In the MCAT syllabus, the respiratory and cardiovascular systems are essential components, constituting a significant part of the Biological and Biochemical Foundations of Living Systems section. Aspiring medical school students in the United States are well aware of the importance of receiving a competitive score on the MCAT exam. This guide is crafted to assist you in your MCAT preparation by providing an overview of the respiratory and cardiovascular systems.

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The Respiratory System for the MCAT

The respiratory system holds a crucial place among the organ systems in the human body. It encompasses the air passages extending from the nose and mouth to the lower part of the lungs. Primarily, its function revolves around the exchange of oxygen and carbon dioxide. Yet, beyond this fundamental role, the respiratory system serves various other purposes such as acting as a blood reserve, regulating blood acidity, supporting the immune system, and facilitating speech. The ordinary respiratory rate, comprising complete cycles of inhaling and exhaling, ranges between 12 to 20 breaths per minute.

The Respiratory System: Structure

The respiratory system is intricately divided into two main sections: the upper respiratory tract, spanning from the nose and mouth down to the larynx, and the lower respiratory tract, extending from the trachea to the alveoli.

Upper Respiratory Tract

The respiratory journey begins as air enters either the nose or mouth. In the nasal cavity, bone shelves known as ‘conchae’ play a pivotal role in warming and humidifying the air, aiding its conduction and diffusion through the alveoli. Beyond the conchae, the air passes through the pharynx and encounters the epiglottis—a cartilage flap that separates the respiratory and digestive systems. During swallowing, muscles connected to the epiglottis contract, ensuring its descent over the larynx entrance, preventing food from entering the respiratory system. Within the larynx, vocal folds contribute to phonation.

Lower Respiratory Tract

Continuing through the larynx, air travels down the trachea, a singular tube composed of C-shaped cartilages preventing collapse due to air pressure changes. At the trachea’s base, bifurcation occurs at the carina, leading to the right and left bronchi, directing air to the corresponding lungs. It’s crucial to note that in anatomy, ‘right’ and ‘left’ refer to the patient’s perspective, not the physician’s face-on view. The bronchi further branch into bronchioles, each connecting to a lung lobe, ultimately terminating in alveoli. These air sacs, responsible for gas exchange, possess a one-cell-thick epithelium. Two types of pneumocytes line the alveoli: Type I facilitating gas exchange, and Type II producing surfactant—composed of phospholipids—to maintain surface tension and prevent alveolar collapse under varying air pressure.

The Respiratory System: Functions 

Gas Exchange

The respiratory system has several functions but it primarily engages in gas exchange, a well-known process where oxygen enters the bloodstream while carbon dioxide exits. Oxygen is vital for aerobic respiration, and carbon dioxide, a byproduct, must be expelled. As oxygenated blood travels through the body, it relinquishes oxygen and picks up carbon dioxide, necessitating elimination. The adaptability of gaseous exchange ensures homeostasis, adjusting during activities like exercise. Different forms of respiratory failure exist; Type I hampers oxygen diffusion, while Type II impedes both carbon dioxide and oxygen diffusion.

Thermoregulation

Maintaining the body’s optimal temperature (~98°F or ~37°C) is achieved through thermoregulation. Heat removal from tissues relies on blood circulation. Thermal energy transfers to cooler blood, which is then transported to the skin’s surface for heat expulsion via sweat evaporation or back to the lungs for release into the air. Continuous blood flow ensures a dynamic process, sustaining the body’s thermal homeostatic state.

Immune Function

The respiratory system serves as a vital physical barrier against infections. While not in direct contact with blood, oxygen and carbon dioxide undergo diffusion across epithelial membranes. This prevents organisms and particles of a certain size from entering the bloodstream and spreading. Breathing, by its constant air flow, reduces the clustering and propagation of organisms, expelling them during expiration. The production of mucus in the lungs traps particles, preventing them from reaching the alveoli, and various phagocytes and mast cells within the respiratory system aid in breaking down pathogens.

