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Starting with gaining an understanding of the relationship between Capillaries and Veins.
Original Author(s): Josh Turiccki
Last updated: 7th February 2021
Exchanging molecules from the bloodstream to tissues forms the basis of life, delivering nutrients and carrying waste products to be disposed of. This occurs in blood vessels known as capillaries. In this article, we shall look at how molecules move between capillaries and tissues as well as related clinical conditions.
Fick’s law states that the rate of diffusion is proportional to the concentration difference and area available for diffusion. It also states that the rate of diffusion is inversely proportional to the diffusion distance.
The structure of capillaries facilitates efficient exchange, by optimising Fick’s law. To maximise the area available for diffusion, there are many capillaries supplying the same tissue. Moreover, a constant blood flow through the capillaries maintains a large concentration gradient to allow the molecules to be rapidly exchanged with the tissue. Additionally, although a singular capillary has greater resistance, many capillaries in parallel reduce the resistance and allow efficient blood supply to the tissues. Finally, the diffusion distance is minimised as the endothelium of the capillaries is just one cell thick and measures a few micrometres in diameter.
Small, lipid-soluble molecules, such as oxygen and carbon dioxide are able to freely diffuse across the membrane. However, molecules can also be exchanged via specialised channels or pores. The number of these channels or pores can vary depending on the function of the tissue. For example, the renal capillary bed is able to exchange water and electrolytes much more efficiently and selectively than in other capillaries. This is because the kidneys function to regulate ion concentrations and osmolarity while receiving approximately 25% of cardiac output. By OpenStax College [CC BY 3.0
(http://creativecommons.org/licenses/by/3.0)%5D, via Wikimedia Commons
Fig 1 – Diagram showing the structure of capillary walls.
A vital example of gas exchange occurs between the terminal portions of the lungs and pulmonary capillaries. Therefore, pulmonary capillaries possess characteristics that allow for rapid and efficient diffusion. The capillaries optimise the diffusion rate by receiving a constant blood supply. They also have an average membrane thickness of only 0.6 micrometres and form a network of capillaries over the alveoli. Furthermore, the alveoli themselves have an extremely large surface area of seventy square metres to further increase the surface area available for diffusion.
However, common diseases can interfere with this optimisation. A useful way of thinking about these diseases is to frame them with respect to the variables of Fick’s law. For example, some pulmonary diseases cause fibrosis or oedema. This increases the diffusion distance that the molecule has to travel, thus decreasing the diffusion rate. Other diseases, such as emphysema, result in damage to the walls of the alveoli causing them to rupture. This consequently forms one larger air space and decreases the surface area available for gas exchange.
Finally, if the lungs are unable to ventilate correctly, such as in restrictive lung diseases, a shallower concentration gradient is established, and the diffusion rate is impaired.
Fluid movement between the capillaries and tissues is controlled by four forces:
- Blood hydrostatic pressure: the pressure exerted by blood in the capillaries against the capillary wall. This pressure forces fluid out of the capillary.
- Blood colloid osmotic (oncotic) pressure: the pressure exerted by proteins in the blood, mostly by albumin in the capillaries. This pressure is attempting to pull fluid into the blood. Proteins in the plasma are normally too large to diffuse into the interstitium, however, in certain scenarios, such as in inflammation, these proteins can.
- Interstitial hydrostatic pressure: the pressure of the fluid in the interstitium. This pressure forces fluid back into the capillary.
- Interstitial colloid osmotic (oncotic) pressure: the pressure of the proteins in the interstitium. This pressure pulls fluid out of the capillary.By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)%5D, via Wikimedia CommonsFig 2 – Diagram showing the Starling forces that take place over a capillary bed.
Moving on to factors which influence venous return.
Venous return is defined as the flow of blood back to the heart. It is therefore important in maintaining normal circulation.
The heart is a myogenic pump, meaning it is responsible for its own stimulation to pump blood out to the rest of the body. In order for blood to be pumped out of the heart, enough blood must be returned to the heart so that it can be pumped around again in the next cardiac cycle.
Veins are blood vessels which return blood to the heart. The pressure in these veins is the driving force for the filling of the heart. This is known as venous pressure. Venous pressure is affected by two main parameters:
- The rate of blood entering the veins – this is linked to the peripheral resistance in the body.
When resistance is high, there is a slower rate of blood entering the veins, which causes a decrease in venous pressure.
When resistance is low, there is a faster rate of blood entering the veins, which will increase venous pressure.
- The rate at which the heart pumps out blood – this is linked to the cardiac output of the heart.
When cardiac output increases, blood is rapidly pumped out of veins, which reduces venous pressure (as it does not get a chance to rise).
