Welcome to the Mammalian Circulatory System!
Hello and welcome to your revision notes for the Circulatory System in Mammals. Whether you find biology effortless or a bit daunting, this guide will break down each concept step by step. We will explore why large animals need a dedicated transport network, how the heart powers blood flow, how vessels are adapted to their jobs, and how oxygen is delivered right to where it is needed.
Don't worry if some terms look intimidating at first—take it one section at a time, use the analogies, and watch for the memory aids!
1. Why Do Mammals Need a Circulatory System?
Tiny single-celled organisms, such as amoebas, have a large surface area to volume ratio (\(\text{SA:V}\)). For them, simple diffusion across their outer body surface is fast enough to supply oxygen and remove waste. However, mammals are large, active organisms with a very small \(\text{SA:V}\) and high metabolic rates.
The Main Challenges for Large Mammals:
• Long diffusion distances: Cells deep inside the body are too far from the surface for diffusion to supply nutrients.
• High metabolic demand: Mammals are endothermic (they maintain a warm body temperature) and have active tissues requiring large volumes of oxygen and glucose continuously.
• Mass transport solution: A specialised circulatory system carries substances rapidly over long distances via mass flow (the bulk movement of fluids driven by pressure differences).
Open vs. Closed Circulatory Systems
• Closed Circulatory System: In mammals, blood is always enclosed inside blood vessels (arteries, capillaries, and veins). This allows blood to be pumped at high pressure, ensuring rapid delivery to metabolically active tissues.
• Open Circulatory System: Found in insects, where blood-like fluid (haemolymph) fills a body cavity and bathes organs directly at low pressure.
Single vs. Double Circulatory Systems
Mammals possess a closed double circulatory system. This means blood passes through the heart twice for each complete circuit of the body.
The Two Circuits:
1. Pulmonary Circulation: Transports deoxygenated blood from the right side of the heart to the lungs to pick up oxygen and release carbon dioxide, then returns oxygenated blood to the left side of the heart.
2. Systemic Circulation: Pumps oxygenated blood from the left side of the heart at high pressure to the rest of the body tissues and organs, returning deoxygenated blood back to the right side of the heart.
Why is a double circulatory system advantageous?
If blood went straight from the delicate lung capillaries to the rest of the body (a single circulation, like in fish), its pressure would drop dramatically. By returning to the heart after the lungs, the pressure is boosted again. This means oxygenated blood travels quickly to active muscles and organs at high pressure, while the lungs are protected from damage by receiving blood at a lower pressure.
Key Takeaway: Mammals have a closed, double circulatory system to overcome small surface-area-to-volume ratios and deliver oxygen rapidly to active tissues at high pressure, while keeping pulmonary pressure low to protect delicate lung tissue.
2. Blood Vessels: Structure and Function
Blood vessels act as the plumbing network of the body. Each vessel type has a unique wall structure tailored to its specific role.
The Three Main Tissue Layers (Tunics)
Most blood vessels (except capillaries) have walls made of three layers:
• Tunica Intima (Inner layer): Made of a smooth layer of endothelium (flat epithelial cells) that minimises friction as blood flows.
• Tunica Media (Middle layer): Contains smooth muscle and elastic fibres. Elastic fibres stretch under high pressure and recoil to maintain blood pressure. Smooth muscle contracts (vasoconstriction) or relaxes (vasodilation) to regulate blood flow and pressure.
• Tunica Externa / Adventitia (Outer layer): Made of tough collagen fibres that provide structural strength, preventing the vessel from over-stretching or bursting under high pressure.
Comparing the Blood Vessels
1. Arteries:
• Function: Carry blood away from the heart at high, fluctuating pressure.
• Wall Structure: Thick muscular and elastic wall; narrow lumen.
• Adaptation: Thick layer of elastic tissue stretches when blood surges from the heart (systole) and recoils during relaxation (diastole) to smooth out blood flow and maintain high pressure.
