Physiology of Gasses of the Cardiovascular System
The cardiovascular system's exchange machinery: hemoglobin oxygen transport, the Bohr effect, CO₂ transport as bicarbonate, Starling forces, and the coagulation cascade.
Beyond blood pressure, the cardiovascular system's other central job is exchange: getting oxygen, nutrients, and hormones into tissues while clearing out carbon dioxide and waste — and sealing off any breach in the system when a vessel is damaged. This page covers that exchange machinery: oxygen and carbon dioxide transport, fluid balance across the capillary wall, and the coagulation cascade.
Key Takeaways
Gas and solute exchange happens at the capillaries, driven by blood pressure and diffusion down concentration gradients.
Each hemoglobin molecule carries up to four oxygen molecules, one per heme group, with cooperative binding making each successive binding easier.
The oxygen-hemoglobin dissociation curve shifts right (easier oxygen release) with increased CO₂, increased H⁺ (lower pH), increased temperature, and increased 2,3-DPG — the Bohr effect.
Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin.
CO₂ travels in the blood dissolved in plasma, bound to hemoglobin, and — most significantly — converted to bicarbonate.
The same capillary networks that handle gas exchange also deliver nutrients and hormones and remove waste.
Starling forces balance hydrostatic pressure (pushes fluid out, strongest at the arteriole end) against oncotic pressure (pulls fluid in, dominant at the venule end); imbalance causes edema, resolved by the lymphatic system.
Coagulation begins with platelet aggregation at exposed collagen, proceeds through a clotting cascade that converts fibrinogen to fibrin via thrombin, and ends with plasmin (from plasminogen) dissolving the clot once healing is complete.
Gas and Solute Exchange at the Capillaries
Gas and solute exchange happens primarily in the capillaries, driven by both blood pressure and concentration gradients. Blood pressure pushes blood forward into the narrow capillary beds, where oxygen and nutrients leave the blood and enter the tissues, while waste products like carbon dioxide leave the tissues and move into the blood. Hormones can also diffuse in or out at this level, depending on what the body needs at that moment.
Diffusion always moves substances from high concentration to low concentration. That's why oxygen — at a higher concentration in blood — leaves the blood and enters tissue, where oxygen is lower. The reverse is true for carbon dioxide: it builds up in tissues as a waste product and moves into the blood, where its concentration is lower.
Oxygen Transport
Hemoglobin and Cooperative Binding
Most of the oxygen in blood is carried by hemoglobin, a protein found in red blood cells. Each hemoglobin molecule has four subunits, and each subunit contains a heme group that can bind one oxygen molecule — so one hemoglobin molecule can carry up to four oxygen molecules at once.
Oxygen binds to hemoglobin cooperatively: once one oxygen molecule binds, hemoglobin changes shape slightly, making it easier for the next one to bind. With each oxygen added, binding becomes progressively easier.
The Oxygen-Hemoglobin Dissociation Curve
The oxygen-hemoglobin dissociation curve describes how tightly hemoglobin holds onto oxygen. Along the x-axis is the partial pressure of oxygen; along the y-axis is the percentage of hemoglobin saturated with oxygen.
In the lungs, where oxygen pressure is high, hemoglobin is nearly fully saturated — it grabs onto oxygen tightly. In the tissues, where oxygen pressure is lower, hemoglobin releases oxygen so it can be used by cells. During exercise, when active tissues are using more oxygen, tissue oxygen pressure drops even further, causing hemoglobin to release oxygen more readily — exactly when those tissues need a steady oxygen supply.
The Bohr Effect
Hemoglobin's affinity for oxygen isn't fixed — it responds to the surrounding environment. This is the Bohr effect: increased levels of carbon dioxide and hydrogen ions, both found in metabolically active tissue, cause hemoglobin to release oxygen more readily. In other words, higher CO₂ or lower pH decrease hemoglobin's affinity for oxygen, making it easier to offload oxygen exactly where it's needed most.
What shifts the dissociation curve:
Right shift (hemoglobin holds oxygen less tightly, releases it more easily): increased pCO₂, increased [H⁺] (lower pH), increased temperature, increased 2,3-DPG.
Left shift (hemoglobin holds oxygen more tightly): decreased pCO₂, decreased [H⁺] (higher pH), decreased temperature, decreased 2,3-DPG.
2,3-DPG is a molecule produced by red blood cells during glycolysis; it binds to hemoglobin and lowers its oxygen affinity. When tissues are active and generating heat, carbon dioxide, and acid, all of those signals push hemoglobin to release oxygen where it's needed.
Fetal Hemoglobin
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, which helps the fetus pull oxygen across the placenta from the mother's circulation.
Carbon Dioxide Transport
Carbon dioxide is carried in the blood in three forms:
Three forms of CO₂ transport:
A small amount dissolved directly in the plasma.
Some CO₂ bound to hemoglobin, though not at the oxygen-binding site.
Most importantly, CO₂ converted to bicarbonate ions — carbon dioxide reacts with water to form carbonic acid, which rapidly dissociates into hydrogen ions and bicarbonate.
This bicarbonate-forming process is fully reversible and is central to maintaining blood pH.
Transport of Nutrients, Waste, and Hormones
Beyond gas exchange, blood also moves nutrients, waste, and hormones through the same capillary networks. Nutrients like glucose, amino acids, and fats are absorbed from the digestive system and delivered to tissues throughout the body. Waste products like urea and carbon dioxide are picked up from tissues and transported to the kidneys or lungs for elimination. Hormones — chemical messengers secreted by endocrine glands — travel through the bloodstream from their site of release to distant target cells, where they trigger responses by binding to specific receptors.
