination of the Heart
Cardiac muscle is myogenic, meaning it can initiate its own electrical depolarisation without nerve input.
The sequence of electrical excitation is as follows:
1. Sinoatrial Node (SAN): Located in the top wall of the right atrium. Known as the pacemaker, it generates a wave of electrical excitation (depolarisation) that spreads across both atria, stimulating atrial systole.
2. Non-Conducting Fibrous Layer: A band of non-conductive tissue between the atria and ventricles prevents the electrical wave from spreading directly from atria to ventricles.
3. Atrioventricular Node (AVN): Located in the lower interatrial septum. The AVN picks up the electrical excitation and delays the impulse by approximately \(0.1\text{ s}\). This delay is crucial because it ensures the atria have completely emptied and the ventricles have fully filled with blood before ventricular contraction starts.
4. Bundle of His & Purkyne Fibres: The impulse travels rapidly down the Bundle of His located within the septum to the heart's apex. It then branches out into the Purkyne fibres in the ventricular walls, spreading excitation from the apex upwards. This ensures the ventricles contract bottom-up, squeezing blood efficiently up and out through the major arteries.
Common Mistake to Avoid: Do not state that the AVN creates or initiates the heartbeat. The SAN initiates the impulse; the AVN delays and conducts it!
---4. Structure and Function of Blood Vessels
The walls of blood vessels (except capillaries) consist of distinct layers: the inner endothelium (tunica intima), the middle tunica media (smooth muscle and elastic fibres), and the outer tunica externa (collagen fibres).
1. Arteries
• Function: Transport blood away from the heart at high, pulsatile pressure.
• Thick Tunica Media: Contains abundant elastic fibres that stretch during ventricular systole to absorb high pressure surges, and recoil during diastole to smooth out blood flow and maintain pressure. Also contains smooth muscle to provide strength.
• Folded Endothelium: Unfolds to accommodate stretching without tearing under high pressures.
• Thick Tunica Externa: Rich in tough collagen to prevent the vessel from overstretching and bursting under high hydrostatic pressure.
2. Arterioles
• Function: Regulate blood flow and pressure into capillary networks.
• Thick Smooth Muscle Layer: Relatively less elastic tissue and more smooth muscle. Can contract (vasoconstriction) to restrict blood flow or relax (vasodilation) to increase blood flow to specific capillary beds depending on physiological demand.
3. Capillaries
• Function: Exchange of respiratory gases, nutrients, and metabolic wastes between blood and tissues.
• Single Layer of Squamous Endothelial Cells: Resting on a basement membrane (only 1 cell thick), providing an extremely short diffusion distance.
• Narrow Lumen (\(\approx 7\text{–}8\,\mu\text{m}\)): Matches the diameter of a single red blood cell. This forces erythrocytes to pass in single file, slowing transit time and pressing cells close to the capillary wall to maximise diffusion efficiency.
• Extensive Branching: Huge total cross-sectional area provides a massive surface area for rapid exchange and lowers blood velocity.
4. Veins
• Function: Return blood to the heart under low hydrostatic pressure.
• Large Lumen: Offers minimal peripheral resistance to low-pressure blood flow.
• Thinner Walls: Much less smooth muscle and elastic tissue compared to arteries because pressure is low and non-pulsatile.
• Semilunar Pocket Valves: Prevent the backflow of blood, ensuring unidirectional flow towards the heart.
• Skeletal Muscle Pump: Contraction of surrounding skeletal muscles compresses veins, squeezing blood forward through open valves.
5. Tissue Fluid Formation and the Lymphatic System
All body cells are bathed in a watery liquid called tissue fluid (interstitial fluid). It allows exchange of substances between blood plasma and cells.
How Tissue Fluid Forms (Ultrafiltration)
Fluid movement across capillary walls is governed by the balance between two opposing pressures:
1. Hydrostatic Pressure: The pressure generated by heart contraction pushing fluid out of the capillary pores.
2. Oncotic (Colloid Osmotic) Pressure: The osmotic pull created by large, retained plasma proteins (like albumin) that cannot cross the capillary wall, pulling water in by osmosis.
At the Arteriole End of the Capillary Bed:
• Hydrostatic pressure inside the capillary (\(\approx 4.3\text{ kPa}\)) is higher than the opposing inward oncotic pressure (\(\approx 3.3\text{ kPa}\)).
• There is a net filtration pressure outwards.
• Water, dissolved glucose, amino acids, fatty acids, ions, and oxygen are forced out through the capillary pores into intercellular spaces, forming tissue fluid.
• Crucial Rule: Large plasma proteins, platelets, and red blood cells are too large to fit through the pores and remain inside the capillary lumen.
At the Venule End of the Capillary Bed:
• Hydrostatic pressure inside the capillary drops significantly (\(\approx 1.6\text{ kPa}\)) due to fluid loss and resistance along the narrow capillary lumen.
• The oncotic pressure remains constant (\(\approx 3.3\text{ kPa}\)) because plasma proteins were retained.
• The inward oncotic pressure is now higher than the outward hydrostatic pressure.
• There is a net inward pressure, causing water to re-enter the capillary by osmosis, carrying dissolved waste products (e.g., urea, carbon dioxide). Roughly \(90\%\) of the fluid is reabsorbed this way.
The Lymphatic Drainage System
• The remaining \(\approx 10\%\) of tissue fluid does not reabsorb into capillaries. If left behind, it would lead to tissue swelling (oedema).
• This remaining fluid drains into blind-ending lymphatic capillaries and is now referred to as lymph.
• Lymph moves through lymphatic vessels (assisted by valves and muscle contraction) and passes through lymph nodes.