Mechanisms of Breathing 

The lungs are enveloped by two layers of pleura: the parietal pleura covering the chest wall and the visceral pleura enveloping the lungs. Between them lies the ‘intrapleural space,’ housing intrapleural fluid to prevent friction. Breathing hinges on pressure changes within the pleura.

Breathing Process Overview

Breathing mechanism involves pressure fluctuations directing air in and out of the chest cavity. Air moves along pressure gradients, so negative pressure draws air into the lungs, while higher pressure expels it.

Inhalation

Inhalation commences with the chest wall expanding. The external intercostal muscles contract, causing the ribs to move upward and outward. Simultaneously, the diaphragm contracts, flattening and expanding the chest wall. While the lungs remain the same size, intrapleural space widens, decreasing pressure. This prompts lung expansion, equalizing intrapleural and lung pressures. Air is drawn into the lungs, aligning pressures with the atmosphere.

Expiration

Expiration begins with relaxation of external intercostal muscles and the diaphragm. The chest wall moves downward, and the diaphragm returns to its relaxed, domed state. Intrapleural space pressure rises, pushing pleura against the lungs. Lung pressure surpasses atmospheric pressure, forcing air out to equalize. Expiration is typically passive but can become active during stress, like exercise, when certain muscles induce forced expiration, increasing respiratory rate.

Surface Tension

Adaptations in the respiratory system accommodate rapid pressure changes during breathing. The trachea employs C-shaped cartilages to maintain openness despite pressure changes. Bronchi and bronchioles contain diminishing cartilage concentrations. At the alveoli, lacking cartilage, surfactant—a lipid-based substance—reduces surface tension. High compliance decreases lung expansion ability, while reduced surface tension enhances compliance, aiding in alveolar openness and preventing collapse.

Thermoregulation

When body tissues surpass the blood temperature, thermal energy transfers from tissues to blood. Continuous blood flow allows ongoing heat extraction until tissues cool down. To release this heat and enforce thermoregulation, blood circulates near capillary beds, close to the nose, trachea, and skin surfaces. These surfaces can secrete water or sweat. Capillaries, with an endothelium just one cell thick, enable efficient thermal energy transfer to the water. As a surface liquid’s thermal energy reaches a peak, it evaporates into the air (skin) or airways (nose and trachea), eventually being exhaled. During exercise, panting increases exhales, enhancing thermal energy loss to the environment.

Filtration of Particles

Vibrissae, small nasal hairs moving through muscle-driven protraction and retraction, play a crucial role in preventing particle inhalation. Goblet cells in the upper respiratory system secrete mucus, trapping microorganisms entering the airways. The mucus, enriched with lysozyme enzyme, combats pathogens. Cilia, tiny hair cells lining the respiratory tract, facilitate the ‘muco-ciliary escalator’—an elliptical motion moving mucus up the tract. This process allows mucus to be expelled through coughing, sneezing, or swallowing. Alveoli in the lungs harbor macrophages for pathogen phagocytosis and mast cells promoting an immune response.

Alveolar Gas Exchange

Diffusion

Partial pressure, denoting the pressure a gas would exert if it were the sole gas in a solution, is crucial for comprehending gas movement across alveoli from the bloodstream. Deoxygenated blood, carrying cell respiration byproducts, enters the pulmonary system through the pulmonary artery from the right ventricle, with both having approximately 6kPa partial pressures. Alveoli present vastly different partial pressures—oxygen around 13kPa (inhaled) and carbon dioxide comparatively low at 5.3kPa. These pressure disparities create a gradient facilitating particle diffusion. Oxygen readily moves into the bloodstream, while carbon dioxide exits the bloodstream, diffusing into the alveolus.

