Premed · Premed · General Biology 2
Lecture 17: Circulation and Gas Exchange
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Compare open and closed circulatory systems across animal phyla
- Describe the structure and function of the mammalian heart, including the cardiac cycle
- Trace the path of blood through the pulmonary and systemic circuits
- Explain the structure and function of arteries, veins, and capillaries
- Describe the mechanisms of gas exchange in lungs and tissues
- Explain how O2 and CO2 are transported in the blood
Lecture Content
I. Circulatory Systems Across Animals
Circulatory systems evolved to transport oxygen, carbon dioxide, nutrients, metabolic wastes, hormones, and immune cells throughout an animal's body. Small or very flat organisms -- cnidarians and flatworms, for example -- can rely on diffusion alone and require no dedicated circulatory system.
In an open circulatory system, found in arthropods and most mollusks, a heart pumps hemolymph (which serves as both blood and interstitial fluid) into open sinuses called the hemocoel, where it bathes the organs directly before returning to the heart. This system operates at relatively low pressure with slower flow rates. A closed circulatory system, found in annelids, cephalopods, and all vertebrates, confines blood within a continuous network of arteries, capillaries, and veins. The closed system generates higher pressure, delivers oxygen and nutrients more efficiently, and allows independent regulation of blood flow to different organs.
The vertebrate heart has evolved progressively greater separation of oxygenated and deoxygenated blood. Fish have a two-chambered heart (one atrium, one ventricle) driving a single circulatory loop. Amphibians have a three-chambered heart (two atria, one ventricle) with partial mixing. Most reptiles add a partial ventricular septum. Crocodilians, birds, and mammals have independently evolved fully four-chambered hearts with complete separation.
II. The Mammalian Heart
The mammalian heart is a muscular pump divided into four chambers that maintain complete separation between oxygenated and deoxygenated blood. The right side receives deoxygenated blood from the body and pumps it to the lungs (the pulmonary circuit), while the left side receives oxygenated blood from the lungs and pumps it to the rest of the body (the systemic circuit).
Blood follows a precise pathway through the heart. Deoxygenated blood from the body enters through the superior and inferior vena cava into the right atrium, passes through the tricuspid valve into the right ventricle, and is ejected through the pulmonary semilunar valve into the pulmonary arteries bound for the lungs. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium, passes through the bicuspid (mitral) valve into the left ventricle, and is ejected through the aortic semilunar valve into the aorta for distribution to the body. The left ventricular wall is substantially thicker than the right because it must generate the much higher pressures needed to drive blood through the extensive systemic circulation. The heart's valves prevent backflow, and their closures produce the characteristic heart sounds: S1 ("lub") when the atrioventricular valves close at the onset of ventricular contraction, and S2 ("dub") when the semilunar valves close at the onset of ventricular relaxation.
The Cardiac Cycle
The cardiac cycle alternates between systole (the contraction phase, during which ventricles eject blood) and diastole (the relaxation phase, during which ventricles fill). Cardiac output, the volume of blood pumped per minute, equals heart rate multiplied by stroke volume. At a typical resting heart rate of 72 beats per minute and a stroke volume of 70 mL, cardiac output is approximately 5 liters per minute -- meaning the heart pumps the body's entire blood volume every minute.
Electrical Conduction System
The heart is myogenic, meaning it generates its own electrical impulses without requiring input from the nervous system. The sinoatrial (SA) node, located in the wall of the right atrium, serves as the natural pacemaker, setting the heart rate. Each impulse spreads across both atria (causing atrial contraction), then reaches the atrioventricular (AV) node, which introduces a brief delay to ensure the atria finish emptying before the ventricles begin to contract. The impulse then travels rapidly through the bundle of His, splits into left and right bundle branches, and spreads through the Purkinje fibers to coordinate ventricular contraction from the apex upward. An electrocardiogram (ECG or EKG) records this electrical activity: the P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization.
