Pulmonary Blood Flow

Pulmonary circulation is defined as the blood flow that is directed towards and through the lungs. It is not the flow to several other tissues and organs around the body known as systemic circulation. The blood flow through the pulmonary circulation is as follows; the right ventricle, pulmonary artery, alveoli, pulmonary vein, left atria. This is essential for gas exchange at the lung. It is important to note that bronchial circulation is not the same as pulmonary circulation, as it is still part of the systemic circulation. It is also functionally different as it supplies the pulmonary structures with the exception of the alveoli and therefore is not crucial for gas exchange.

Pulmonary Circulation

The vascular radius, as you may recall, is a crucial player in pulmonary circulation. With this, you may also remember that blood vessel radius is vital for predicting the resistance of flow through the system, which is demonstrated in the equation below. In this section, we will examine several ways to regulate vascular radius.

[latex]R=\frac{1}{r^4}[/latex]

This learning object is included to review and solidify the concepts of respiration anatomy discussed in the previous chapter using a 3-dimensional model. The video will further introduce blood flow in the pulmonary system, more specifically pulmonary circulation. A visualization of the respiratory pathway can be seen at the timestamp of 0:15 – 0:30. Make a note of the steps taken to prepare the air before entering the lower respiratory tract, like moistening and warming. The gas exchange process is also touched on within this video at the timestamp of 0:45 – 2:59. Appreciate the intricate network of capillaries that encompass the alveoli, as these vessels are essential moving forward in blood gas exchange.

Change in Smooth Muscle Function

Blood vessels contain smooth muscle, which can be acted on by nervous, endocrine, and paracrine systems to constrict or dilate the vessels. Local control is the first way that smooth muscle function can be altered, meaning that the alveolar partial pressure of oxygen (PO₂ ) can dictate whether a vessel will dilate or constrict within the lung. This regulatory method’s innervation comes from the autonomic nervous system, which further branches into the sympathetic (SNS) and the parasympathetic nervous systems (PNS). Recall that the SNS signals during ‘fight or flight’ and the PNS signals the body to act for ‘rest and digest’ functions. Before moving forward, think about how the body would want to alter blood vessel radius during each stage and why.

We will now look at how the neural pathways mentioned above will influence endocrine stimulation. The endocrine system controls smooth muscle function through alpha and beta-adrenergic receptors.  During SNS innervation, epinephrine and norepinephrine hormones are released to interact with beta-adrenergic receptors, causing blood vessels to vasoconstrict. On the opposing end, during PNS innervation, acetylcholine is released, causing blood vessels to vasodilate. Now moving from a full-body endocrine situation, it is essential to look at how cells communicate with each other. Paracrine singling is a type of local cell communication produced by a single cell to stimulate change in their close-by neighbouring cells. Regarding the respiratory tract, the trachea houses epithelial cells capable of releasing products into the smooth muscle to regulate radius. The products released are known as epithelial-derived relaxing factors (EpDRF) and lead to smooth muscle vasodilation.

Smooth muscle function is also influenced by the gasses we inspire, transmural pressure, and autonomic reflexes. For example, if oxygen availability decreases and altitude increases, it causes vasodilation to allow more oxygen to enter the system. Simultaneously, an alteration in transmural pressure according to inhalation or exhalation results in a change in smooth muscle radius. Finally, a reflex such as a cough to expel a liquid or solid object from our airways demonstrates vasoconstriction. This is more of a defensive action in the process of protecting the respiratory system from foreign particles. As you can see, there are multiple ways to alter the respiratory system’s smooth muscles to either encourage vasoconstriction or dilation. 

This section of the blood gases unit requires you to actively recall which inputs lead to vasoconstriction and vasodilation, respectively. This quiz is designed to help you classify the type of inputs that will lead to constriction and dilation within the respiratory tract. After reviewing the section, attempt this from memory to determine which inputs may require further review.

Transmural Pressure and the Effect of Vasculature Radius

We have two main types of vessels that can affect vasculature radius. Extra alveolar vessels, including arteries, arterioles, or veins that are not directly associated with the alveoli. These vessels sit in the thoracic cavity and are part of the pulmonary circulation but are not directly associated with the capillaries. In contrast, alveolar vessels are capillaries that are directly associated with the alveoli. These vessels wrap around the alveoli and allow oxygen to pass from the alveoli into the blood. The big difference between these two vessels is their relationship with resistance during inspiration. During inspiration, the resistance in the extra alveolar vessels will be reduced. While in the alveolar vessels, an increase in resistance is experienced.

It can be seen that the alveolar-capillary vessel is crossing over and wrapping around the alveoli, as shown in figure 1. The pressure in the alveoli, in this case, becomes the environmental or transmural pressure and is denoted as P_A. When we inspire, the alveoli fill with air and expand, causing an increase in alveolar pressure (P_A).

Since the capillary is wrapped around the alveoli, we need to consider three pressures when thinking about blood flow through the capillary, the pressure in the alveoli (P_A), the pulmonary artery pressure (P_PulmA), and the pressure in the pulmonary vein (P_PulmV). To simulate flow through the alveoli-wrapped capillaries, we must be in a situation that allows for the following pressure relationship.

[latex]P_{PulmA} > P_{PulmV} > P_A[/latex]

This video provides an in-depth explanation of the different zones within the lungs and how transmural pressure influences capillary blood flow within the pulmonary circulation. The Starling Resistor Model showcases the impact of flow on various pressure changes within the pulmonary system and is analyzed at the timestamp 4:05 – 6:44. During 9:13- 10:54 the video showcases different pressure gradients within the lungs and how these zones lead to no flow, reduced or intermittent flow, and continuous blood flow.

Let’s practice how the uneven distribution of pressures across the lungs impacts the pulmonary capillary blood flow. Work through these questions to see what topics may need some additional review. Remember, to get flow (Q) we must have a larger pressure on the pulmonary artery side compared to the pulmonary vein side of a capillary; pressure one must be larger than pressure to according to the flow equation.

[latex]Q=\frac{P_1−P_2}{R}[/latex]

Pressure and Blood Flow Across the Lung

There is an inverse relationship between the height of the lung and blood flow, which means that the top region of the lung has the least amount of blood and the bottom of the lung gets the most blood flow. However, this only holds true when an individual is in an upright position, and we will examine why this is later in this chapter section.

Many structures have an uneven distribution of pressures across their entire surface, and the lung is no different. When we are standing in an upright position, we can think of the lung as having three distinct flow levels due to pressure differences. At the top of our lungs, we have no flow (Q). This is due to the alveoli’s pressure being more significant than the pulmonary artery pressure, shown in figure 2. While in the middle of the lung, we see intermittent or reduced blood flow, and by the time we reach the bottom of the lung, we see a continuous stream of flow through the vasculature.

Subchapter Quiz

The questions below have been created to assess your knowledge within this chapter. You will find multiple-choice questions that will provide you with responses to your answers. Be sure to review any remaining sticky spots and critical points after you’ve completed them.