Henry’s Law

Henry’s Law asserts that the amount of gas dissolving in a liquid is proportional to the gas’s partial pressure in that liquid. Practically, as the external environment pressure increases (e.g., underwater), alveolar gas partial pressures rise. Consequently, more oxygen dissolves into the blood. Divers must grasp this mechanism: deeper dives require less inhaled oxygen for the same amount to diffuse into the bloodstream, reaching tissues effectively.

pH Maintenance

The respiratory system plays a pivotal role in preserving blood acidity and controlling blood pH by removing carbon dioxide from the circulatory system. Carbon dioxide reacts with water in the body to form carbonic acid—a reversible equilibrium reaction. If carbon dioxide isn’t expelled, and water isn’t removed through kidneys or thermoregulatory processes, excess carbonic acid forms, lowering blood pH. This can adversely affect enzymes, membranes, and cells. Expelling carbon dioxide is crucial for maintaining proper acidity. During exercise, panting becomes essential. It aids in thermoregulation, supplies cells with more oxygen, and reduces acid buildup from carbon dioxide and water released during aerobic respiration.

Breathing Regulation

The medulla oblongata’s regulatory centers in the brain monitor breathing rate. Central chemoreceptors within it detect pH and carbon dioxide level changes in the brain’s interstitial fluid. If pH is too low, the medulla increases respiration, and if too high, it decreases it. The dorsal respiratory group (DRG), located in the dorsal medial medulla, controls inspiration independently due to its active nature. The ventral respiratory group (VRG), situated ventrally, manages ‘forced’ breathing—both inspiration and expiration. In high-stress situations requiring intentional or increased breathing, the VRG is activated. These systems collaborate to regulate breathing rates, optimizing the efficiency of the human body.

The Cardiovascular System for MCAT Preparation

The circulatory system  or Cardiovascular System, a prominent component of the human body, holds significance in the MCAT syllabus, specifically in the Biological and Biochemical Foundations of Living Systems section. A comprehensive understanding of the circulatory system is essential for success in the MCAT exam.

Originating from the heart and intricately comprising arteries, veins, and capillaries, the circulatory system is pivotal for transporting nutrients, oxygen, hormones, and water, facilitating crucial cellular processes. Additionally, it efficiently removes waste products generated during these processes. Referred to as a ‘double circulatory system,’ it features two functional divisions within the heart. The first pump transports deoxygenated blood to the lungs for oxygenation, and then back to the heart. The second pump conveys this oxygenated blood to the entire body, eventually returning as deoxygenated blood to the first division. This continuous circulation ensures the effective distribution of essential components throughout the body and the elimination of waste products.

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Cardiovascular System: Functions

The Cardiovascular System serves various crucial functions, including:

Transport of Nutrients:

• The primary role of the circulatory system is to circulate and transport essential substances throughout the body. Nutrients, oxygen, hormones, and drugs are conveyed through the bloodstream to different tissues. Red blood cells, containing hemoglobin, play a key role in transporting oxygen for cellular respiration. Various molecules, depending on their composition, travel freely or are carried bound to proteins like albumin. The cardiovascular system adapts to the body’s needs, enlarging capillaries during exercise, increasing heart rate, and optimizing blood flow to vital organs.

Thermoregulation:

• The circulatory system plays a vital role in thermoregulation, ensuring the body maintains the correct temperature (approximately 98°F) regardless of external factors or stress. Maintaining this temperature is crucial for the optimal function of enzymes and bodily systems. The circulatory system facilitates the transfer of thermal energy from internal body areas to the external environment, aiding in temperature regulation.

Cardiovascular System: The Heart

The heart’s fundamental structure comprises two atria (where blood enters) and two ventricles (which propel blood out).

Blood Flow into the Heart:

• The right side receives blood from the superior and inferior vena cava. The inferior vena cava drains blood from below the heart, while the superior vena cava drains blood from above, including the arms, neck, and head.
• The left side receives oxygenated blood from the pulmonary vein, transporting it from the lungs to the heart.

