Membranes

In this lesson we’re going to look at membranes in the body and what they do.

The overall functions of membranes are to line body cavities, cover organs, and line the heart, blood vessels, and lymph vessels. There are two types of membranes – mucus membranes, which secrete a mucus substance, and serous membranes, which secrete a watery substance. Membranes are composed of epithelial tissue that is attached to another structure like muscle or connective tissue.

So let’s first talk about mucus membranes. Mucus membranes line OPEN body cavities. What does that mean? An open body cavity is one that has a continuity or opens up to the outside of the body. So the buccal cavity is the mouth, the nasal cavity is the nose – they open to the outside, right? The respiratory tract opens to the outside as well through those same openings. We also see an opening at both ends of the digestive tract in the mouth and the anal canal. And the genitourinary system includes both the urinary or excretory system and the reproductive system. All of these body systems are lined with mucus membranes.

So what are mucus membranes made of and what do they do? Mucus membranes are composed of any type of epithelial tissue except simple squamous or stratified cuboidal. If you need a refresher on this, make sure you check out the types of epithelial tissues lesson. In terms of the mucus secreting part of this membrane, it could either be with goblet cells or with actual mucus glands. Then that epithelial tissue is connected by areolar connective tissue to smooth or skeletal muscle. Mucus membranes serve to protect, secrete mucus, and have a role in absorption, especially in the digestive tract.

So now let’s talk about serous membranes. Serous membranes serve to line CLOSED body cavities. So by deduction we can figure out that closed body cavities are ones that DON’T open to the outside, right? This includes our thoracic cavity, which goes from the top of the diaphragm to the top of the lungs and all the way around. And our abdominopelvic cavity which goes from the bottom of the diaphragm to the top of the bladder and all the way around. We also have a sac around our heart called the pericardial sac, which is a serous membrane. We also see our internal organs covered by serous membranes and the inner linings of our heart, blood vessels, and lymph vessels are lined with them as well. So let’s look more specifically at a couple of types of serous membranes you need to know about.

The first is pleura or pleural membranes, which are associated with the lungs. What you’ll see which each of these is you have two types – parietal membranes and visceral membranes. Parietal means the cavity or space. So the parietal pleura lines the entire thoracic cavity. It’s connect to the inside of our ribs and intercostal muscles. Then, visceral means the organ – so the visceral pleura covers the lungs themselves. When you check out the breathing movements lesson in the respiratory module of this course, you’ll also see that there’s a fluid in here between these membranes that helps keep them stuck together – that’s called the pleural fluid and it’s the watery substance secreted by these membranes.

Next is peritoneum – the peritoneum is associated with the abdominopelvic cavity. So, again, parietal means cavity – so the parietal peritoneum surrounds the abdominopelvic cavity. That goes from the bottom of the diaphragm down to the top of the bladder. And visceral means organ – so the visceral peritoneum covers the organs in these cavities. Now, you’ll notice that the bladder is actually below this membrane. Well there is actually something called a retroperitoneal structure. Retro means “behind” – so what we actually see here is that the entire genitourinary system – so the kidneys, ureters, bladder, and all reproductive organs – are OUTSIDE of this parietal peritoneum membrane. So we call them retroperitoneal.

Last is the pericardium, which is associated with the heart. Think peri means around, cardium means heart – so around the heart! Parietal pericardium is actual the sac around the heart, also called the pericardial sac. Then the visceral pericardium surrounds the heart itself – and just like in the pleural space, there’s fluid in between here as well called the pericardial fluid – that helps to cushion and protect the heart from friction while it’s beating. Now when it comes to the heart and blood vessels, we also have endocardium inside the heart and endothelium inside the blood vessels that are serous membranes.

So serous membranes are SUPER thin, guys – they are only made of simple squamous epithelium. Remember from those lessons that this is a single layer of thin, flat cells. These serous membranes serve as protection against friction because of the watery substance they secrete – it’s like a slip and slide. They also help to suspend organs in place and can help with fluid balance because they can filter and absorb fluid. We also see the pleural membranes helping to keep the lungs expanded and the endothelium inside the blood vessels helps to prevent blood from clotting. So these membranes are a HUGE part of keeping all of our organs working right every day – and we don’t even know it!
So make sure you know that membranes serve to line and cover our cavities and organs. There are two main types – mucus and serous. And remember that serous membranes are either visceral or parietal depending on whether they cover an organ or line a cavity.

That’s it for our lesson on membranes. Make sure you check out all the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!