<image>A diagram of the mammalian heart showing blood flow and the electrical conduction system. Panel A: A frontal cross-section of the heart with all four chambers labeled (right atrium, right ventricle, left atrium, left ventricle), along with the valves (tricuspid, bicuspid/mitral, pulmonary semilunar, aortic semilunar), major vessels (superior and inferior vena cava, pulmonary arteries and veins, aorta), and the septum. Deoxygenated blood is shown in blue, oxygenated blood in red, with arrows indicating the direction of flow. Panel B: The electrical conduction system overlaid on the heart — the SA node in the right atrial wall, the AV node at the junction of atria and ventricles, the bundle of His passing through the septum, bundle branches descending along the septum, and Purkinje fibers spreading through ventricular walls. An ECG trace is shown below with P wave, QRS complex, and T wave labeled and correlated to the corresponding events in the heart.</image>
III. Blood Vessels
Arteries carry blood away from the heart under high pressure. Their thick, elastic walls absorb the pulsatile pressure of each heartbeat. Arterioles, the smallest arteries, are the primary regulators of blood flow to capillary beds, adjusting flow through vasoconstriction and vasodilation of the smooth muscle in their walls. Capillaries are the sites of exchange between blood and tissues. Their walls consist of a single layer of endothelial cells, thin enough for gases, nutrients, and wastes to cross by diffusion, transcytosis, or passage through fenestrations (pores). Precapillary sphincters regulate blood flow into capillary beds. Veins carry blood back toward the heart at lower pressure. Their walls are thinner than those of arteries, and they contain one-way valves that prevent backflow. Venous return is assisted by the skeletal muscle pump (contracting muscles squeeze veins) and the respiratory pump (breathing creates pressure changes that help move blood toward the heart).
Blood pressure, measured as systolic over diastolic (typically approximately 120/80 mmHg), is highest in the arteries and drops progressively through the arterioles, capillaries, venules, and veins. Blood pressure is regulated by cardiac output, blood volume, and peripheral resistance (primarily arteriolar diameter). Baroreceptors in the carotid arteries and aortic arch detect pressure changes and relay this information to the brainstem, which adjusts heart rate and vessel tone to maintain homeostasis.
IV. Blood Composition
Blood consists of plasma (approximately 55% by volume) and formed elements (approximately 45%). Plasma is approximately 90% water, with dissolved proteins (albumin for osmotic balance, globulins including antibodies, and fibrinogen for clotting), ions, nutrients, metabolic wastes, hormones, and dissolved gases. Red blood cells (erythrocytes) are biconcave discs packed with hemoglobin that transport oxygen and carbon dioxide. In mammals, they lack nuclei at maturity. They are produced in the bone marrow (erythropoiesis), regulated by erythropoietin (EPO) from the kidneys, and have a lifespan of approximately 120 days before being recycled in the spleen and liver. White blood cells (leukocytes) carry out immune defense (detailed in Lecture 23). Platelets (thrombocytes), cell fragments derived from megakaryocytes, are essential for blood clotting (hemostasis).
Hemostasis proceeds in three overlapping steps. Vascular spasm constricts the damaged vessel. Platelet plug formation occurs as platelets adhere to exposed collagen and aggregate. The coagulation cascade, a series of protease-activating reactions, converts prothrombin to thrombin, which in turn converts fibrinogen to fibrin. The resulting fibrin mesh reinforces and stabilizes the platelet plug.
V. Gas Exchange
Gas exchange requires respiratory surfaces that are thin, moist, and offer a large surface area. Gases move by diffusion down their partial pressure gradients. Different animal groups have evolved distinct respiratory structures suited to their environments: the body surface in earthworms and amphibians (cutaneous respiration), gills in aquatic animals (with fish employing countercurrent exchange, in which blood flows opposite to water flow across the gill lamellae for maximum oxygen extraction), tracheae in insects (branching tubes delivering oxygen directly to cells), and lungs in terrestrial vertebrates (internal respiratory surfaces protected from desiccation).
