Comprehensive Study Notes: Transport in Mammals

Introduction to Cardiovascular Systems and Artificial Hearts

• Cardiovascular Disease Statistics: Approximately $18 \times 10^6$ people die worldwide each year from cardiovascular disease, which relates to the heart and the circulatory system. Many of these deaths result from heart failure.
• Treatment Options: Medical interventions range from pharmaceutical drugs to major heart surgery and heart transplants. However, the demand for heart transplants far exceeds the supply of available donor hearts.
• Total Artificial Heart (TAH):     * Case Study: A patient referred to as Petar Bilic, whose ventricles had severely deteriorated, received a total artificial heart.     * Mechanism: The patient's failing heart is removed and replaced by a pumping device.     * Energy Supply: These devices require an external energy source, typically a battery carried by the patient in a backpack.     * Longevity: Some patients have survived nearly $5$ years with an artificial heart while waiting for a transplant.     * Advancements: Biomedical engineers are developing smaller versions for women and children and seeking designs that could last for a patient’s entire life.

Questions & Discussion: Artificial Hearts

• Question: What do you think might be the advantages and disadvantages of using an artificial heart rather than a heart transplant, to treat a person whose own heart is failing?
• Discussion Points:     * Advantages: Immediate availability (no waiting list), no risk of biological rejection from a donor, and provides a bridge to transplant for those nearing death.     * Disadvantages: Requirement for external power (batteries), noise of the pump, risk of mechanical failure, increased risk of blood clots or infection, and the physical burden of carrying equipment.

Transport Systems in Animals

• Energy and Respiration: Animals require energy for movement (muscle contraction) and nerve impulse transmission. This energy is released from glucose through respiration.
• Aerobic Respiration: The most efficient form of respiration, requiring a constant supply of oxygen and the removal of waste products like carbon dioxide ($CO_2$).
• Diffusion Limits:     * Very small or inactive animals (e.g., jellyfish) rely on simple diffusion because no cell is far from the surface.     * Large animals (mammals) have a much smaller surface area to volume ratio, making diffusion too slow to reach internal cells.
• Mammalian Requirements: Mammals have exceptionally high oxygen demands because they generate internal heat to maintain a constant body temperature (homeostasis).

The Mammalian Circulatory System

• Closed Blood System: The blood remains within interconnecting tubes (blood vessels) and does not come into direct contact with all body cells.
• Double Circulation: Blood passes through the heart twice during one complete circuit of the body.     * Pulmonary Circulation: The right side of the heart pumps deoxygenated blood to the lungs to pick up oxygen and return to the left side.     * Systemic Circulation: The left side of the heart pumps oxygenated blood to the rest of the body (except the lungs) and returns it to the right side.
• Major Vessels:     * Aorta: The main artery carrying oxygenated blood from the left ventricle.     * Vena Cava: The main vein returning deoxygenated blood to the right atrium.     * Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.     * Pulmonary Veins: Carry oxygenated blood from the lungs to the left atrium.

Structure and Function of Blood Vessels

• General Wall Structure (Arteries and Veins):     1. Inner Layer (Tunica Intima): Composed of a single layer of very smooth flat cells (squamous epithelium) called clinical endothelium, plus a layer of elastic fibers. This minimizes friction.     2. Middle Layer (Tunica Media): Contains smooth muscle, collagen, and elastic fibers.     3. Outer Layer (Tunica Adventitia): Primarily collagen fibers and some elastic fibers.

• Arteries and Arterioles:     * Function: Transport blood swiftly at high pressure away from the heart.     * Pressure: Aorta pressure is approximately $120\,mmHg$ ($16\,kPa$).     * Elastic Arteries: Located near the heart (e.g., aorta). They have many elastic fibers to stretch during ventricular systole and recoil during diastole, evening out blood flow.     * Muscular Arteries: Located further from the heart; they have more smooth muscle to control blood volume to specific organs.     * Arterioles: Smallest arteries that provide resistance to flow, slowing blood before it enters capillaries.     * Vasoconstriction/Vasodilation: Smooth muscle in arteriole walls responds to nerves or hormones to narrow ($vasoconstriction$) or widen ($vasodilation$) the lumen.

• Capillaries:     * Structure: Approximately $7\,\mu m$ in diameter (same as an RBC). Walls are one endothelial cell thick with tiny gaps between cells.     * Function: Allow rapid exchange of substances between blood and cells.     * Distribution: Found in nearly every tissue except the cornea, brain, and cartilage.     * Pressure: Drops from $35\,mmHg$ ($4.7\,kPa$) at the arterial end to $10\,mmHg$ ($1.3\,kPa$) at the venous end.

• Veins and Venules:     * Function: Return blood to the heart at low pressure ($< 5\,mmHg$).     * Structure: Thinner walls than arteries with less muscle and elastic tissue.     * Semilunar Valves: Half-moon shaped folds of endothelium that prevent backflow.     * Skeletal Muscle Pump: Contraction of muscles near veins squeezes blood toward the heart.

