OCR GCSE Biology B (J257) — Human Physiology
You can easily deliver a letter to your next-door neighbour by walking, but sending thousands of parcels across the country requires a high-speed transport network. Large multicellular organisms face a similar problem: their low surface area to volume ratio means simple diffusion is too slow to reach internal cells.
To overcome this, humans have a mass transport system driven by a double circulatory system. In this system, blood passes through the heart twice for every one complete circuit of the body, split into two distinct loops. The pulmonary circuit carries deoxygenated blood to the lungs for gas exchange and returns oxygenated blood to the heart. The systemic circuit then pumps this oxygenated blood around the rest of the body, delivering it to metabolising organs and tissues.
This double system is vital for maintaining high blood pressure. As blood passes through the fine capillaries of the lungs, it loses pressure. Returning it to the heart allows it to be re-pressurised before entering the systemic circuit, ensuring the fast delivery of oxygen required for our high metabolic rates.
To describe the flow of blood through the circulatory system, you must follow a step-by-step pathway. First, deoxygenated blood from the body enters the right atrium via the vena cava. Next, it is pumped into the right ventricle and travels through the pulmonary artery to the lungs.
Once oxygenated, the blood returns to the left atrium via the pulmonary vein. Finally, it enters the left ventricle and is pumped out through the aorta to the body organs. Throughout this journey, valves in the heart and veins ensure a one-way flow, preventing any backflow of blood.
The heart walls are adapted for these different circuits. The left ventricle has a significantly thicker muscular wall than the right ventricle. This is because it must generate enough pressure to pump blood around the entire body, whereas the right ventricle only pumps blood to the nearby lungs.
The volume of blood the heart pumps per minute is an important measure of cardiovascular efficiency.
Worked Example: Calculate the cardiac output for a person with a heart rate of 70 bpm and a stroke volume of .
Step 1: Identify the known variables. ,
Step 2: Substitute into the equation.
Step 3: Calculate the final answer with units. (or )
Blood is the transport medium for metabolites (substances involved in metabolism). Human blood is a mixture: .
Red blood cells (erythrocytes) are specialised to carry oxygen (). They contain a protein called haemoglobin, which binds to oxygen in the lungs to form oxyhaemoglobin.
The liquid component, plasma (about 55% of blood volume), transports almost everything else. Carbon dioxide () dissolves into the plasma, as do nutrients like glucose and amino acids. Plasma also transports metabolic waste, such as urea, from the liver to the kidneys.
The circulatory system forms a highly integrated network with exchange surfaces. It continuously replaces "exchanged" blood with "fresh" blood, which maintains a steep diffusion gradient.
At the lungs, dense capillary networks surround the alveoli. Deoxygenated blood in the pulmonary capillaries exchanges for . The alveoli and capillary walls are only one cell thick, providing a very short diffusion distance.
The efficiency of this exchange can be understood using Fick's Law:
The circulatory system's constant blood flow is what maximises the "Concentration Difference" in this relationship.
In the small intestine, nutrients are absorbed into blood capillaries within finger-like projections called villi. This occurs via diffusion and active transport, which requires ATP from mitochondria.
However, large dietary fats (lipids) cannot fit into blood capillary pores. Instead, they are packaged into chylomicrons and absorbed into lacteals—specialised lymphatic capillaries inside each villus. These lipids travel through the lymphatic system as a milky fluid called chyle, eventually joining the main circulatory system near the heart via the thoracic duct.
Water-soluble nutrients take a different route. They travel from the small intestine directly to the liver via the hepatic portal vein. The liver processes this nutrient-rich blood, storing excess glucose as glycogen and breaking down excess amino acids through deamination (producing urea).
Because the liver is highly active, it also needs its own oxygenated blood supply directly from the aorta via the hepatic artery. Processed, deoxygenated blood leaves the liver through the hepatic vein to return to the heart.
The circulatory system is responsible for delivering metabolic waste to the kidneys for removal. Oxygenated, "unfiltered" blood carrying high concentrations of urea arrives at the kidneys via the renal artery.
Inside the kidney, the high pressure of the circulatory system forces small molecules (water, urea, glucose) out of the blood and into the glomerulus in a process called ultrafiltration. As this fluid moves through the kidney tubules (nephrons), selective reabsorption ensures that all glucose and necessary water are pulled back into the blood.
Finally, the "filtered", deoxygenated blood—now containing significantly less urea and balanced ion levels—exits the kidneys through the renal vein to return to the heart. The rate of this continuous filtering can be measured:
Students often state that thin capillary walls provide a 'short diffusion gradient' — you must use the exact phrase 'short diffusion distance' or 'short diffusion path'.
