Biology (9610) | The circulation of blood and the structure of the mammalian heart

Welcome to the Core of Biological Transport!

Hello! This chapter dives into one of the most vital processes in any large organism: the transport system. Since large mammals have a very small surface area to volume ratio (SA:V), simple diffusion isn't fast enough to deliver oxygen and nutrients to all cells.

That’s where the blood circulatory system comes in. Think of it as the body’s highly efficient, pressurised delivery and waste removal network, centered around the incredible pump—the mammalian heart.

Don't worry if the names of vessels and chambers seem confusing at first. We will break down the structure of the vessels and the heart step-by-step, relating every part to its specific function.

1. The Mammalian Blood System: Arteries, Veins, and Capillaries (3.2.6.1)

The mammalian circulatory system is a closed, double circulatory system, meaning blood passes through the heart twice for every circuit of the body (once to the lungs, once to the body). This dual pathway ensures high pressure is maintained for efficient transport.

1.1. Comparing the Structure and Function of Blood Vessels

The three main types of blood vessels—arteries, capillaries, and veins—are perfectly adapted to handle different parts of the transport job.

a) Arteries (High-Pressure Conveyors)

Arteries carry blood Away from the heart (easy mnemonic!). They handle the highest pressure.

• Thick wall: To withstand the high pressure generated by the heart.
• Thick muscle layer: Helps control blood flow (vasoconstriction/vasodilation).
• Thick elastic layer: Allows the vessel to stretch when the pulse hits and recoil, smoothing out blood flow and maintaining pressure.
• Narrow lumen: Maintains high blood pressure.

Key Vessel Names (Required):

• Aorta: Leaves the left ventricle, supplying oxygenated blood to the body.
• Pulmonary Artery: Leaves the right ventricle, supplying deoxygenated blood to the lungs.
• Coronary Arteries: Specialised arteries that supply oxygen and nutrients directly to the cardiac muscle (the heart tissue itself). Blockage here causes heart attacks (myocardial infarction).

b) Capillaries (The Exchange Sites)

Capillaries are the site of all metabolic exchange, transferring substances between blood and tissue cells.

• Wall is one cell thick: This provides a very short diffusion distance for fast exchange of O₂, CO₂, glucose, etc.
• Narrow lumen: Forces red blood cells to travel in single file, maximising their surface area exposure to the capillary wall for efficient gas exchange.
• Highly branched network: Creates a massive total surface area for exchange.

Analogy: If arteries are motorways carrying blood quickly, capillaries are the small local roads where goods (oxygen/nutrients) are actually dropped off.

c) Veins (Low-Pressure Return)

Veins carry blood towards the heart. They operate under very low pressure.

• Wide lumen: Reduces resistance to blood flow.
• Thin wall: Less elastic and muscular tissue needed because pressure is low.
• Valves: Essential feature! They prevent the backflow of blood, especially when blood moves against gravity (e.g., from the legs).

Key Vessel Names (Required):

• Vena Cava (Superior and Inferior): Enters the right atrium, carrying deoxygenated blood from the body.
• Pulmonary Vein: Enters the left atrium, carrying oxygenated blood from the lungs.

2. Capillaries and the Formation of Tissue Fluid (3.2.6.1)

The efficient transfer of nutrients and waste relies on an intermediary fluid called tissue fluid (or interstitial fluid).

2.1. Formation of Tissue Fluid

Tissue fluid is the liquid environment surrounding the body cells, providing the medium for exchange. It forms at the capillary network due to two main opposing forces:

1. Hydrostatic Pressure (HP): This is the pressure exerted by the blood itself (like water pushing on the sides of a hose). At the arteriole end of the capillary, HP is high. This high pressure forces plasma fluid (containing water, oxygen, glucose, and ions) out of the capillary pores.
2. Oncotic/Water Potential Gradient: Large molecules, especially plasma proteins (like albumin), are too big to leave the capillary. They remain in the blood, creating a low water potential inside the capillary compared to the tissue fluid outside.

At the arteriole end, HP is stronger than the water potential gradient, so fluid is pushed out.

2.2. Return of Tissue Fluid

As blood flows towards the venule end of the capillary:

• Hydrostatic pressure falls (because some fluid has already leaked out).
• The water potential gradient becomes dominant. Since the water potential inside the capillary is now lower (more concentrated) than the tissue fluid, water re-enters the capillary by osmosis.

2.3. The Role of the Lymphatic System

Not all tissue fluid returns directly to the capillary. The excess fluid drains into the lymphatic system, forming lymph, which eventually returns to the blood circulatory system near the heart.

Did you know? Tissue fluid is essentially blood plasma minus the large proteins and blood cells.

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3. Gross Structure and Function of the Mammalian Heart (3.2.6.2)

The heart is a muscular organ enclosed by a tough membrane (pericardium) that pumps blood around the double circulatory system.