2. Arterioles:
• Function: Branch from arteries and control blood distribution to capillary beds.
• Wall Structure: Relatively thicker smooth muscle layer and fewer elastic fibres than arteries.
• Adaptation: Muscle contraction narrows the lumen (vasoconstriction), reducing blood flow; muscle relaxation widens the lumen (vasodilation), increasing blood flow to active tissues.
3. Capillaries:
• Function: Site of exchange of gases, nutrients, and wastes between blood and tissue cells.
• Wall Structure: Extremely thin—consisting of a single layer of endothelium (only one cell thick) and a narrow lumen (around \(7\text{–}8\,\mu\text{m}\), just wide enough for red blood cells to pass single file).
• Adaptation: Very short diffusion pathway; huge total cross-sectional area across capillary beds slows blood velocity, giving ample time for diffusion.
4. Venules:
• Function: Collect blood from capillary beds and channel it into veins.
5. Veins:
• Function: Return blood towards the heart under low pressure.
• Wall Structure: Thin muscular/elastic wall, large wide lumen, and pocket valves (semilunar valves).
• Adaptation: Wide lumen offers low resistance to blood flow. Semilunar valves prevent the backflow of blood. Contraction of surrounding skeletal muscles squeezes veins, pushing blood back towards the heart.
Memory Aid for Vessels:
• Arteries go Away from the heart.
• VeINs go INto the heart and contain Valves.
Key Takeaway: Arteries have thick, elastic walls to withstand and smooth out high pressure; veins have wide lumens and valves for low-pressure return; capillaries are single-cell-thick for rapid exchange.
3. Structure of the Mammalian Heart
The heart is a muscular double pump made of specialized cardiac muscle. Cardiac muscle is myogenic (it initiates its own contractions without needing nerve signals from the brain).
Chambers and Vessels
The heart has four chambers:
• Right Atrium: Receives deoxygenated blood from the body via the vena cava.
• Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery.
• Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
• Left Ventricle: Pumps oxygenated blood to the entire body via the aorta.
Common Mistake to Avoid: When looking at a heart diagram, remember you are viewing it as if looking at a patient's chest. The left side of the diagram is the heart's right side, and the right side of the diagram is the heart's left side!
Why is the Left Ventricle Wall Thicker?
The wall of the left ventricle contains significantly thicker cardiac muscle than the right ventricle. This is because the left ventricle must generate enough high hydrostatic pressure to pump blood all the way around the systemic circulation (the whole body), whereas the right ventricle only needs to pump blood a short distance to the nearby lungs under low pressure.
Heart Valves
Valves ensure that blood flows in only one direction:
• Atrioventricular (AV) Valves: Located between atria and ventricles.
— Tricuspid valve: On the right side (has 3 flaps).
— Bicuspid (Mitral) valve: On the left side (has 2 flaps).
Memory Trick: LAB RAT \(\rightarrow\) Left Atrium = Bicuspid; Right Atrium = Tricuspid.
• Semilunar Valves: Located at the base of the major exit arteries:
— Pulmonary valve: Between the right ventricle and pulmonary artery.
— Aortic valve: Between the left ventricle and aorta.
• Tendinous Cords (Valve Tendons / "Heartstrings"): Attach the AV valves to papillary muscles on the ventricle walls. They prevent the valve flaps from inverting (turning inside out) into the atria during ventricular contraction.
• Coronary Arteries: Branch from the base of the aorta and supply the cardiac muscle itself with oxygenated blood and glucose for continuous aerobic respiration.
Key Takeaway: The heart has four chambers; the left ventricle has the thickest wall to generate systemic pressure; valves and tendinous cords prevent backflow.
4. The Cardiac Cycle
The cardiac cycle is the continuous sequence of contraction (systole) and relaxation (diastole) that constitutes one complete heartbeat.
Step-by-Step Stages of the Cardiac Cycle
Stage 1: Atrial Systole (Ventricular Diastole)
• The atria contract, reducing their volume and increasing atrial pressure.
