# Capillaries --- **Capillaries** are the smallest of [[blood vessels]], and are the structure that connect where [[arteries]] end and [[veins]] begin. The amount of blood entering the capillaries, and thus the amount of nutrients available to the tissues, is called [[tissue perfusion|perfusion]]. Interwoven into the capillary beds are [[lymphatic vessels|lymphatic capillaries]]. ![[capillary bed.png]] >[!image]- TEM scan of a capillary ![[RBC inside capillary.png]] [[electron microscope|Transmission electron microscope]] image of a thin section cut through the pancreas(mammalian). This image shows a capillary within the pancreatic tissue (acinar cells in this image). Note the abundance of rough endoplasmic reticulum in the acinar cells. There is a red blood cell within the capillary. The capillary lining consists of long, thin endothelial cells, connected by tight junctions. The image shows fenestration of these endothelial cells. The image also shows synaptic vesicles in the neuron(nerve cell) next to the capillary. JEOL 100CX ## Anatomy A **capillary bed** is a network of 50-100 individual capillaries that function together. A single capillary bed connects each [[arteries|arteriole]] to it's mate [[veins|veniole]]. Although it is not always so readily visually apparent (as it is with the below image), the single main vessel connecting the arteriole and the veniole is called the **thoroughfare channel**. There are several other vessels that branch off of this channel that are called **true capillaries**. Each true capillary branches off and merges back together in a sort of jumble, before finally all remerging with the thoroughfare channel. ![[capillary.png]] ### Shunting A small band of smooth muscle called a **precapillary sphincter** around where each true capillary branches off of the thoroughfare channel acts as an "On/Off" switch for the capillary. If the sphincters are open more blood cn flow through the capillary, and when it's closed only a small amount of blood flows. Even without any outside influences the sphincters cycle open and close at a rate of 5-10 cycles per minute, which is called **vasomotion**. At any given time only about 1/4 of capillary beds in the body are open. If they all opened at once the amount of blood entering the capillary beds would cause [[shock|hypovolemic shock]]. (My question is, does vasodilation/constriction affect the opening and closing of the capillaries themselves? Or does that have more to do with the arteries?) ## Physiology Capillary beds are where **capillary exchange** occurs. This is the process of exchanging fluids, nutrients, [[tissue gas exchange|gasses]] and waste products between the blood in the capillaries and the [[body fluids|interstitial fluids]]. **Net filtration** Trade gap = the stuff leaving the blood outways the amount that comes back in. this stuff goes to the lymphatic system ![[lecture notes - bulk flow.png]] ## Types of Capillary There are three types of capillary, depending on how permeable they are to substances. - **Continuous capillaries** are the most common, and only allow fluids and small solutes to pass through. - **Fenestrated capillaries** have tiny holes in the membranes, which allow larger solutes through—an example is the [[glomerular filtration|filtration membrane]] of the [[kidneys]]. - **Sinusoidal capillaries** have big gaps that allow formed units in and out of the blood, found in places like [[bone marrow|red bone marrow]], where [[red blood cells|RBCs]] enter the bloodstream. ![[capillary types.png]] ## Capillary Refill **Capillary refill** refers to how quickly a person's capillary beds fill back up again after being squished out. Quick refill is a good indication that the body's tissues are well [[tissue perfusion|perfused]]. To test for capillary refill you take the fingernail and press gently and firmly on it so that it blanches (i.e. looses it's color). When you remove the pressure, observe how long it takes for the color to return. Normal capillary refill is in under two seconds. Bruises that do not blanch indicate damage to the capillaries themselves, such as with a [[pressure injury]]. ___