Transport in Humans

As organisms grow larger, diffusion alone cannot deliver substances fast enough — the distance from the surface to the innermost cells becomes too great. Multicellular organisms therefore develop specialised transport systems to move materials quickly over long distances.

Why Transport Systems Are Needed¶

A small organism has a high surface area to volume ratio, so diffusion supplies every cell adequately. In a large organism the ratio is much lower, and cells deep inside would starve of oxygen or accumulate waste. Transport systems solve this by:

• delivering oxygen, glucose, and hormones to every tissue
• removing carbon dioxide, urea, and other metabolic wastes
• distributing heat evenly around the body

The Human Circulatory System¶

Humans have a closed, double circulatory system:

• closed — blood stays within blood vessels at all times
• double — blood passes through the heart twice per complete circuit (once for the lungs, once for the body)

Pulmonary circulation: right ventricle → lungs → left atrium. Blood picks up oxygen and loses CO₂.

Systemic circulation: left ventricle → body → right atrium. Blood delivers oxygen and nutrients; returns with CO₂ and waste.

The Heart¶

The heart is a muscular pump with four chambers: two atria (receive blood) and two ventricles (pump blood out). Valves prevent backflow.

• Right side receives deoxygenated blood from the body and pumps it to the lungs
• Left side receives oxygenated blood from the lungs and pumps it to the body
• The left ventricle has thicker walls than the right because it pumps blood the greater distance around the entire body

Blood Vessels¶

Capillaries are so thin that red blood cells must squeeze through in single file. Their walls allow plasma to leak out, forming tissue fluid that bathes cells.

Blood¶

Blood is a liquid connective tissue. It has four main components:

Plasma¶

Plasma is the pale yellow liquid portion of blood (about 55%). It transports:

• dissolved glucose, amino acids, fatty acids
• hormones
• CO₂ (partly as hydrogencarbonate ions)
• urea
• heat

Red Blood Cells (Erythrocytes)¶

Red blood cells carry oxygen using haemoglobin, an iron-containing protein that combines reversibly with oxygen:

haemoglobin + oxygen ⇌ oxyhaemoglobin

In the lungs (high O₂): oxyhaemoglobin forms. In the tissues (low O₂): oxygen is released.

Adaptations of red blood cells:

• biconcave disc shape — large surface area relative to volume; short diffusion distance to centre
• no nucleus — maximum space for haemoglobin
• flexible membrane — squeezes through narrow capillaries
• produced in vast numbers — high oxygen-carrying capacity

White Blood Cells (Leucocytes)¶

White blood cells defend the body. Two main types:

Phagocytes — engulf and destroy pathogens by phagocytosis (the phagocyte surrounds the pathogen, encloses it in a vacuole, and enzymes from lysosomes digest it).

Lymphocytes — produce antibodies. B-lymphocytes differentiate into:

• plasma cells — secrete antibodies specific to the antigen
• memory cells — remain in the blood long-term; allow a much faster response to the same pathogen on re-exposure

T-lymphocytes coordinate the immune response and attack infected or abnormal cells.

Platelets¶

Platelets are cell fragments that trigger blood clotting when a vessel is damaged:

damaged tissue → platelets release thromboplastin → prothrombin → thrombin → fibrinogen → fibrin mesh → clot forms

Clotting prevents excessive blood loss and blocks entry of pathogens.

Immunity and Vaccination¶

Antigens are foreign molecules (usually on pathogen surfaces) that trigger an immune response.

Antibodies are proteins produced by B-lymphocytes that are specific to one antigen. They may neutralise toxins, cause pathogens to clump (agglutination), or mark them for destruction.

Vaccination works by introducing weakened or dead pathogens, or their antigens, so the immune system produces antibodies and memory cells without the person suffering the disease. If the pathogen is later encountered, the memory cells allow a rapid secondary immune response.

FLASHCARD SET • 9 CARDSTransport in Humans