Fluid Balance and Starling Forces
Hydrostatic Pressure vs. Oncotic Pressure
Inside the capillaries, fluid movement is controlled by two opposing forces:
Hydrostatic pressure — the force blood pushes against capillary walls. It's highest at the arteriole end and pushes fluid out into surrounding tissue.
Oncotic pressure — created by large plasma proteins, mainly albumin, that stay inside the capillaries and pull fluid back in.
As blood flows through the capillary, the balance between these two forces shifts. At the arteriole end, hydrostatic pressure is stronger than oncotic pressure — for example, about 36 mmHg pushing out versus 25 mmHg pulling in — so fluid moves out. By the time blood reaches the venule end, hydrostatic pressure has dropped while oncotic pressure stays roughly the same, so oncotic pressure wins — for example, 25 mmHg pulling in versus only 15 mmHg pushing out — and fluid is pulled back into the capillary.
This push-pull balance is called Starling forces, and it regulates how much fluid leaves the bloodstream and how much returns.
Edema and the Lymphatic System
If this process doesn't work properly — if too much fluid stays in the tissues — the result is swelling, or edema. The lymphatic system picks up the extra fluid and returns it to circulation through vessels that eventually drain into the thoracic duct.
Coagulation
Coagulation is the body's process for forming blood clots to seal an injured vessel.
Platelet Aggregation
When a blood vessel is damaged, platelets are the first responders. These cell fragments detect exposed collagen beneath the vessel lining and immediately adhere to the injury site. This contact activates the platelets, causing them to change shape and release chemical messengers. These signals amplify the response, recruiting even more platelets in a process called platelet aggregation.
The Clotting Cascade
At the same time, a complex cascade of clotting factors is triggered. This cascade ends with the enzyme thrombin converting the soluble protein fibrinogen into fibrin, an insoluble mesh that weaves through the platelet plug. This fibrin mesh stabilizes the clot, sealing the breach and stopping blood loss.
Clot Resolution
Once the vessel has healed, the clot is no longer needed. The body activates plasmin, an enzyme that digests fibrin and dissolves the clot. Plasmin is formed from plasminogen, its inactive precursor, which gets incorporated into the clot early on so it can be activated later during clot resolution.
Common MCAT Mistakes
Thinking hemoglobin binds oxygen independently at each subunit. Binding is cooperative — once one oxygen binds, hemoglobin shifts shape and each subsequent oxygen binds more easily, not at a constant rate.
Mixing up which direction the dissociation curve shifts. A right shift (increased CO₂, increased H⁺/lower pH, increased temperature, increased 2,3-DPG) means hemoglobin releases oxygen more easily — it does not mean hemoglobin binds oxygen more tightly.
Forgetting that most CO₂ travels as bicarbonate, not dissolved gas. Only a small amount of CO₂ dissolves directly in plasma; the majority is converted to bicarbonate ions after reacting with water to form carbonic acid.
Assuming hydrostatic pressure dominates the entire capillary. Hydrostatic pressure wins at the arteriole end, pushing fluid out, but oncotic pressure takes over by the venule end, pulling fluid back in — it's a shifting balance, not a fixed one.
MCAT-Style Concept Check
Question: A patient's tissues are actively metabolizing, producing more CO₂ and lowering local pH. According to the Bohr effect, how does this change hemoglobin's behavior, and what is the direction of the shift on the oxygen-hemoglobin dissociation curve?
A) Hemoglobin's oxygen affinity increases, and the curve shifts left.
B) Hemoglobin's oxygen affinity decreases, and the curve shifts right.
C) Hemoglobin's oxygen affinity is unaffected by CO₂ or pH changes.
D) Hemoglobin's oxygen affinity decreases, and the curve shifts left.
Answer: B
Explanation: The Bohr effect describes how increased CO₂ and increased H⁺ (lower pH) in metabolically active tissue lower hemoglobin's affinity for oxygen, causing it to release oxygen more readily exactly where it's needed. On the oxygen-hemoglobin dissociation curve, this decreased affinity corresponds to a right shift. A left shift (options A and D mischaracterize the direction) would instead mean hemoglobin holds oxygen more tightly, which happens with decreased CO₂ and decreased H⁺ — the opposite of this scenario.
FAQ
How many oxygen molecules can one hemoglobin molecule carry?
Up to four. Each hemoglobin molecule has four subunits, and each subunit contains one heme group capable of binding one oxygen molecule.
What is the Bohr effect?
The Bohr effect is the drop in hemoglobin's oxygen affinity caused by increased carbon dioxide and increased hydrogen ions (lower pH) — conditions found in metabolically active tissue. It causes hemoglobin to release oxygen more readily exactly where tissues need it most.
How is most carbon dioxide transported in the blood?
Most CO₂ is converted to bicarbonate ions: it reacts with water to form carbonic acid, which rapidly dissociates into hydrogen ions and bicarbonate. Smaller amounts travel dissolved in plasma or bound to hemoglobin.
What causes edema?
Edema is swelling caused by excess fluid remaining in tissues when the balance between hydrostatic pressure (pushing fluid out) and oncotic pressure (pulling fluid in) is disrupted. The lymphatic system normally clears this extra fluid and returns it to circulation.
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