Transport in Humans

The human circulatory system is an intricate network responsible for transporting essential substances to and from the trillions of cells in the body. It consists of a pump (the heart), a series of tubes (blood vessels), and a transport medium (blood). Humans possess a double circulatory system, a highly efficient design where blood passes through the heart twice for each complete circuit of the body.

The Double Circulation

This system is divided into two distinct circuits:

1. Pulmonary Circuit: This circuit handles gas exchange. Deoxygenated blood is pumped from the right side of the heart to the lungs via the pulmonary artery. In the lungs, carbon dioxide is released, and oxygen is absorbed. Oxygenated blood then returns to the left side of the heart via the pulmonary vein.
2. Systemic Circuit: This circuit serves the rest of the body. Oxygenated blood is pumped from the left side of the heart to all other body tissues via the aorta. After delivering oxygen and nutrients and collecting waste products like carbon dioxide, the deoxygenated blood returns to the right side of the heart through the vena cava.

The primary advantage of a double circulation is that it allows blood to be pumped to the body at a much higher pressure than to the lungs. This ensures rapid, efficient delivery of oxygen to tissues, while the lower pressure in the pulmonary circuit protects the delicate capillary networks in the lungs.

The Heart: The Central Pump

The heart is a muscular organ with four chambers that work in a coordinated cycle.

• Right Atrium: Receives deoxygenated blood from the body via the superior and inferior **vena cava**.
• Right Ventricle: Receives blood from the right atrium and pumps it to the lungs via the **pulmonary artery**.
• Left Atrium: Receives oxygenated blood from the lungs via the **pulmonary vein**.
• Left Ventricle: Receives blood from the left atrium and pumps it to the entire body via the **aorta**. Its muscular wall is significantly thicker and stronger than the right ventricle's because it must generate much higher pressure to pump blood throughout the systemic circuit.

Valves are crucial for ensuring one-way blood flow. The atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow into the atria when the ventricles contract. The semilunar valves (pulmonary and aortic) prevent backflow into the ventricles after they have contracted. The muscular wall separating the left and right sides of the heart is the septum, which prevents the mixing of oxygenated and deoxygenated blood.

Blood Vessels: The Network of Tubes

There are three main types of blood vessels:

• Arteries: Carry high-pressure blood **away** from the heart. They have thick, muscular, and elastic walls to withstand and maintain this pressure. The elastic recoil helps to smooth out the pulsatile flow from the heart. They have a relatively narrow **lumen** (internal channel). The aorta is the largest artery.
• Veins: Carry low-pressure blood **towards** the heart. Their walls are thinner and less muscular than arteries, and they have a wider lumen. Most veins, especially in the limbs, contain **valves** to prevent the backflow of blood, which is assisted by the contraction of surrounding skeletal muscles.
• Capillaries: These are the smallest blood vessels, forming vast networks (**capillary beds**) that link arteries and veins. Their walls are only **one cell thick**, providing a very short diffusion path for efficient exchange of substances like oxygen, glucose, carbon dioxide, and urea between the blood and tissue cells. Their lumen is so narrow that red blood cells pass through in single file, maximising the time for exchange.

Exam Trap: The pulmonary artery is the only artery that carries deoxygenated blood. The pulmonary vein is the only vein that carries oxygenated blood. The defining feature is the direction of blood flow relative to the heart, not its oxygen content.

Blood: The Transport Medium

Blood is composed of several components suspended in a liquid matrix called plasma.

• Plasma: This straw-coloured liquid is mostly water and transports dissolved substances, including digested food (glucose, amino acids), hormones, carbon dioxide (as hydrogencarbonate ions), and waste products like urea.
• Red Blood Cells (Erythrocytes): These cells are specialised for oxygen transport. They contain the protein **haemoglobin**, which binds reversibly with oxygen to form **oxyhaemoglobin**. Their **biconcave disc shape** increases the surface area-to-volume ratio for faster diffusion, and the absence of a nucleus provides more space for haemoglobin.
• White Blood Cells (Leucocytes): Part of the immune system. There are two main types to know:
• Phagocytes: Engulf and digest pathogens through phagocytosis.
• Lymphocytes: Produce **antibodies** that target specific pathogens.
• Platelets (Thrombocytes): These are small cell fragments that initiate **blood clotting**. When a blood vessel is damaged, platelets adhere to the site and release chemicals that trigger a cascade of reactions, converting the soluble plasma protein **fibrinogen** into insoluble **fibrin** threads. These threads form a mesh that traps red blood cells, forming a clot to seal the wound and prevent blood loss.

Cardiovascular Health

Pulse rate is the rhythmic expansion of an artery that can be felt, corresponding to each heartbeat. During exercise, the heart rate increases to supply working muscles with more oxygen and glucose for respiration and to remove carbon dioxide more quickly, thus the pulse rate also increases.

Coronary Heart Disease (CHD): This is a major health issue globally, including in Pakistan. It is caused by the blockage of the **coronary arteries**—the vessels that supply the heart muscle itself with oxygenated blood. This blockage is typically due to **atherosclerosis**, the build-up of fatty plaques (containing cholesterol) on the inner walls of the arteries, narrowing the lumen. Risk factors include a diet high in saturated fats, smoking, and a sedentary lifestyle. A complete blockage can lead to a heart attack (myocardial infarction) as a section of the heart muscle is deprived of oxygen and dies.