Difficulty: Introductory | Prerequisites: Basic understanding of diffusion and cell membrane transport.
This set of notes covers how the heart pumps blood, how vessels differ in structure and function, and how gases move between the lungs and the bloodstream. You will also meet the Bohr shift (how acidity affects oxygen binding) and fetal haemoglobin. If you are comfortable with the idea that molecules move down concentration gradients, you are ready.
Blood moves through a closed circuit of arteries, capillaries, and veins, each built differently for its job. Gas exchange depends on partial pressure differences between the alveoli and capillaries. The Bohr shift explains why active tissues (which are more acidic) get more oxygen. Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin, which lets the foetus pull oxygen from maternal blood.
Systolic pressure
The first (higher) number in a blood pressure reading. It measures the pressure in arteries when the heart contracts (systole). Think of it as the peak pressure.
Diastolic pressure
The second (lower) number in a blood pressure reading. It measures the pressure in arteries when the heart relaxes (diastole). Think of it as the resting pressure between beats.
Arteries
Vessels that carry blood away from the heart. They have thick, muscular walls to withstand high pressure.
Capillaries
The smallest blood vessels. Their walls are just one cell thick, which allows exchange of gases, nutrients, and waste between blood and tissues. The total surface area of all capillaries is enormous, and blood velocity slows here to give time for exchange.
Veins
Vessels that carry blood back to the heart. They have thinner walls than arteries and contain one-way valves to prevent backflow.
Partial pressure
The pressure exerted by a single gas in a mixture of gases. Gases diffuse from regions of higher partial pressure to regions of lower partial pressure. In the lungs, oxygen's partial pressure in the alveoli is higher than in the capillaries, so oxygen moves into the blood.
Alveoli
Tiny air sacs in the lungs where gas exchange takes place. Their thin walls and large combined surface area maximise diffusion.
Bohr shift (Bohr effect)
The phenomenon in which a drop in blood pH (increased acidity, from CO2 and lactic acid) reduces haemoglobin's affinity for oxygen, causing it to release more oxygen to active tissues. In simple terms, harder-working tissues are more acidic, and that acidity makes haemoglobin let go of oxygen right where it is needed most.
Fetal haemoglobin
A variant of haemoglobin found in foetal blood that has a higher affinity for oxygen than adult haemoglobin. This lets the foetus pull oxygen from the mother's blood across the placenta.
A blood pressure reading has two numbers: systolic / diastolic (e.g. 120/80 mmHg).
Systolic = heart contracting (pumping). Diastolic = heart relaxing (filling).
The first number is always higher than the second.
Each vessel type is built for its specific job:
Arteries: thick, muscular, elastic walls. They handle the high pressure of blood leaving the heart.
Capillaries: walls one cell thick (endothelium only). No muscle, no elastic tissue. Designed purely for exchange.
Veins: walls are thinner than arteries but thicker than capillaries. They contain one-way valves that prevent blood from flowing backward, which is important because venous blood is under low pressure.
Although individual capillaries are tiny, the body has an enormous number of them. Their combined cross-sectional area is much larger than that of arteries or veins. Because the same volume of blood is spread across a much larger total area, blood velocity slows dramatically in the capillary beds. This is functionally important: slow flow gives more time for gases, nutrients, and waste to diffuse across capillary walls.
Oxygen moves from the alveoli into the capillaries because the partial pressure of O2 is higher in the alveoli than in the blood.
CO2 moves in the opposite direction, from capillary blood into the alveoli, because the partial pressure of CO2 is higher in the blood.
This is passive diffusion, driven entirely by partial pressure gradients. No energy input is required.
When tissues are metabolically active, they produce CO2 and lactic acid. This lowers the local blood pH (makes it more acidic). The drop in pH changes the shape of haemoglobin, reducing its affinity for oxygen, and causing it to release more O2 to those tissues.
The result is elegant: the tissues that need the most oxygen (because they are working hardest) automatically receive more of it. The Bohr shift is a built-in delivery prioritisation system.
A foetus cannot breathe on its own. It obtains oxygen from maternal blood via the placenta. For this to work, fetal haemoglobin must have a higher affinity for oxygen than adult haemoglobin. It does. When maternal and fetal blood flow close to each other in the placenta, oxygen preferentially binds to fetal haemoglobin, crossing from mother to foetus.
The pig foetus example in the source material illustrates this same principle.