• Blood is forced through the open AV valves into the relaxed ventricles.
• Semilunar valves remain closed.
Stage 2: Ventricular Systole (Atrial Diastole)
• After a slight delay, ventricles contract from the bottom (apex) upwards.
• Ventricular pressure rises sharply above atrial pressure, forcing the AV valves shut (producing the first heart sound, "lub").
• When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves are pushed open.
• Blood is pumped forcefully into the aorta and pulmonary artery.
Stage 3: Diastole (Complete Relaxation)
• Both atria and ventricles relax.
• Pressure in the ventricles falls below the pressure in the aorta and pulmonary artery.
• Blood starts to fall back towards the heart, filling the valve pockets and snapping the semilunar valves shut (producing the second heart sound, "dub").
• Blood from the vena cava and pulmonary veins flows passively into the relaxed atria and trickles down through relaxed AV valves into the ventricles.
Calculating Cardiac Output
Cardiac Output (\(\text{CO}\)) is the total volume of blood pumped by one ventricle in one minute (usually expressed in \(\text{cm}^3\,\text{min}^{-1}\) or \(\text{dm}^3\,\text{min}^{-1}\)).
Formula:
\(\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}\)
\(\text{CO} = \text{HR} \times \text{SV}\)
• Heart Rate (\(\text{HR}\)): Number of beats per minute (\(\text{bpm}\)).
• Stroke Volume (\(\text{SV}\)): Volume of blood pumped out by a ventricle during each beat (\(\text{cm}^3\)).
Example: If a resting student has a heart rate of \(70\,\text{bpm}\) and a stroke volume of \(75\,\text{cm}^3\):
\(\text{CO} = 70 \times 75 = 5250\,\text{cm}^3\,\text{min}^{-1}\) (or \(5.25\,\text{dm}^3\,\text{min}^{-1}\)).
Key Takeaway: Valve opening and closing is driven entirely by pressure differences between chambers. The "lub-dub" sounds correspond to AV valves closing, followed by semilunar valves closing.
5. Coordination of the Cardiac Cycle
Heart muscle is myogenic, meaning it generates its own electrical impulses without requiring stimulation from the nervous system. To ensure efficient pumping, contractions must be coordinated in an orderly sequence.
The Electrical Pathway Step-by-Step
1. Sinoatrial Node (SAN):
Located in the wall of the right atrium, the SAN acts as the heart's natural pacemaker. It initiates a wave of electrical excitation (depolarisation) at regular intervals.
2. Atrial Contraction:
The electrical wave spreads rapidly across both atria, causing the atrial walls to contract simultaneously from top to bottom, pushing blood into the ventricles.
3. Non-Conducting Fibrous Tissue:
A band of non-conducting collagenous tissue between the atria and ventricles prevents the electrical impulse from spreading directly from atria to ventricles. This prevents ventricles from contracting at the same time as atria!
4. Atrioventricular Node (AVN) Delay:
The electrical wave can only pass to the ventricles via the AVN. The AVN introduces a crucial delay of approximately \(0.1\text{–}0.15\,\text{seconds}\). This delay allows the atria to finish emptying their blood completely into the ventricles before ventricular contraction begins.
5. Bundle of His and Purkyne Fibres:
From the AVN, the impulse travels rapidly down the Bundle of His (located in the septum) to the bottom of the heart (the apex). The impulse then branches into Purkyne fibres spreading upwards through the muscular ventricular walls.
6. Ventricular Contraction from the Apex Upwards:
This pathway causes the ventricles to contract from the apex upwards, squeezing blood efficiently up and out through the semilunar valves into the major arteries.
Key Takeaway: SAN initiates impulse \(\rightarrow\) Atria contract \(\rightarrow\) AVN delays impulse \(\rightarrow\) Bundle of His conducts to apex \(\rightarrow\) Purkyne fibres conduct through walls \(\rightarrow\) Ventricles contract from bottom up.
