What is the Difference Between Sinusoids and Capillaries

The main difference between sinusoids and capillaries is that sinusoids are small blood vessels resembling capillaries, whereas capillaries are the blood vessels with the smallest diameter in the vascular system. Furthermore, sinusoids occur in the liver, adrenal glands, spleen, and bone marrow while capillaries occur in all types of animal organs. 

Sinusoids and capillaries are the smallest types of blood vessels in the vascular system. Generally, their main function is to facilitate the interchange of nutrients and wastes between blood and organs. 

Key Areas Covered 

1. What are Sinusoids
     – Definition, Structure, Function
2. What are Capillaries
     – Definition, Structure, Function
3. What are the Similarities Between Sinusoids and Capillaries
     – Outline of Common Features
4. What is the Difference Between Sinusoids and Capillaries
     – Comparison of Key Differences

Key Terms 

Capillaries, Continuous Capillaries, Discontinuous Capillaries, Fenestrated Capillaries, Sinusoids

What are Sinusoids 

Sinusoids are a slightly larger type of capillaries characterized by the presence of large gaps between endothelial cells and discontinuous or completely absent basal lamina. Therefore, sinusoids are considerably more permeable, allowing rapid exchange of nutrients. Furthermore, sinusoids mainly occur in the liver, adrenal glands, spleen, and bone marrow. Generally, sinusoids are  30–40 µm wide. Moreover, they are open-pore capillaries without a diaphragm to cover the pore. Additionally, due to the presence of a discontinuous basal lamina, they are known as discontinuous capillaries.

Figure 1: Hepatic Sinusoid

Furthermore, the large pores of the sinusoids allow passing blood cells, including red and white blood cells out of the blood. Moreover, they allow escaping serum proteins from the blood. In the liver, sinusoids serve as a location for mixing oxygen-rich blood from the hepatic artery and nutrient-rich blood from the portal vein. Also, Kupffer cells that line the sinusoids in the liver plays a vital function in immunity. 

What are Capillaries  

Capillaries are the smallest blood vessels made up of one-cell thick endothelial layer, supported by a basal lamina. Generally, they convey blood between arterioles and venules. The main function of blood capillaries is to exchange many substances with the interstitial fluid surrounding them. There are three types of blood capillaries: continuous capillaries, fenestrated capillaries, and discontinuous capillaries or sinusoids. Here, the diameter of a continuous capillary is around 5-10 µm. Usually, continuous capillaries occur in many tissues including skeletal muscles, gonads, fingers, and other organs. Their main function is to transport oxygen and glucose out of the capillary while transporting carbon dioxide, uric acid, lactic acid, urea and creatinine into the capillary.

Figure 2: Types of Arteries

Moreover, fenestrated capillaries are an especial type of blood capillaries that occur in endocrine glands, intestines, pancreas, and the glomeruli of the kidney. Basically, the main characteristic feature of these capillaries is the presence of pores known as fenestrae in the endothelial cells. Furthermore, the diameter of these pores is 60-80 nm. However, these pores are covered with diaphragms made up of fibrils, limiting the size of molecules to pass through the capillary wall. Also, the fenestrated capillaries in the renal glomerulus do not contain diaphragms. Cells called podocytes cover the capillaries, acting as diaphragms. However, fenestrated capillaries contain a continuous basal lamina.  

Similarities Between Sinusoids and Capillaries 

• Sinusoids and capillaries are two types of small blood vessels of the vascular system. 
• They occur inside organs. 
• Moreover, they are made up of endothelial cells supported by a basal lamina. 
• Both are classified based on the structure of endothelial cells. 
• Their main function is to facilitate the exchange of nutrients and wastes between the blood and organs through passive diffusion. 

Difference Between Sinusoids and Capillaries 

Definition 

Sinusoids refer to the small, irregularly-shaped blood vessels found in certain organs, especially the liver, while capillaries refer to any of the fine branching blood vessels, which form a network between the arterioles and venules. Thus, this is the main difference between sinusoids and capillaries.

Correspondence 

Moreover, sinusoids or discontinuous capillaries are a type of capillaries, while the three types of capillaries are the continuous capillaries, fenestrated capillaries, and discontinuous capillaries. 

Occurrence 

Sinusoids occur in some organs, including the liver, adrenal glands, spleen, and bone marrow, while other capillaries occur in skeletal muscles, skin, gonads, renal glomerulus, etc. 

Diameter 

The diameter of sinusoids is 30–40 µm,  while the diameter of a regular capillary is 5-10 µm   

Structure 

Furthermore, sinusoids are an especial type of fenestrated capillaries that have larger openings (30–40 μm in diameter) in the endothelium, while capillaries made up of a single layer of endothelial cells supported by a basal lamina. 

Basal Lamina 

The basal lamina is another difference between sinusoids and capillaries. Sinusoids have a discontinuous basal lamina, while capillaries have a basal lamina, supporting endothelial cells. 

Pores 

Besides, sinusoids contain open pores while some other capillaries have pores covered by diaphragms.   

Function 

Also, the sinusoids allow passing both red and white blood cells and various serum proteins while other capillaries only allow passing smaller molecules but not the blood cells and serum proteins. 

Conclusion 

In brief, sinusoids are an especial type of blood capillaries characterized by the presence of a discontinuous basal lamina and large, open-pores between endothelial cells. Moreover, they mainly occur in the liver, spleen, and bone marrow. Due to their structure, they allow passing blood vessels and serum proteins out of the circular system. On the other hand, capillaries are the smallest blood vessels in the vascular system. Apart from sinusoids, the other two types of capillaries are continuous capillaries and fenestrated capillaries. Basically, continuous capillaries occur in many tissues, including skeletal muscles, and other organs while fenestrated capillaries occur in the renal glomerulus. However, they only allow passing small molecules out of the plasma. Therefore, the main difference between sinusoids is the occurrence, structure, and function. 

References:

1. Bengochea, K. “Capillaries.” Kenhub, Kenhub, 11 July 2019, Available Here.

Image Courtesy:

1. “Hepatic structure2” By Originally by Frevert U, Engelmann S, Zougbédé S, Stange J, Ng B, et al. Converted to SVG by Viacheslav Vtyurin who was hired to do so – Based on the research article “Intravital Observation of Plasmodium berghei Sporozoite Infection of the Liver”, PLoS Biology, doi:10.1371/journal.pbio.0030192.g011 (CC BY 2.5) via Commons Wikimedia  
2. “Different Types of Capillaries” By Elizabeth2424 – Own work (CC BY-SA 3.0) via Commons Wikimedia