###### Created: 2024-01-12 14:59 ###### Areas & Topics: #medicine #anatomy #physiology ###### Note Type: #permanent ###### Connected to: [[]] - A capillary is a type of blood vessel. - Capillaries are the smallest type of blood vessel and act as the connection and exchange site between arteries and veins. ### History - The word capillary comes from the Latin 'capillaris' meaning 'of or resembling hair' due to how small they are. - Capillaries were first theorised to exist by William Harvey as he saw the need for a connection between the arterial and venous system, but were only discovered and correctly described in 1661 by the Italian physician Marcello Malpighi when he saw them in frog's lungs. ### Embryology - In the womb, the foetus develops new capillaries via a process known as [[Vasculogenesis]] - After birth, capillaries are still able to form from existing blood vessels in a process known as [[Angiogenesis]] ### Structure - Capillaries are made up of only a tunica intima (the inner lining of blood vessels), making them very small and thin. - Due to this, they are typically 5 to 10 micrometres in diameter and one cell thick. - Being incredibly small and thin allows the simple diffusion of gases and various blood products quickly to the tissues they are supplying. - The network of capillaries which form as the vessels pass through a tissue is known as a capillary bed. - The more metabolically active a tissue is (i.e. the more activity that goes on within it and therefore the more substances it needs or waste it produces), the larger the network of capillaries or capillary bed there will be to supply it. ### How Capillaries Connect Arteries and Veins 1. As [[Arteries]] begin to enter tissues, they branch off into smaller vessels known as [[Arterioles]] 2. These [[Arterioles]] then branch off further and become capillaries as they pass by tissues 3. The structure of capillaries allows a wide range of substances contained within the blood (including oxygen, other gases and various solutes) to diffuse into the tissues as needed. 4. The type of capillary and its structure determines what substances are able to pass from the blood and into the tissues. 5. Once the substances in the blood have passed by and diffused into the tissues, the blood the continues through, leaving the capillaries and entering the Venules and then [[Veins]], before recirculating and becoming oxygenated and infused with solutes once again. ### Capillary Types - All types of capillary have a basic underlying structure including some form on endothelial layer (tunica intima), basement membrane and intercellular clefts or gaps. - The amount of gaps or pores in the endothelium and basement membrane is the main feature which differentiates the types of capillary. - Places in the body which require larger substances to be passed from the blood (such as blood cells or larger proteins) will typically contain a larger amount of capillaries which possess bigger gaps in their endothelium. **Continuous** - Continuous capillaries have endothelial linings which are uninterrupted. - Because they are uninterrupted, they only allow the passage of very small molecules through their intercellular clefts, such as water or ions, to pass through them. **Fenestrated** - Fenestrated capillaries have small openings or pores in their endothelium. - This allows the passage of slightly larger substances, such as small molecules or some proteins to pass through. - The word fenestrated comes from the Latin 'fenestrae' meaning 'windows', so the small gaps can be thought of as little windows in the epithelium. **Sinusoidal** - Sinusoidal capillaries are also known as sinusoids or open-pore capillaries. - They are characterised by the large fenestrations in their epithelium and their incomplete basement membrane. - These large gaps allow much bigger solutes, such as red and white blood cells or larger proteins to pass through ![[2104_Three_Major_Capillary_Types.jpg]] ![[21788-continuous-capillary-illustration.webp]] ### Function **Regulation** - Due to their structure, capillaries are relatively weak vessels. - This means that for them to function effectively, the pressure and blood flow within them needs to be kept relatively constant. - Too little pressure won't allow substances to adequately diffuse into tissues and too much pressure can lead to the capillaries bursting or becoming damaged. - Capillaries have therefore evolved methods of autoregulation, which is where blood flow is kept constant despite changing central blood pressure. - This is mainly achieved in the body by a myogenic (muscular) response (see [[The Myogenic Response of Blood Vessels (The Bayliss Effect)]]) in [[Arteries]] and [[Arterioles]], and by tubuloglomerular feedback in the kidneys. **Transport** - Transport of substances through capillaries depends on multiple factors. - Larger substances (typically bigger than 3nm, such as albumin) require cells and protein to help facilitate their transport through capillary walls (this is known as transcellular transport). - Smaller substances (typically smaller than 3nm, such as water molecules or ions) are small enough to pass through the intercellular clefts of capillaries unassisted (this is known as paracellular transport). - The main mechanism which drives movement of these smaller substances is diffusion. - The [[Starling Equation]] describes the different factors which influence the diffusion of substances between the blood in capillaries and the outside tissues and fluids. ![[2108_Capillary_Exchange.jpg]] ### Resources https://en.wikipedia.org/wiki/Capillary https://www.elsevier.com/resources/anatomy/hepatic-lobule/micro-anatomy/sinusoid/15748