Atria and Atrioventricular Valves:

• Atria, smaller chambers with pectinate muscles and appendages, facilitate blood passage into the ventricles through atrioventricular valves.
• Tricuspid valve (right) and bicuspid (mitral) valve (left) prevent blood backflow during ventricular contraction. Papillary muscles and chordae tendineae play a crucial role in valve function.

Ventricles, Semilunar Valves, and Arteries:

• Ventricles, the heart’s largest chambers, pump blood at high pressure. The left ventricle has thicker muscle layers, given its role in pumping blood throughout the body.
• Semilunar valves at the base of the pulmonary artery (right) and aorta (left) prevent blood from re-entering the ventricles. They open due to internal pressure, ensuring unidirectional blood flow.
• Blood from the right side, deoxygenated, heads to the pulmonary artery, branching into lungs. Blood from the left side, oxygenated, exits through the aorta, supplying the upper body before branching into the abdomen and legs.

Endothelial Cells for the MCAT

Definition and Location:

• Endothelial cells are flat, thin cells lining blood vessels.

Functions:

• Release factors like nitrogen oxide for vessel dilation, ligands for blood clotting, and receptors for phagocytes.
• Role variation in different vessels, influencing nutrient transport and tissue interaction.

Pathological Impact:

• Atherosclerosis can lead to fibrous endothelium, reducing artery elasticity and lumen size.

Blood Pressure for the MCAT

Definition:

Blood pressure is the force exerted by blood on arteries during the cardiac cycle, measured in mmHg.

Representation:

Presented as a fraction (e.g., 120/80mmHg), with the numerator denoting systolic pressure (during ventricular contraction) and the denominator denoting diastolic pressure (during ventricular filling).

Normal Range:

Standard blood pressure is 120/80mmHg.

Clinical Significance:

Hypotension: Under 90/60mmHg.

Hypertension: Around 130/85mmHg or above.

Blood Vessels Overview for the MCAT

To navigate the complexities of blood vessels for the MCAT, one must delve into their dual classification as arteries and veins, each boasting unique characteristics. These vessels exhibit a three-layered structure: the tunica intima, housing the endothelial layer with responsibilities spanning vasodilation, blood clotting, and immune functions; the tunica media, composed primarily of smooth muscle cells facilitating vessel stretching; and the tunica externa (or adventitia), a layer of collagenous connective tissue acting as a safeguard against overexpansion.

Arteries:

Arteries, robust conduits primarily responsible for transporting oxygenated blood from the heart throughout the body, showcase distinctive features. With thick walls, a small lumen, high pressure, and a rapid flow rate, arteries, excluding the pulmonary artery delivering deoxygenated blood to the lungs, efficiently navigate systemic circulation. The substantial tunica media, characterized by a thick layer of smooth muscle, enables arteries to dynamically expand and contract with each heartbeat. The aorta, the body’s largest artery, branches into smaller arterioles, leading further to capillaries, facilitating nutrient exchange.

Veins:

Contrastingly, veins, tasked with returning deoxygenated blood to the heart, exhibit contrasting traits. Notably, veins possess a larger lumen and a thin smooth muscle layer, resulting in lower pressure and a slower flow compared to arteries. While this low pressure poses challenges for blood return to the heart against gravity, veins adapt ingeniously. Valves within most veins prevent the backflow of blood, and the easily compressible thin walls, accompanied by the pulsations of adjacent arteries, assist in forcing blood back to the heart. Despite the predominantly deoxygenated blood carried by veins, the pulmonary vein stands as an exception, ushering oxygenated blood from the lungs back to the heart.

Capillary Beds 

Exchange of Solutes:

Capillary beds, the tiniest blood vessels, play a pivotal role in solute exchange, a critical MCAT concept. Differing from larger vessels, capillaries consist of a single endothelial cell layer adapted to specific functions. Continuous capillaries permit only non-polar molecules, like oxygen, to traverse the membrane. Fenestrated capillaries, featuring gaps, facilitate the passage of molecules such as glucose. Sinusoid capillaries, with incomplete membranes, enable the exchange of larger molecules like proteins and hormones. Aquaporins, specialized pores, facilitate water movement across the endothelium. Filtration is more prominent at the arteriolar end, while resorption is common at the venous end, preventing fluid buildup.