3.1. The Chambers and Double Circulation

The heart is divided into four chambers:

• Right Atrium (RA): Receives deoxygenated blood from the body via the Vena Cava.
• Right Ventricle (RV): Pumps deoxygenated blood to the lungs via the Pulmonary Artery. (This is the Pulmonary Circuit.)
• Left Atrium (LA): Receives oxygenated blood from the lungs via the Pulmonary Vein.
• Left Ventricle (LV): Pumps oxygenated blood around the entire body via the Aorta. (This is the Systemic Circuit.)

3.2. Relating Structure to Function

The thickness of the heart muscle (myocardium) reflects the force required to pump blood to different destinations:

Left Ventricle Wall > Right Ventricle Wall > Atrial Walls

• The atria only need thin walls because they only pump blood a short distance into the ventricles below them.
• The Right Ventricle pumps blood to the lungs, a relatively short distance, requiring moderate pressure.
• The Left Ventricle has the thickest, most muscular wall because it must generate massive pressure to pump blood all the way around the rest of the body (systemic circulation).

3.3. The Heart Valves

Valves ensure unidirectional flow of blood, preventing backflow due to pressure changes.

a) Atrioventricular (AV) Valves

Located between the atria and the ventricles. They are held in place by tendinous cords (heart strings).

• Tricuspid Valve: On the right side (Right Atrium to Right Ventricle).
• Bicuspid (Mitral) Valve: On the left side (Left Atrium to Left Ventricle).

b) Semilunar Valves (SL)

Located at the exit of the ventricles, where they meet the major arteries. They look like half-moons.

• Pulmonary Semilunar Valve: Between the Right Ventricle and the Pulmonary Artery.
• Aortic Semilunar Valve: Between the Left Ventricle and the Aorta.

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4. The Cardiac Cycle (3.2.6.2)

The cardiac cycle is the sequence of events that occurs during one complete heartbeat. This involves precise changes in pressure and volume that control the opening and closing of the valves.

4.1. Phases of the Cardiac Cycle

Step 1: Atrial Systole (Contraction)

The atria contract simultaneously, pushing the remaining blood into the ventricles.

• Pressure Changes: Atrial pressure increases slightly. Ventricular pressure is still low.
• Valve Movement: The AV valves are open. The Semilunar valves are closed.

Step 2: Ventricular Systole (Contraction)

This is the powerful phase where the ventricles contract, starting from the base upwards.

a) Isovolumetric Contraction:

• Ventricular pressure rapidly rises, exceeding atrial pressure.
• The high pressure forces the AV valves shut (this closure produces the first heart sound, "lub").
• For a brief moment, all four valves are closed, and blood volume in the ventricles is constant (isovolumetric).

b) Ejection:

• Ventricular pressure continues to rise rapidly, eventually exceeding the pressure in the aorta and pulmonary artery.
• The Semilunar valves are forced open, and blood is ejected into the arteries.

Step 3: Diastole (Relaxation)

The entire heart muscle relaxes, allowing blood to flow back in passively.

• Pressure Changes: Ventricular pressure drops sharply as blood leaves and the muscle relaxes.
• Once the ventricular pressure falls below the arterial pressure, the backflow of blood immediately snaps the Semilunar valves shut (this closure produces the second heart sound, "dub").
• As the heart continues to relax and fill, atrial pressure eventually exceeds ventricular pressure, and the AV valves passively open, allowing blood to flow freely from the atria into the ventricles (filling phase).

5. Cardiac Output (CO) (3.2.6.2)

Cardiac Output is the total volume of blood pumped by one ventricle (usually measured from the left ventricle) in one minute. It is a key measure of circulatory efficiency.

5.1. The Formula

Cardiac output is determined by two factors:

Cardiac Output = Heart Rate (HR) \(\times\) Stroke Volume (SV)

Or, mathematically:

$$ CO = HR \times SV $$

• Heart Rate (HR): The number of beats per minute (bpm).
• Stroke Volume (SV): The volume of blood pumped out by one ventricle in one contraction (usually measured in cm³ or dm³).

Example: If a person has a heart rate of 70 beats per minute and a stroke volume of 75 cm³ per beat, their Cardiac Output is: \(70 \times 75 = 5250 \text{ cm}^3 \text{ per minute}\), or 5.25 dm³/min.

Understanding CO is essential, particularly when studying physical exertion or heart disease, as it directly relates to the body's ability to supply oxygen and remove waste efficiently.

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Key Takeaway from Circulation and Heart Structure

The structure of every component in the circulatory system—from the thick, elastic walls of the arteries to the single-cell thick walls of the capillaries and the one-way valves of the heart—is an elegant adaptation ensuring that blood moves efficiently under controlled pressure, maintaining the vital environment needed for cell survival.