Welcome to Transport in Mammals!
In this chapter, we are going to explore how the body moves essential "supplies" (like oxygen and glucose) to every single cell and how it picks up "trash" (like carbon dioxide) to be taken away. Think of the mammalian transport system as a high-speed, 24/7 delivery and waste-collection service. Because mammals are big and very active, we can't just rely on simple diffusion; we need a powerful heart and a complex network of "pipes" to keep everything running.
8.1 The Circulatory System
The mammalian circulatory system is described as a closed double circulation.
• Closed: The blood stays inside vessels (it doesn't just splash around the body).
• Double: The blood passes through the heart twice for every one complete circuit of the body. There are two "loops":
1. Pulmonary circulation: Heart → Lungs → Heart (to pick up oxygen).
2. Systemic circulation: Heart → Body → Heart (to deliver oxygen to cells).
The "Pipes": Blood Vessels
Each type of vessel is designed for a specific job. Don't worry if the names seem similar; focus on their roles!
Arteries: These carry blood Away from the heart (remember A for Away). They have thick walls to handle high pressure.
• Elastic Arteries (like the Aorta): Have lots of elastic fibers to "stretch and recoil," which helps push blood along and smooth out the pressure from the heart's beats.
• Muscular Arteries: Have more smooth muscle to help control blood flow to different parts of the body.
Arterioles: Smaller versions of arteries that lead into capillaries.
Capillaries: The "business end" of the system. They are one cell thick (made of squamous endothelium) to allow substances to diffuse in and out easily.
Venules and Veins: These carry blood back to the heart. Because the pressure is low, they have valves to prevent blood from flowing backward. They also have a wider lumen (the space inside) to reduce friction.
Who's in the Blood?
You need to be able to recognize these cells under a microscope:
• Red Blood Cells (Erythrocytes): Shaped like biconcave discs with no nucleus (more room for oxygen!).
• Neutrophils: White blood cells with a lobed nucleus (looks like a string of sausages). They eat bacteria!
• Lymphocytes: Smaller white blood cells with a very large, round nucleus that takes up most of the cell.
• Monocytes: The largest white blood cells, often with a kidney-bean shaped nucleus.
Tissue Fluid: The Bridge to the Cells
Blood doesn't touch the cells directly. Instead, a liquid called tissue fluid acts as a middleman.
How it forms:
At the start of a capillary (the arterial end), the blood pressure is high. This "pushes" water and small molecules out through the tiny gaps in the capillary wall. Large proteins and blood cells stay inside because they are too big.
Quick Review: Tissue fluid is basically plasma minus the big proteins. Its job is to bathe the cells in nutrients and take away waste.
Key Takeaway:
The double circulation ensures that oxygenated blood is pumped at high pressure to the body, making the system very efficient for active mammals.
8.2 Transport of Oxygen and Carbon Dioxide
Oxygen is carried by haemoglobin (Hb), a amazing protein found inside red blood cells. One Hb molecule can carry four oxygen molecules.
The Oxygen Dissociation Curve
This is a graph that shows how much oxygen the haemoglobin holds at different "pressures" of oxygen (\(pO_2\)). It is S-shaped (sigmoidal).
Why the S-shape?
When the first oxygen molecule binds to Hb, it changes the shape of the protein, making it easier for the next three to join. This is called cooperative binding.
• In the lungs: \(pO_2\) is high, so Hb is "greedy" and loads up with oxygen.
• In respiring tissues: \(pO_2\) is low, so Hb releases the oxygen to the cells that need it.
The Bohr Shift
When you exercise, your cells produce more \(CO_2\). This makes the environment more acidic. Haemoglobin reacts to this by giving up its oxygen more easily. On a graph, the curve shifts to the right.
Memory Aid: "Exercise is Right!" - The Bohr curve shifts to the right during exercise to help muscles get more oxygen.
Transporting Carbon Dioxide (\(CO_2\))
\(CO_2\) is transported in three ways:
1. Dissolved in plasma (about 5%).
2. Bound to haemoglobin as carbaminohaemoglobin (about 10-20%).
3. As hydrocarbonate ions (\(HCO_3^-\)) in the plasma (about 75-85%).
The Chemistry of \(CO_2\) (Step-by-Step):
1. \(CO_2\) enters the red blood cell.
2. It reacts with water to form carbonic acid: \(CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3\).
3. The acid splits into \(H^+\) and \(HCO_3^-\).
4. The \(HCO_3^-\) leaves the cell. To keep the electrical charge balanced, chloride ions (\(Cl^-\)) enter the cell. This is called the Chloride Shift.
5. The \(H^+\) ions bind to haemoglobin to form haemoglobinic acid. This prevents the cell from becoming too acidic!
Key Takeaway:
Haemoglobin is not just an oxygen carrier; it also acts as a buffer to maintain blood pH while helping transport \(CO_2\).
8.3 The Heart
The heart is a double pump. The right side pumps deoxygenated blood to the lungs, and the left side pumps oxygenated blood to the body.
Heart Structure and Wall Thickness
• Atria: Have thin walls because they only push blood a short distance into the ventricles.
• Ventricles: Have much thicker walls than atria.
• Left Ventricle vs. Right Ventricle: The left ventricle has the thickest wall of all! It needs to generate enough pressure to pump blood all the way to your toes, whereas the right side only pumps to the nearby lungs.
The Cardiac Cycle
This is the sequence of one heartbeat. It has three main stages:
1. Atrial Systole: The atria contract, pushing blood into the ventricles. (Valves between them are open).
2. Ventricular Systole: The ventricles contract. The pressure closes the "middle" valves (AV valves) with a "lub" sound and forces open the "exit" valves (semilunar valves) to push blood into the arteries.
3. Diastole: Everything relaxes. The heart fills with blood again. The "exit" valves close with a "dub" sound to prevent backflow.
The Electrical "Remote Control"
The heart beat is myogenic, meaning it starts from within the muscle itself. It follows a specific path:
1. Sinoatrial Node (SAN): The pacemaker. It sends out an electrical wave to start the atria contracting.
2. Atrioventricular Node (AVN): This node catches the signal but delays it for a fraction of a second.
Why the delay? To make sure the atria have finished emptying before the ventricles start squeezing!
3. Purkyne Tissue: The signal travels down the septum and spreads into the ventricle walls, causing them to contract from the bottom up (like squeezing a tube of toothpaste from the bottom).
Key Takeaway:
The heart's structure and electrical timing ensure that blood flows in one direction and that the pumping is perfectly coordinated.
Common Mistakes to Avoid:
• Confusing the sides: Remember, diagrams are usually drawn as if you are looking at someone else's heart. The Left side of the heart is on the Right side of your paper!
• Artery vs. Vein: Not all arteries carry oxygenated blood (the Pulmonary Artery carries deoxygenated blood). Always define them by the direction of flow (Away vs. Towards the heart).
• The Delay: Forgetting to mention why the AVN delays the impulse. It’s a favorite exam question!
Don't worry if the names of the nodes or the chemistry of the chloride shift feel complicated at first. Draw the heart, trace the path of a red blood cell, and the logic will start to click!