Heat Exchange:

Capillaries contribute significantly to thermoregulation, crucial for enzyme and cellular function. During high thermal energy conditions, capillaries expand and move closer to the skin and bronchi, enhancing heat transfer. Conversely, in cold environments, vasoconstriction occurs, reducing thermal energy loss through the skin.

Peripheral Resistance:

Peripheral resistance, controlled by smaller arterioles, is a pivotal MCAT topic. Poiseuille’s Law defines factors influencing resistance. The equation emphasizes the impact of vessel radius on resistance, highlighting higher resistance in arteries due to their smaller lumen radius compared to veins. 

Blood Composition for MCAT

Understanding Blood Components:

The composition of blood, a central focus on the MCAT, comprises three main elements: plasma (55%), red blood cells (41%), and white blood cells/platelets (4%). Plasma transports antibodies, hormones, and nutrients, while red blood cells, lacking a nucleus, carry oxygen through hemoglobin. White blood cells and platelets play crucial roles in immune responses and blood clotting.

Erythrocyte Life Cycle:

Erythropoiesis, the creation of red blood cells, is initiated by erythropoietin from the kidneys. Stem cells differentiate into reticulocytes, maturing into red blood cells with a lifespan of approximately 120 days. The spleen filters out aging erythrocytes, completing their life cycle.

Regulation of Plasma Volume:

Mastering plasma volume regulation is vital for the MCAT. Oncotic pressure, driven by proteins like albumin, draws fluids into capillaries, countering hydrostatic pressure. Venous systems, influenced by gravity, may lead to conditions such as edema. Salts, especially sodium, contribute to fluid balance. Natriuretic peptides modulate sodium reabsorption and aldosterone levels to manage plasma volume effectively.

Blood Clotting

Understanding blood clotting is crucial for the MCAT, and key concepts include:

Formation and Purpose of Blood Clots:

Blood clots, composed of clotting factors and platelets, play a vital role in wound healing by preventing excessive bleeding and forming a protective seal. This process initiates in response to endothelial damage, leading to the release of tissue factor.

Sequential Activation of Clotting Factors:

The clotting cascade involves the sequential activation of clotting factors. Tissue factor initiates the process, activating successive factors until prothrombin is converted to thrombin. Thrombin, in turn, converts fibrinogen into fibrin, forming a meshwork that traps red blood cells and platelets at the injury site.

Platelet-Endothelium Interactions:

Interactions between platelets and the cut endothelium are vital for clot formation. Antigens on platelets bind to receptors on the endothelium, and vice versa, ensuring that platelets are anchored to the injured vessel. This anchoring mechanism secures the clot in place until healing is complete.

Clot Resolution:

Once the injured area heals, plasminogen is converted to plasmin. Plasmin is responsible for breaking down the fibrin meshwork, leading to clot resolution. This final step ensures that the clot is removed, allowing for proper tissue repair.

Oxygen Transport

Hemoglobin Structure and Oxygen Transport:

Oxygen transport relies significantly on hemoglobin, a 4-peptide structure within red blood cells. Each peptide, composed of 141-146 amino acids, contains a heme group with an iron ion at its center. Adult hemoglobin comprises 2 alpha and 2 beta chains, while fetal hemoglobin consists of 2 alpha and 2 gamma chains. Hemoglobinopathies, like thalassemia, can result from alterations in these chains. A single hemoglobin molecule can bind up to 4 oxygen molecules, and each red blood cell, carrying about 250 million hemoglobin molecules, can transport up to 1 billion oxygen molecules.

Hematocrit and Oxygenation Measurement:

Pulse oximetry and arterial blood gas (ABG) tests are standard clinical methods to assess oxygenation. Hematocrit, representing the percentage of red blood cells in blood volume (usually around 40%), serves as an indirect marker of oxygenation. Hemoglobin levels, measured in raw molecules per red blood cell, complement hematocrit evaluations.