Do not confuse the ureter (the tube carrying urine from the kidney to the bladder) with the urethra (the tube where urine exits the body).
In 6-mark questions describing the heart, explicitly link the thicker muscular wall of the left ventricle to the need to generate 'higher pressure' to pump blood to the 'entire body'.
OCR examiners expect you to clearly distinguish between 'blood flow' (mass transport of fluids over distance) and 'diffusion' (the specific exchange of particles at capillaries).
Never use the phrase 'blue blood' in your exam; always use the correct scientific terms: 'deoxygenated' and 'oxygenated' blood.
Mass transport system
A system that moves substances over large distances to overcome the limitations of simple diffusion in multicellular organisms.
Double circulatory system
A system where blood flows through the heart twice in two separate loops (pulmonary and systemic) for each full circuit of the body.
Pulmonary circuit
The portion of the circulatory system that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.
Systemic circuit
The portion of the circulatory system that carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.
Metabolites
Substances involved in or produced by metabolism, such as glucose, oxygen, and urea.
Plasma
The liquid component of blood that carries dissolved substances including carbon dioxide, urea, glucose, and amino acids.
Diffusion gradient
The difference in concentration between two areas; a steeper gradient increases the rate of diffusion.
Villi
Finger-like projections in the small intestine that increase surface area for the absorption of nutrients.
Lacteals
Specialised lymphatic capillaries located in the centre of intestinal villi that absorb dietary fats.
Chyle
A milky fluid containing lymph and emulsified fats (lipids) that travels through the lymphatic system.
Hepatic portal vein
The blood vessel that transports deoxygenated but nutrient-rich blood directly from the small intestine to the liver.
Hepatic artery
The blood vessel that delivers oxygenated blood directly from the aorta to provide oxygen for the liver's high metabolic activity.
Hepatic vein
The blood vessel that returns processed, deoxygenated blood from the liver to the heart.
Deamination
The process in the liver where excess amino acids are broken down, producing urea as a metabolic waste product.
Renal artery
The blood vessel that carries oxygenated, waste-rich blood from the aorta to the kidneys.
Renal vein
The blood vessel that carries deoxygenated, filtered blood away from the kidneys back to the heart.
Ultrafiltration
The pressure-driven filtration of small molecules like urea, water, and glucose out of the blood into the kidney tubule at the glomerulus.
Selective reabsorption
The recovery of useful molecules, such as all glucose and required water, from the kidney nephron back into the blood.
Put your knowledge into practice — try past paper questions for Biology B
Mass transport system
A system that moves substances over large distances to overcome the limitations of simple diffusion in multicellular organisms.
Double circulatory system
A system where blood flows through the heart twice in two separate loops (pulmonary and systemic) for each full circuit of the body.
Pulmonary circuit
The portion of the circulatory system that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.
Systemic circuit
The portion of the circulatory system that carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.
Metabolites
Substances involved in or produced by metabolism, such as glucose, oxygen, and urea.
Plasma
The liquid component of blood that carries dissolved substances including carbon dioxide, urea, glucose, and amino acids.
Diffusion gradient
The difference in concentration between two areas; a steeper gradient increases the rate of diffusion.
Villi
Finger-like projections in the small intestine that increase surface area for the absorption of nutrients.
Lacteals
Specialised lymphatic capillaries located in the centre of intestinal villi that absorb dietary fats.
Chyle
A milky fluid containing lymph and emulsified fats (lipids) that travels through the lymphatic system.
Hepatic portal vein
The blood vessel that transports deoxygenated but nutrient-rich blood directly from the small intestine to the liver.
Hepatic artery
The blood vessel that delivers oxygenated blood directly from the aorta to provide oxygen for the liver's high metabolic activity.
Hepatic vein
The blood vessel that returns processed, deoxygenated blood from the liver to the heart.
Deamination
The process in the liver where excess amino acids are broken down, producing urea as a metabolic waste product.
Renal artery
The blood vessel that carries oxygenated, waste-rich blood from the aorta to the kidneys.
Renal vein
The blood vessel that carries deoxygenated, filtered blood away from the kidneys back to the heart.
Ultrafiltration
The pressure-driven filtration of small molecules like urea, water, and glucose out of the blood into the kidney tubule at the glomerulus.
Selective reabsorption
The recovery of useful molecules, such as all glucose and required water, from the kidney nephron back into the blood.