6. Tissue Fluid and Lymph Formation
Cells do not come into direct contact with blood in vessels. Instead, they are bathed in tissue fluid (interstitial fluid), which provides an exchange medium for oxygen, glucose, amino acids, and metabolic wastes.
How Tissue Fluid Forms (Ultrafiltration)
Tissue fluid formation relies on the balance between two opposing forces:
1. Hydrostatic Pressure: Blood pressure generated by the pumping heart, which pushes fluid out of capillaries through tiny gaps between endothelial cells.
2. Oncotic Pressure (Colloid Osmotic Pressure): Plasma proteins (like albumin) remain inside capillaries because they are too large to pass through the capillary wall. These proteins lower the water potential of blood plasma, creating an osmotic pull drawing water into the capillaries.
Step-by-Step at the Capillary Bed
At the Arteriole End of the Capillary:
• Hydrostatic pressure inside the capillary is high (approx. \(4.3\,\text{kPa}\)).
• Oncotic pressure drawing water in is lower (approx. \(-2.7\,\text{kPa}\)).
• Net filtration pressure is positive (outward force): Fluid containing water, dissolved oxygen, ions, and glucose is forced out of the capillary into the surrounding intercellular spaces to form tissue fluid.
• Red blood cells, platelets, and large plasma proteins stay inside the capillary.
At the Venule End of the Capillary:
• Hydrostatic pressure falls dramatically due to fluid loss and resistance along the capillary wall (approx. \(1.6\,\text{kPa}\)).
• Oncotic pressure remains high (approx. \(-2.7\,\text{kPa}\)) because plasma proteins are still trapped in the blood.
• Net pressure is negative (inward force): Most water moves back into the capillary from the tissue fluid by osmosis, carrying dissolved waste products (like carbon dioxide and urea) with it.
The Lymphatic System
Not all fluid that leaves at the arteriole end returns at the venule end; roughly \(10\%\) remains in the tissue spaces. If this fluid accumulated, it would cause swelling (oedema).
• The excess tissue fluid drains into blind-ended lymph capillaries, where it is called lymph.
• Lymph vessels contain valves to prevent backflow and rely on surrounding muscle contractions to move lymph towards the neck.
• Lymph passes through lymph nodes (which contain white blood cells to filter bacteria and pathogens) and is eventually returned to the blood circulation via the subclavian veins.
Summary of Fluid Compositions:
• Blood Plasma: Contains red blood cells, white blood cells, platelets, large plasma proteins, glucose, and amino acids.
• Tissue Fluid: Water, glucose, amino acids, ions, dissolved gases, and white blood cells (no red blood cells or large proteins).
• Lymph: Similar to tissue fluid, but contains more lipids/fats (absorbed from the lacteals of the small intestine) and many lymphocytes.
Key Takeaway: Hydrostatic pressure forces fluid out at the arteriole end; oncotic pressure draws most fluid back in at the venule end; excess drains as lymph into the lymphatic system.
7. Oxygen Transport and Haemoglobin
Oxygen has low solubility in water, so blood relies on haemoglobin (\(\text{Hb}\)) inside red blood cells (erythrocytes) to transport oxygen efficiently.
Structure of Haemoglobin
Haemoglobin is a globular protein with a quaternary structure:
• Composed of four polypeptide chains (two \(\alpha\)-globins and two \(\beta\)-globins in adult human haemoglobin).
• Each polypeptide chain contains a non-protein prosthetic haem group containing an iron ion (\(\text{Fe}^{2+}\)).
• Each \(\text{Fe}^{2+}\) ion binds reversibly to one molecule of oxygen (\(\text{O}_2\)).
• Therefore, one complete haemoglobin molecule can bind up to four \(\text{O}_2\) molecules (\(8\) oxygen atoms):
\(\text{Hb} + 4\text{O}_2 \rightleftharpoons \text{Hb}(\text{O}_2)_4\) (oxyhaemoglobin)
The Oxygen Dissociation Curve
The oxygen dissociation curve is a graph showing the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (\(p\text{O}_2\), measured in \(\text{kPa}\)).