Mammalian Respiratory System
Inhaled air follows a path from the nasal cavity through the pharynx, larynx, trachea, bronchi, and bronchioles to the approximately 300 million alveoli in the human lungs. These tiny air sacs provide an enormous surface area of approximately 70 square meters. Their walls are a single cell layer thick (type I pneumocytes) and are surrounded by a dense network of capillaries, minimizing the diffusion distance for gases. Type II pneumocytes secrete surfactant, which reduces surface tension and prevents alveolar collapse.
Breathing is accomplished through negative pressure ventilation. During inhalation, the diaphragm contracts and flattens while the external intercostal muscles lift and expand the rib cage, increasing thoracic volume and creating a pressure gradient that draws air into the lungs. Exhalation at rest is largely passive: the diaphragm and intercostal muscles relax, and the elastic recoil of the lungs compresses the air back out. At the alveolar surface, oxygen diffuses from the alveolar air (where its partial pressure is approximately 104 mmHg) into the blood (where it is approximately 40 mmHg), while carbon dioxide diffuses in the opposite direction, from blood (PCO2 approximately 46 mmHg) into the alveolar air (approximately 40 mmHg).
<image>A diagram of gas exchange in the mammalian lung. Panel A: Overview of the respiratory system showing the trachea branching into bronchi and bronchioles, terminating in clusters of alveoli. Panel B: Magnified view of an alveolus surrounded by capillaries. The thin alveolar wall (type I pneumocyte) and the capillary endothelium are shown with a shared basement membrane. Arrows show O2 diffusing from the alveolar air space into the capillary blood (turning blood from blue to red), and CO2 diffusing from capillary blood into the alveolar air space. Partial pressures are labeled: alveolar PO2 = 104 mmHg, incoming blood PO2 = 40 mmHg; alveolar PCO2 = 40 mmHg, incoming blood PCO2 = 46 mmHg. Panel C: Countercurrent exchange in a fish gill — showing water flow and blood flow in opposite directions across the gill lamellae, with a graph demonstrating how PO2 gradients are maintained along the entire length for efficient extraction.</image>
VI. O2 and CO2 Transport in Blood
Approximately 98.5% of oxygen in the blood is transported bound to hemoglobin (Hb) in red blood cells. Hemoglobin is a tetramer with four subunits, each containing a heme group with an iron atom (Fe2+) that binds one molecule of O2. Oxygen binding is cooperative: the attachment of O2 to one subunit increases the affinity of the remaining subunits, producing the characteristic sigmoidal oxygen-hemoglobin dissociation curve. In the lungs, where PO2 is high, hemoglobin is nearly fully saturated (approximately 98%). In the tissues, where PO2 is low, hemoglobin releases oxygen. The Bohr effect describes how conditions in metabolically active tissues -- lower pH from CO2 and lactic acid, higher temperature, higher PCO2, and increased 2,3-BPG (bisphosphoglycerate, produced by RBCs during glycolysis) -- shift the dissociation curve rightward, promoting oxygen release precisely where it is most needed. Fetal hemoglobin (HbF) has a higher oxygen affinity than adult hemoglobin, facilitating oxygen transfer from maternal to fetal blood across the placenta.
Carbon dioxide is transported in three forms. A small fraction (approximately 7%) dissolves directly in plasma. About 23% binds to hemoglobin at amino groups (not the heme iron), forming carbaminohemoglobin. The majority -- approximately 70% -- is converted to bicarbonate (HCO3-) through a reaction catalyzed by the enzyme carbonic anhydrase in red blood cells: CO2 combines with water to form carbonic acid (H2CO3), which immediately dissociates into hydrogen ions (H+) and bicarbonate. The bicarbonate is transported out of the red blood cell into the plasma via the chloride shift (Cl- moves in to maintain electrical neutrality). At the lungs, these reactions reverse: bicarbonate re-enters the RBC, is converted back to CO2, and the CO2 diffuses into the alveolar air for exhalation. The bicarbonate buffer system (CO2 <-> H2CO3 <-> H+ + HCO3-) also plays a crucial role in maintaining blood pH.
