Structure of the heart, blood and circulation

Organisation • Animal tissues, organs and systems

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Explain why capillaries are adapted for exchange.

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Extremely thin walls (one cell thick) and extensive networks give short diffusion distances and large surface area for exchange of gases and nutrients.

Key concepts

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Heart anatomy and blood flow

The heart consists of two atria (upper chambers) and two ventricles (lower chambers). Valves between chambers and at major arteries prevent backflow so contractions move blood forward through the circuit. The left and right sides of the heart remain separated so oxygenated and deoxygenated blood do not mix. Blood from the lungs enters the left atrium, flows into the left ventricle and is then pumped into the aorta for systemic distribution. Blood returning from the body enters the right atrium, flows into the right ventricle and is pumped to the lungs via the pulmonary artery for gas exchange. The pulmonary vein returns oxygenated blood to the left atrium.

Double circulatory system and chamber roles

The circulation is double: the pulmonary circuit carries blood between heart and lungs; the systemic circuit carries blood between heart and body. Blood passes through the heart twice each full circuit, which takes about one minute at rest. This arrangement maintains higher pressure to tissues while allowing lower-pressure flow through delicate lung capillaries. The left ventricle has a thicker muscular wall than the right ventricle because it generates higher pressure to deliver blood throughout the body. The right ventricle has thinner walls because it only needs to propel blood to the nearby lungs. Differences in muscle thickness therefore match distance and required pressure.

Control of heart rate: pacemaker and artificial pacemakers

A specialised group of cells in the wall of the right atrium generates the electrical impulses that set the natural resting heart rate; this group of cells acts as the cardiac pacemaker. Nerve tissue conducts these impulses through the heart, synchronising atrial and ventricular contraction. Faulty pacemaker cells change the timing of impulses and cause irregular heart rhythms. An artificial pacemaker is an implanted device that generates timed electrical impulses when natural pacemaker activity is insufficient or irregular. The device restores a controlled heart rate by delivering electrical stimulation to the heart muscle.

Major blood vessels and coronary supply

The aorta carries oxygenated blood at high pressure from the left ventricle to the body. The vena cava returns deoxygenated blood from the body to the right atrium. The pulmonary artery carries deoxygenated blood to the lungs; the pulmonary vein returns oxygenated blood to the left atrium. Coronary arteries supply oxygenated blood directly to the heart muscle cells, supporting their respiration. Blockage of coronary arteries restricts oxygen and glucose delivery to heart muscle cells, causing cell death and potentially a heart attack. Atherosclerotic plaques and high blood pressure increase the risk of such blockages.

Blood as a tissue: components and functions

Blood consists of plasma (about 55% by volume) and cellular components (about 45%): red blood cells (RBCs), white blood cells (WBCs) and platelets. Plasma is a straw-coloured fluid that transports dissolved molecules such as glucose, amino acids, hormones, carbon dioxide and urea. Plasma composition supports transport, heat distribution and medium for clotting factors. Red blood cells contain haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases oxygen in tissues. White blood cells defend against pathogens by engulfing them or producing antibodies. Platelets are cell fragments that release clotting factors to form fibrin mesh and stop bleeding.

Adaptations of blood cells

Red blood cells are biconcave and lack a nucleus; the shape increases surface area to volume ratio and the absence of a nucleus increases space for haemoglobin, both adaptations that maximise oxygen transport. White blood cells retain nuclei and contain organelles to synthesise enzymes and antibodies needed for immune responses. Platelets are small fragments that aggregate rapidly to start clot formation. Recognition of blood cell types in images depends on size, shape and nucleus: RBCs are small, circular and non‑nucleated; WBCs are larger and nucleated with variable internal structure; platelets appear as tiny fragments. These visual features link to each cell type’s function.

Structure and function of arteries, veins and capillaries

Arteries have thick muscular and elastic walls and a relatively narrow lumen to withstand and maintain high pressure from ventricular contraction; elastic tissue allows stretching and recoil that helps maintain continuous flow. Veins have wider lumens, thinner walls and one‑way valves to assist low-pressure return to the heart and to prevent backflow, especially from lower limbs. Capillaries are one cell layer thick with extremely thin walls to provide short diffusion distances for exchange of gases, nutrients and wastes. Capillary networks provide large surface area and slow blood flow to increase time for diffusion, allowing plasma to form tissue fluid that bathes cells and supports exchange of oxygen, glucose and wastes.

Gas exchange in lungs and use of glucose in respiration

Alveoli in the lungs present a large, moist surface with thin cell layers and a dense capillary supply so oxygen diffuses from air into blood and carbon dioxide diffuses out. Continuous blood flow maintains steep concentration gradients that drive diffusion. Ventilation renews alveolar air and supports ongoing exchange. Glucose absorbed from the small intestine enters the blood and travels to tissues where cells use it with oxygen during cellular respiration to release energy. Higher cellular respiration in active tissues lowers local oxygen and glucose concentrations, causing diffusion of these molecules from blood into cells.

Simple rate and flow calculations

Rate calculations express a quantity divided by a time interval (for example, volume per minute). Human total blood volume is approximately 5.0 litres and a full circuit through the heart takes about one minute at rest; these values give an average circulation rate near 5.0 L per minute for total blood volume over that time. Use consistent units when calculating flow rates (e.g., litres per minute). Example calculation: total blood volume (5.0 L) ÷ circulation time (1.0 min) = 5.0 L min−1. Cardiac output can also be estimated as stroke volume × heart rate when those measurements are available.

Key notes

Important points to keep in mind

Left ventricle walls are thicker than right ventricle walls because higher pressure is required to pump blood around the whole body.

Blood is approximately 55% plasma and 45% cells; plasma volume ≈ 3 L of total ≈ 5 L blood.

Arteries: thick muscular and elastic walls → withstand high pressure and produce pulse.

Veins: wide lumen and valves → low-pressure return and prevention of backflow.

Capillaries: one-cell-thick walls → short diffusion distance for gas and nutrient exchange.

Red blood cell biconcave shape and no nucleus → increased haemoglobin capacity and rapid oxygen diffusion.

Pacemaker cells in the right atrium set resting heart rate; artificial pacemakers replace or support this function when necessary.

Coronary arteries supply oxygen and glucose to heart muscle; blockage reduces supply and risks heart attack.

Alveoli adaptations: large surface area, moist thin lining, dense capillaries and ventilation → efficient gas exchange.

Use consistent units in rate calculations: convert ml to L or seconds to minutes before dividing.

Cardiac output can be estimated as stroke volume × heart rate when both quantities are known.