Why is the curve S-shaped (Sigmoid)?
• Cooperative Binding: When haemoglobin is unoxygenated, its four subunits are tightly packed, making it difficult for the first \(\text{O}_2\) molecule to bind (the curve starts off shallow).
• Once the first \(\text{O}_2\) molecule binds, it induces a conformational change (change in 3D shape) in the haemoglobin molecule.
• This shape change uncovers the remaining haem groups, making it significantly easier for the second and third \(\text{O}_2\) molecules to bind (the curve rises steeply).
• As saturation nears \(100\%\), binding the fourth \(\text{O}_2\) molecule becomes harder simply due to probability (fewer empty binding sites available), causing the curve to level off.
Significance of the Sigmoid Curve in the Body:
• In the lungs (high \(p\text{O}_2\), around \(13\text{–}14\,\text{kPa}\)), haemoglobin has a high affinity for oxygen and becomes almost fully saturated (\(\approx 98\%\)).
• In respiring tissues (low \(p\text{O}_2\), around \(2\text{–}4\,\text{kPa}\)), haemoglobin has a lower affinity and readily dissociates (releases) its oxygen to supply the working cells.
The Bohr Effect
When tissues respire actively, they produce large amounts of carbon dioxide (\(\text{CO}_2\)).
How it works:
1. \(\text{CO}_2\) dissolves in blood plasma and red blood cells, reacting with water to form carbonic acid (\(\text{H}_2\text{CO}_3\)), catalysed by the enzyme carbonic anhydrase.
2. Carbonic acid dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogencarbonate ions (\(\text{HCO}_3^-\)).
3. The accumulation of \(\text{H}^+\) lowers the pH (makes the environment more acidic).
4. \(\text{H}^+\) ions bind to oxyhaemoglobin, altering its tertiary structure and reducing its affinity for oxygen (forming haemoglobinic acid, \(\text{HHb}\)).
5. This causes haemoglobin to release oxygen more readily at the same \(p\text{O}_2\).
• Visual on the Graph: The entire dissociation curve shifts to the right.
Memory Aid: Bohr Shift = Shift to the Right (Right \(\rightarrow\) Release oxygen for Respiring tissues!).
Fetal Haemoglobin and Myoglobin
• Fetal Haemoglobin: Has a higher affinity for oxygen than adult maternal haemoglobin at all partial pressures. Its dissociation curve is shifted to the left of adult haemoglobin. This allows the fetus to absorb oxygen across the placenta from the mother's blood.
• Myoglobin: A single polypeptide pigment with one haem group found in muscle tissue. It has a very high affinity for oxygen and will only release its oxygen at extremely low \(p\text{O}_2\) levels (such as during severe exercise). Its curve is shifted far to the left, acting as an oxygen store.
Key Takeaway: Cooperative binding gives the oxygen dissociation curve its S-shape. High \(\text{CO}_2\) / low pH shifts the curve right (Bohr effect) to release \(\text{O}_2\) to working tissues, while fetal haemoglobin and myoglobin sit to the left to bind \(\text{O}_2\) tightly.
Quick Review Summary Checklist
Make sure you can confidently:
• Explain why large mammals need a double closed circulatory system.
• Relate the histological structure of arteries, veins, and capillaries to their functions.
• Describe the path of blood through the heart, including all 4 chambers, valves, and associated vessels.
• Trace the electrical excitation from SAN \(\rightarrow\) AVN \(\rightarrow\) Bundle of His \(\rightarrow\) Purkyne fibres.
• Calculate cardiac output using \(\text{CO} = \text{HR} \times \text{SV}\).
• Explain the formation and reabsorption of tissue fluid in terms of hydrostatic and oncotic pressures.
• Explain the S-shaped oxygen dissociation curve, the Bohr effect, and comparisons with fetal haemoglobin and myoglobin.