Transport in Mammals
The mammalian double circulatory system's structure and function in transporting vital substances.
Large, active mammals have a low surface area to volume ratio and a high metabolic rate, necessitating a specialised transport system to efficiently deliver oxygen and nutrients while removing metabolic waste. This is achieved through a closed, double circulatory system.
### The Double Circulatory System
This system involves two distinct circuits for blood flowing from the heart:
This dual system is highly efficient as it maintains high pressure for rapid delivery of blood to the body tissues and allows for a lower pressure in the delicate lung capillaries.
### The Heart and Cardiac Cycle
The mammalian heart is a four-chambered muscular pump. The two upper chambers are the atria (sing. atrium), and the two lower, more muscular chambers are the ventricles. A muscular septum separates the right and left sides, preventing the mixing of oxygenated and deoxygenated blood.
Valves ensure unidirectional blood flow:
* Atrioventricular (AV) valves are located between the atria and ventricles (tricuspid on the right, bicuspid/mitral on the left).
* Semilunar (SL) valves are found at the exit of the ventricles (the pulmonary valve and aortic valve).
The cardiac cycle describes the events of a single heartbeat (~0.8 seconds):
### Blood Vessels
The structure of each blood vessel is adapted to its function:
* Arteries and Arterioles: Carry high-pressure blood away from the heart. They have a thick wall with a layer of elastic fibres to stretch and recoil, maintaining pressure, and a thick layer of smooth muscle to control blood flow. They have a relatively narrow lumen.
* Capillaries: Are the site of exchange between blood and tissues. Their walls are only one-cell thick (endothelium) to provide a short diffusion path. Their narrow lumen slows blood flow, maximising time for exchange. They form extensive networks called capillary beds.
* Veins and Venules: Carry low-pressure blood towards the heart. They have thin walls with less muscle and elastic tissue. Their wide lumen reduces resistance to blood flow, and they contain valves to prevent the backflow of blood, which is aided by the contraction of surrounding skeletal muscles.
### Blood Composition and Gas Transport
Blood is composed of plasma (the liquid matrix), erythrocytes (red blood cells), leucocytes (white blood cells), and platelets.
Oxygen Transport:
Oxygen is primarily transported by haemoglobin (Hb), a globular protein in red blood cells. Each Hb molecule can bind to four oxygen molecules, forming oxyhaemoglobin. This binding is reversible: Hb + 4O₂ ⇌ Hb(O₂)₄.
The relationship between oxygen availability (partial pressure of O₂, pO₂) and haemoglobin saturation is shown by the oxygen dissociation curve. This curve is sigmoid (S-shaped) due to cooperative binding: once the first O₂ molecule binds, the Hb molecule changes shape, making it easier for subsequent O₂ molecules to bind.
In respiring tissues, the high partial pressure of carbon dioxide (pCO₂) lowers the pH of the blood. This causes the dissociation curve to shift to the right, a phenomenon known as the Bohr effect. This shift indicates that haemoglobin has a lower affinity for oxygen at a lower pH, thus promoting the release of oxygen where it is most needed.
Carbon Dioxide Transport:
CO₂ is transported in three ways:
Within red blood cells, the enzyme carbonic anhydrase rapidly catalyses the reaction: CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid). Carbonic acid then dissociates into hydrogen ions (H⁺) and hydrogencarbonate ions (HCO₃⁻). The HCO₃⁻ ions diffuse out of the red blood cell into the plasma. To maintain electrical neutrality, chloride ions (Cl⁻) move from the plasma into the red blood cell, an exchange known as the chloride shift. The H⁺ ions produced are buffered by binding to haemoglobin, which also triggers the Bohr effect, facilitating oxygen release.
Key Points to Remember
- 1Mammals possess a closed, double circulatory system (pulmonary and systemic) for efficient transport to meet high metabolic demands.
- 2The four-chambered heart acts as a dual pump, with valves ensuring unidirectional blood flow through the cardiac cycle (systole and diastole).
- 3Arteries, capillaries, and veins are structurally adapted for their specific functions: withstanding high pressure, efficient substance exchange, and low-pressure blood return.
- 4Blood is a composite tissue of plasma, erythrocytes (for O₂ transport), leucocytes (for immunity), and platelets (for clotting).
- 5Oxygen binds cooperatively to haemoglobin, resulting in a sigmoid-shaped oxygen dissociation curve.
- 6The Bohr effect describes how increased CO₂ (lower pH) reduces haemoglobin's affinity for oxygen, enhancing its delivery to respiring tissues.
- 7Most carbon dioxide is transported as hydrogencarbonate ions (HCO₃⁻) in blood plasma, a conversion catalysed by carbonic anhydrase in red blood cells.
- 8The chloride shift maintains electrochemical neutrality in red blood cells as hydrogencarbonate ions are exchanged for chloride ions.
Pakistan Example
Hypertension and Cardiovascular Disease in Pakistan
The principles of the mammalian circulatory system are crucial for understanding public health crises in Pakistan, such as the high prevalence of cardiovascular disease (CVD). A significant portion of the adult population suffers from hypertension (high blood pressure). This condition forces the heart's left ventricle to work harder to pump blood into the systemic circulation, leading to cardiac hypertrophy (enlargement) and eventual heart failure. Furthermore, lifestyle factors common in Pakistan, such as diets high in saturated fats and salt, contribute to atherosclerosis—the buildup of fatty plaques in arteries. This narrows the lumen, increases blood pressure, and can lead to coronary artery disease, heart attacks, and strokes, illustrating the critical balance required for a healthy transport system.
Quick Revision Infographic
Biology — Quick Revision
Transport in Mammals
Key Concepts
Hypertension and Cardiovascular Disease in Pakistan
The principles of the mammalian circulatory system are crucial for understanding public health crises in Pakistan, such as the high prevalence of cardiovascular disease (CVD). A significant portion of the adult population suffers from hypertension (high blood pressure). This condition forces the heart's left ventricle to work harder to pump blood into the systemic circulation, leading to cardiac hypertrophy (enlargement) and eventual heart failure. Furthermore, lifestyle factors common in Pakistan, such as diets high in saturated fats and salt, contribute to atherosclerosis—the buildup of fatty plaques in arteries. This narrows the lumen, increases blood pressure, and can lead to coronary artery disease, heart attacks, and strokes, illustrating the critical balance required for a healthy transport system.