Tissue Fluid and Oedema

• Formation: Plasma leaks through capillary gaps into the spaces between cells. Tissue fluid is plasma minus large proteins and red blood cells.
• Forces Influencing Formation:     1. Hydrostatic Pressure: High pressure at the arterial end pushes fluid out.     2. Water Potential Gradient: Higher protein concentration in the blood ($solutes$) pulls water back in via osmosis.
• Net Movement: At the arterial end, hydrostatic pressure exceeds osmotic pressure (fluid leaves). At the venous end, osmotic pressure exceeds hydrostatic pressure (fluid returns).
• Oedema: Build-up of fluid in tissues caused by high blood pressure or low plasma protein levels.
• Homeostasis: Tissue fluid provides the immediate environment for cells, which is regulated for glucose, water, pH, and temperature.

Components of Blood

• Physicality: Adult humans have approx. $5\,dm^3$ of blood ($5\,kg$).
• Plasma: $95\%$ water. Contains nutrients (glucose), wastes (urea), hormones, and plasma proteins (e.g., albumin). It also transports heat.
• Red Blood Cells (Erythrocytes):     * Count: $2.5 \times 10^{13}$ in the body.     * Features: Biconcave disc (increases surface area to volume ratio), no nucleus/mitochondria/ER (more room for haemoglobin), flexible (specialized cytoskeleton).     * Dimensions: $7\,\mu m$ diameter.
• White Blood Cells (Leukocytes):     * Phagocytes: Neutrophils (lobed nucleus, granular cytoplasm) and Monocytes/Macrophages (bean-shaped nucleus).     * Lymphocytes: Large round nucleus, small cytoplasm; produce antibodies.
• Platelets: Small fragments of cells with no nuclei, involved in clotting.

Oxygen Transport and Haemoglobin

• Molecular Structure: Each haemoglobin molecule ($Hb$) has four polypeptide chains, each with a haem group containing iron ($Fe^{2+}$).
• Reaction: $Hb + 4O_2 \rightarrow HbO_8$ ($oxyhaemoglobin$).
• Dissociation Curve: An S-shaped (sigmoid) curve showing percentage saturation of Hb against partial pressure of oxygen ($pO_2$).     * Cooperativity: The first $O_2$ molecule changes the Hb shape, making it easier for the second and third to bind.     * Lungs: At high $pO_2$ ($12\,kPa$), Hb is $95-97\%$ saturated.     * Tissues: At low $pO_2$ ($2\,kPa$), Hb is only $20-25\%$ saturated, releasing oxygen to cells.
• Bohr Shift: High partial pressure of $CO_2$ ($pCO_2$) causes the dissociation curve to shift down and to the right, meaning Hb releases oxygen more easily in active tissues.

Carbon Dioxide Transport

• Transport Methods:     1. Hydrogencarbonate Ions ($85\%$): $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$ (catalyzed by carbonic anhydrase in RBCs).     2. Carbaminohaemoglobin ($10\%$): $CO_2$ binds directly to amine groups on Hb.     3. Dissolved in Plasma ($5\%$).
• Chloride Shift: Negatively charged chloride ions ($Cl^-$) enter RBCs from plasma to balance the exit of hydrogencarbonate ions ($HCO_3^-$), maintaining electrical neutrality.
• Buffering: Hb binds with $H^+$ ions to form haemoglobinic acid ($HHb$), preventing the blood from becoming too acidic.

The Structure and Control of the Heart

• Anatomy:     * Cardiac Muscle: Myogenic (contracts without nerve impulses), interconnecting cells for rapid electrical transmission.     * Chambers: Right/Left Atria and Right/Left Ventricles separated by the muscular septum.     * Atrioventricular Valves: Tricuspid (right) and Bicuspid/Mitral (left).     * Coronary Arteries: Supply the heart muscle itself with oxygenated blood.
• The Cardiac Cycle:     1. Atrial Systole ($0.1\,s$): Atria contract, pushing blood into ventricles.     2. Ventricular Systole ($0.3\,s$): Ventricles contract; AV valves shut; semilunar valves open; blood enters aorta/pulmonary artery.     3. Diastole: Entire heart relaxes; semilunar valves shut; blood flows from veins into atria.
• Electrical Control:     * Sinoatrial Node (SAN): The pacemaker; initiates waves of excitation.     * Atrioventricular Node (AVN): Delays the impulse for $0.1\,s$ to allow atria to empty.     * Purkyne Tissue: Conducting fibers in the septum that carry the impulse to the apex of the heart, causing contraction from the bottom up.

Questions and Discussion: Coronary Health and Circulation

• Question: Suggest why there are no blood capillaries in the cornea of the eye. How might the cornea be supplied with oxygen and nutrients?
• Answer: Capillaries would block light and obscure vision. The cornea receives oxygen directly from the air and nutrients from the aqueous humor via diffusion.
• Question: Why does blood pressure rise in the feet when standing motionless?
• Answer: Gravity causes blood to pool in the lower extremities, and without the skeletal muscle pump, venous return is poor, increasing hydrostatic pressure.
• Question: Describe and explain how blood pressure varies in the circulatory system.
• Answer: Pressure is highest in the aorta, oscillates in large arteries due to heartbeats, drops significantly in the arterioles and capillaries due to resistance, and reaches its lowest in the veins.
• Question: Explain the impact of weakened heart valves.
• Answer: This leads to backflow (regurgitation), reducing the efficiency of the heart's pumping action, causing tiredness and potential heart failure.