• Lymph nodes contain high concentrations of lymphocytes and macrophages that filter and destroy foreign microorganisms and debris.
• The lymph is eventually returned to the blood circulatory system via the subclavian veins near the base of the neck.
6. Oxygen Transport and Haemoglobin
Structure of Haemoglobin
Haemoglobin (\(\text{Hb}\)) is a quaternary globular protein designed for transport. Its structure includes:
• 4 Polypeptide Chains: Two alpha (\(\alpha\)) chains and two beta (\(\beta\)) chains.
• 4 Prosthetic Haem Groups: Each containing an iron ion (\(\text{Fe}^{2+}\)).
• Each \(\text{Fe}^{2+}\) ion can reversibly bind one molecule of oxygen (\(\text{O}_2\)). Therefore, one complete haemoglobin molecule carries up to \(4\text{ O}_2\) molecules (8 oxygen atoms):
\(\text{Hb} + 4\text{O}_2 \rightleftharpoons \text{Hb}(\text{O}_2)_4\)
(Deoxyhaemoglobin + Oxygen \(\rightleftharpoons\) Oxyhaemoglobin)
The Oxygen Dissociation Curve and Cooperative Binding
An oxygen dissociation curve plots the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (\(p\text{O}_2\)).
Rather than a straight diagonal line, the curve is S-shaped (sigmoid). Why?
• At low \(p\text{O}_2\), the haemoglobin subunits are tightly packed, making it difficult for the first \(\text{O}_2\) molecule to bind. The initial slope is shallow.
• Cooperative Binding: When the first \(\text{O}_2\) molecule binds, it changes the tertiary and quaternary shape (a conformational change) of the haemoglobin molecule. This uncovers the other haem groups, making it much easier for the second and third \(\text{O}_2\) molecules to bind rapidly. The curve steepens sharply.
• At high \(p\text{O}_2\), three of the four sites are occupied. It becomes statistically harder for the fourth \(\text{O}_2\) to collide with the last empty site, so the curve plateaus near \(100\%\) saturation.
Physiological Advantage: In the lungs (high \(p\text{O}_2\)), haemoglobin readily saturates with oxygen (\(\approx 98\%\)). In respiring tissues (lower \(p\text{O}_2\)), a small drop in \(p\text{O}_2\) causes a large unloading of oxygen where it is needed most.
The Bohr Effect
The Bohr Effect describes how changes in carbon dioxide concentration affect haemoglobin's affinity for oxygen.
Step-by-Step Mechanism:
1. Actively respiring tissues produce large amounts of \(\text{CO}_2\).
2. \(\text{CO}_2\) diffuses into red blood cells and reacts with water to form carbonic acid (\(\text{H}_2\text{CO}_3\)), which dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogen carbonate ions (\(\text{HCO}_3^-\)):
\(\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-\)
3. The accumulation of \(\text{H}^+\) lowers the intracellular pH (makes it more acidic).
4. The \(\text{H}^+\) ions bind to oxyhaemoglobin to form haemoglobinic acid (\(\text{HHb}\)), acting as a buffer.
5. Binding of \(\text{H}^+\) changes haemoglobin's tertiary shape, reducing its affinity for oxygen. Oxygen is displaced and released into the active tissues.
6. This causes the oxygen dissociation curve to shift to the RIGHT.
Memory Trick: Bohr Shift RIGHT: More \(\text{CO}_2\) / Lower pH / High respiration \(\rightarrow\) Curve moves to the RIGHT \(\rightarrow\) Releases oxygen readily!
Fetal Haemoglobin vs. Adult Haemoglobin vs. Myoglobin
• Fetal Haemoglobin (\(\text{HbF}\)):
— Has a higher affinity for oxygen than adult haemoglobin (\(\text{HbA}\)) at all partial pressures.
— Its dissociation curve is shifted to the LEFT.
— Biological Importance: In the placenta, maternal and fetal blood run close together. Fetal haemoglobin must bind oxygen at the lower partial pressures where maternal adult haemoglobin is releasing it, allowing oxygen transfer from mother to fetus.
• Myoglobin:
— A single polypeptide chain with only one haem group (found inside muscle cells).
— Has an extremely high affinity for oxygen and does not show cooperative binding (hyperbolic curve shifted far to the LEFT).
— It will not release its oxygen until \(p\text{O}_2\) drops to extremely low levels (during intense muscular exercise and hypoxia), acting as an emergency oxygen store in muscle tissue.
7. Quick Review: Summary of Common Pitfalls & Key Terms
• Left Ventricle vs Right Ventricle Thickness: Left ventricle is thicker to generate higher pressure against systemic peripheral resistance, not to pump a higher volume of blood.
• Direction of Contraction: Ventricles contract from the apex upwards via Purkyne fibres to push blood towards the arterial outlets.
• Role of the AVN: Delays the electrical impulse (by \(\approx 0.1\text{ s}\)) so atria empty completely before ventricles contract; it does not initiate the heartbeat.
• Tissue Fluid Ultrafiltration: Plasma proteins (e.g., albumin) never leave the capillary under normal conditions; they generate the oncotic pressure required to reabsorb fluid at the venule end.
• Curve Shifts:
— Shift to the RIGHT: Lower affinity; unloads \(\text{O}_2\) easily (Bohr effect, high \(p\text{CO}_2\), lower pH, higher temperature).
— Shift to the LEFT: Higher affinity; loads \(\text{O}_2\) more tightly at lower partial pressures (fetal haemoglobin, organisms adapted to low-oxygen environments, myoglobin).