Students often think arteries always carry oxygenated blood and veins always carry deoxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood back. The defining feature of arteries is direction (away from the heart), not oxygen content.
Students sometimes believe gas exchange requires energy. It does not. It is entirely passive diffusion, driven by partial pressure gradients.
The Bohr shift is frequently confused with the Bohr model (the atomic model). They are unrelated. The Bohr shift is named after Christian Bohr, a physiologist; the atomic model after his son Niels Bohr.
Students sometimes think fetal haemoglobin is a different molecule entirely. It is a variant of the same protein, with a slightly different structure that gives it a higher oxygen affinity.
⚠️ Know which number is systolic and which is diastolic. "Systolic = squeezing" is a reliable mnemonic.
⚠️ Be able to compare vessel wall thickness: arteries (thickest) > veins (middle, with valves) > capillaries (thinnest, one cell thick).
⚠️ Understand why blood velocity is slowest in capillaries (largest total cross-sectional area), and why that matters for exchange.
⚠️ The partial pressure relationship (O2 higher in alveoli than in capillaries) is a standard exam question. Be ready to explain the direction of gas movement based on gradients.
⚠️ The Bohr shift is frequently tested. Know the chain: active tissue produces CO2/lactic acid, pH drops, haemoglobin releases more O2.
⚠️ Fetal haemoglobin and its higher oxygen affinity is a common short-answer or multiple-choice question.
True or False: The diastolic number is the first number in a blood pressure reading. (False, systolic is first.)
Fill in the blank: Blood velocity is slowest in ______ because their total cross-sectional area is the ______. (capillaries; largest)
True or False: Oxygen moves from capillaries into alveoli during gas exchange in the lungs. (False, oxygen moves from alveoli into capillaries.)
Fill in the blank: The Bohr shift causes haemoglobin to release ______ oxygen when blood pH ______. (more; drops/decreases)
True or False: Fetal haemoglobin has a lower oxygen affinity than adult haemoglobin. (False, it has a higher affinity.)
Q: In a blood pressure reading of 120/80, what does each number represent?
A: 120 is the systolic pressure (pressure when the heart contracts). 80 is the diastolic pressure (pressure when the heart relaxes).
Q: Compare the wall structure of arteries, capillaries, and veins.
A: Arteries have the thickest walls (muscular and elastic) to handle high pressure. Capillaries have walls just one cell thick for exchange. Veins have walls of intermediate thickness and contain valves to prevent backflow.
Q: Why does blood slow down in capillary beds?
A: The total cross-sectional area of all capillaries combined is far greater than that of arteries or veins. The same volume of blood spread over a larger area moves more slowly. This slow flow allows time for gas and nutrient exchange.
Q: Explain the direction of oxygen movement in the lungs using partial pressure.
A: The partial pressure of O2 in the alveoli is higher than in the pulmonary capillaries. Oxygen diffuses down its concentration gradient, from alveoli into the blood.
Q: What is the Bohr shift, and why is it physiologically useful?
A: The Bohr shift is the decrease in haemoglobin's oxygen affinity when blood pH drops (becomes more acidic). Active tissues produce CO2 and lactic acid, lowering pH locally. This causes haemoglobin to release more oxygen precisely where it is needed most.
Q: How does a foetus obtain oxygen from its mother's blood?
A: Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. In the placenta, where maternal and fetal blood flow close together, oxygen preferentially binds to fetal haemoglobin, transferring from mother to foetus.
The cardiovascular system connects to the respiratory system (gas exchange in the lungs), the musculoskeletal system (the heart is a muscle that contracts via similar calcium-dependent mechanisms), and the excretory system (the kidneys filter blood arriving through renal arteries). The Bohr shift ties into cellular respiration, since CO2 is a waste product of aerobic metabolism. Blood glucose regulation (covered in the hormones notes) also depends on the circulatory system to deliver insulin and glucagon throughout the body.
Cardiovascular system, circulatory system, blood pressure, systolic, diastolic, arteries, veins, capillaries, valves, blood vessel structure, gas exchange, alveoli, partial pressure, diffusion, pulmonary circulation, Bohr shift, Bohr effect, haemoglobin, oxygen affinity, fetal haemoglobin, oxygen-haemoglobin dissociation curve, pH, blood velocity, capillary beds, surface area, BIO 101, introductory biology, University of Washington