Oxygen Content and Solubility:

Oxygen content is influenced by both hemoglobin-bound and dissolved oxygen. While the majority binds to hemoglobin, a smaller portion is soluble, with solubility inversely proportional to temperature. Warm-blooded organisms, like humans, exhibit lower oxygen solubility compared to cold-blooded counterparts.

Oxygen Affinity and Cooperative Binding:

Cooperative binding, governing hemoglobin’s varying affinity for oxygen, involves the Taut (T) and Relaxed (R) states. In the T state, without oxygen, affinity is low; upon oxygen binding, hemoglobin shifts to the R state, facilitating increased oxygen saturation. The sigmoid-shaped dissociation curve demonstrates the cooperative nature. Fetuses, with alpha and gamma chains, showcase a higher oxygen affinity. Factors affecting affinity include pH (lower pH shifts the curve right), carbon dioxide levels (lower levels shift left), 2,3 DPG (lower levels shift left), and temperature (lower temperature shifts left).

Carbon Dioxide Transport

Methods of Carbon Dioxide Transport:

Carbon dioxide, a crucial waste product, undergoes diverse transport mechanisms within the blood. It can dissolve directly in the blood, bind to hemoglobin, or most commonly, exist as bicarbonate ions. The latter, catalyzed by carbonic anhydrase within red blood cells, is particularly efficient due to increased solubility.

Bicarbonate Ions: Efficient Transport Strategy:

The conversion of carbon dioxide to bicarbonate ions involves the enzyme carbonic anhydrase within red blood cells. This process enhances solubility and efficiency. Bicarbonate ions can subsequently combine with hydrogen ions, forming carbonic acid. Additionally, bicarbonate can revert to carbon dioxide in lung capillaries, facilitating its diffusion across the alveolar membrane during exhalation. This intricately regulated system allows for effective carbon dioxide elimination.

Regulation and Normal Levels:

Maintaining homeostasis is vital, and normal carbon dioxide levels in the blood typically range between 35-45mEq/l. Precise regulation ensures that carbon dioxide, a byproduct of cellular respiration, is efficiently transported and eliminated, preventing disruptions in blood pH and overall physiological balance.

Cardiovascular System Regulation

Control Nodes:

The rhythmic beating of the heart is orchestrated by specialized nodes that generate electrical signals. The sinoatrial (SA) node, situated in the right atrium, serves as the pacemaker, initiating the heartbeat at a rate of 60-100 beats per minute. The self-excitatory cells in the SA node eliminate the need for external signals. The atrioventricular (AV) node, located in the atrial septum, receives impulses from the SA node, directing them down the septum and coordinating atrial contraction. Following the AV node, the Bundle of His in the ventricular septum and Purkinje fibers in the ventricles further propagate impulses, facilitating ventricular contraction and the ejection of blood from the heart.

Impulse Speeds:

Impulse speeds vary across these nodes, with the SA node at approximately 0.5m/s, AV node at 0.05m/s, Bundle of His at 2m/s, and Purkinje fibers at 4m/s. Despite differing natural impulse speeds, the SA node’s signal predominates, ensuring a coherent and efficient heartbeat.

Thyroid Hormone Influence:

Thyroid hormone, released from the thyroid gland, plays a crucial role in cardiovascular regulation. Elevated thyroid levels (hyperthyroidism) result in tachycardia and increased inotropy, intensifying each heartbeat. Conversely, decreased thyroid activity (hypothyroidism) leads to bradycardia and reduced inotropy. The delicate balance of thyroid hormone levels is pivotal for maintaining optimal cardiovascular function.

Conclusion

Here we have provided a comprehensive overview of the respiratory and cardiovascular systems, essential topics in the MCAT syllabus. This guide equips you with fundamental knowledge crucial for success in the Bio section of the MCAT exam. 

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