Respiratory and Cardiovascular Systems for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about respiratory and cardiovascular systems, plus practice questions and answers
(Note: This guide is part of our MCAT Biology series.)
Part 1: Introduction to respiratory and cardiovascular systems
Part 2: Structure and function of the respiratory system
a) Structure of the respiratory system
b) Gas exchange
c) Thermoregulation
d) Protection against disease
Part 3: Breathing mechanisms
a) Diaphragm, rib cage, and differential pressure
b) Resiliency and surface tension effects
c) Volumes and capacities
Part 4: Gas exchange and regulation
a) Diffusion and differential partial pressure
b) Henry’s law
c) pH control and nervous system control
Part 5: Overview of the circulatory system
a) Components of the circulatory system
b) The heart
Part 6: Overview of blood
a) Blood composition
b) Blood flow
c) Gas exchange by blood
Part 7: Regulation from other systems
a) Nervous system control
b) Endocrine system control
Part 8: High-yield terms
Part 9: Practice passage and answers
Part 10: Practice standalone questions and answers
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Part 1: Introduction to respiratory and cardiovascular systems
The respiratory and circulatory systems are two of the most important body systems to know for the MCAT, and they often go hand in hand. This section will first talk about the respiratory system and then move to the circulatory system, detailing the structure and functions of both. Finally, we’ll provide an MCAT-style practice passage along with a set of standalone questions.
(Suggested Reading: MCAT Biology Practice Questions)
This topic is medium to high yield, so it is important to understand it for the MCAT. Let’s get started!
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Part 2: Structure and function of the respiratory system
The respiratory system is traditionally associated with breathing, but for the MCAT we need to go into a little more detail. This section will first cover the components and structure of the respiratory system, and then we’ll talk about the general functions of the respiratory system, which include gas exchange, thermoregulation, and protection against disease and particulate matter.
a) Structure of the respiratory system
Air enters the body first through the nose and mouth and then travels through the pharynx, which is located at the back of the mouth and also carries food. Following the pharynx is the epiglottis, which is the cap that covers the larynx when swallowing so that food goes down the esophagus instead of the larynx. Air continues on into the larynx, which contains the vocal cords that vibrate during speaking, and it then moves to the trachea, which is commonly referred to as the windpipe.
From the trachea, bronchi branch out and carry the air in and out of the lungs, and these further branch out into secondary bronchi, tertiary bronchi, and bronchioles. These bronchioles finally end with alveoli, which are microscopic sacs that are covered with blood capillaries to facilitate gas exchange. This entire succession of branching out into smaller tubes serves the purpose of increasing surface area for gas exchange. Alveoli contain a coating layer called surfactant, which is a soapy substance that prevents the alveoli from collapsing in on themselves.
Figure 1. Structure of the respiratory system.
The lungs are the main organ of the respiratory system, and they are the site of gas exchange between bronchi/alveoli and the blood. The lungs are covered by a membrane known as pulmonary pleura as well as an outer membrane known as the parietal pleura that keeps the lungs in place. These pleural membranes prevent the lungs from collapsing in on themselves, and they have a space in between them known as the pleural cavity, which is a thin layer of lubrication that allows for sliding movement between the two pleurae. Because of this cavity, there is a pressure between the two pleura that keeps them together and anchors the lungs to the chest.
b) Gas exchange
The most basic function of the lungs and the respiratory system as a whole is gas exchange. Cells need oxygen to engage in aerobic respiration, and the waste product is carbon dioxide. As a result, there must be a constant exchange of oxygen into the body and carbon dioxide out of the body.
The detailed description of gas exchange appears in Part 4 of this chapter, but the general mechanism is very simple. Once blood returns from the body to the heart, it is then sent to the lungs to interact with the alveoli. Each of the tiny alveoli has many capillaries running across it, and thus there is ample surface area for gas to be exchanged. Due to the microscopic nature of the alveoli and capillaries, there is only about one cell distance that carbon dioxide and oxygen have to diffuse to provide oxygen to the body.
c) Thermoregulation
The respiratory system plays a large role in the maintenance of homeostasis. Both the air passing through the lungs and blood in the body can maintain thermoregulation by dissipating heat to the external environment through evaporative cooling. In some animals, such as dogs, the process is made more efficient by panting. Panting brings warm air from the lungs and warm blood in the tongue in contact with the cooler external environment.
For the respiratory system, the nasal and tracheal capillary beds are close to the outside of the body, which causes heat to be released. These capillaries can also be expanded or contracted as a response to being too cold or too hot. When the body is too hot, the capillaries expand, causing more blood to flow through these external capillaries and lose heat. When this occurs, it is known as vasodilation, and the opposite, capillaries getting smaller, is known as vasoconstriction. The role of blood vessels in thermoregulation is not limited to the respiratory system, but it is one of the ways the respiratory system is able to help control body temperature.
d) Protection against disease
The respiratory system can also prevent diseases and small particles from entering the body. In the path of airflow in and out of the body, there are two zones: the conduction zone and the respiratory zone.
The conduction zone consists of the anatomy spanning from the nose/mouth to the primary bronchi. This is known as the conduction zone because the cells are too thick to facilitate meaningful gas exchange, and the function is to simply bring the air to the alveoli.
In the conduction zone, particles are filtered using two primary methods: mucus membranes and cilia. Small nose hairs known as cilia waft mucus and any trapped particles upwards and outwards using the ciliary escalator. Specialized cells known as goblet cells secrete a sticky mucus, and other cells known as epithelial cells have cilia which sweep the mucus towards the pharynx. Together, the mucus traps the particles and pathogens and moves to the pharynx, where it can be either swallowed or coughed out.
In the respiratory zone, which spans from the bronchi to the alveoli, a mucus membrane or cilia would interfere with gas exchange, so another mechanism of disease protection must be present. Here, alveolar macrophages, which roam around in the alveoli, engulf any foreign particles that they encounter.
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