When cardiac output decreases, blood backs up into the venous system. Therefore, the blood volume increases which raises venous pressure.
Factors Affecting Central Venous Pressure
Central venous pressure (CVP) is the blood pressure in the vena cava near the right atrium. Under normal circumstances, the CVP ranges from 2-6mmHg.
Veins are low pressure, low resistance vessels and have high capacitance. These properties allow veins to distend with the increasing pressure of blood, allowing them to maintain the heart’s venous pressure.
Additionally, veins have valves which act to maintain the unidirectional flow of blood. The competency of these valves is important in maintaining venous return as they ensure blood is always flowing towards the heart.
There are various factors which can affect venous pressure and venous return:
- Skeletal Muscle Pump – Peripheral veins work in concert with the muscular contraction to increase venous return to the heart. When muscles (such as the quadriceps) contract (during walking, running etc), the valves are forced open to increase the venous return.
By OpenStax College [CC BY 3.0], via Wikimedia Commons
Fig 1 Skeletal muscle pump of the venous system
- Respiration – During inspiration, venous return increases as the thoracic cavity’s pressure becomes more negative. This reduced intrathoracic pressure draws more blood into the right atrium. This results in greater venous return.
- Venous Compliance – Increased sympathetic activity will reduce venous compliance. This increases the venous pressure and venous return as when blood flow into the veins increases, it cannot dilate to accommodate the increased blood. Instead, pressure in the veins rises and blood flow through the vessels increases to empty the veins faster.
- Blood Volume – The greater the blood volume in the veins, the greater the blood flow and venous pressure. The heart can accommodate the increase in blood volume because of the Frank-Starling mechanism (the greater the stretch, the greater the contractility of the heart).
- The Heart- must be working efficiently to pump blood out of the veins and maintain CVP.
Clinical Relevance – Chronic Varicose Veins
In this condition, the competency of the valves in veins in compromised. This means the valves do not sufficiently close, allowing blood in the veins to flow backwards and accumulate in the veins. This can decrease the venous return.
This commonly affects the superficial veins of the legs, which look engorged and twisted. Blood can pool in the veins to cause bruising and ulceration of the tissue if the pressure becomes excessive.By Blausen Medical Communications, Inc.
(Donated via OTRS, see ticket for details) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)%5D, via Wikimedia Commons
Fig 2 – Diagram showing the difference between normal veins and varicose veins.
TeachMe Physiology – Part of the TeachMe Series
Capillary-Venous Malformation in the Upper Limb
- July 2013
- Pediatric Dermatology 30(5)
Abstract and Figures
Regional capillary malformation of a lower extremity is associated with the overgrowth of bone or soft tissue in several disorders, most commonly Klippel-Trenaunay syndrome and Parkes Weber syndrome. We have observed a subset of patients with a capillary malformation of the leg, minor growth disturbance, and prominent veins. The objective of the current study is to describe a series of patients with regional capillary malformation of the lower extremity in association with phlebectasia. This is a retrospective series of 17 patients diagnosed with capillary-venous malformation of the lower extremity. We excluded patients with clinical or radiographic evidence of lymphatic or arteriovenous malformation. Age, presentation, associated features, radiographic findings, and management were documented. In most patients the capillary malformation covered a large area without sharply demarcated borders. Four patients had one or more discrete, well-defined capillary stains involving less than 5% of the total surface area of the affected lower limb. Prominent veins were most common in the popliteal fossa and on the knee and dorsal foot. Approximately two-thirds of patients had a leg length discrepancy, with the affected leg being longer (n = 6) or shorter (n = 4); in many the affected leg was also slightly larger (n = 8) or smaller (n = 4) in girth. Radiographic imaging showed dilatation of superficial (n = 16), muscular (n = 9), and deep veins (n = 6). We characterize a subset of patients with regional capillary-venous malformation of the lower extremity with prominent veins and minor hypotrophy/hypertrophy that differs from Klippel-Trenaunay syndrome (capillary-lymphatic-venous malformation) but belongs at the minor end of the spectrum of vascular disorders with overgrowth.
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Pediatric Dermatology Vol. 32 No. 2 287–294, 2015
Capillary-Venous Malformation in the Upper Limb
We present a group of patients with regional capillary malformations of the upper limbs and few additional ﬁndings other than prominent veins. We believe that this entity is the upper extremity equivalent of capillary-venous malformation of the lower limb and, likewise, belongs at the minor end of the spectrum of vascular disorders with overgrowth. Capillary-venous malformations (CVMs) are slow-ﬂow vascular anomalies comprised of abnormally dilated capillaries and veins. Patients with CVM exhibit a regional stain, minor limb hypertrophy or hypotrophy, and prominent veins in the absence of a lymphatic anomaly. We recently described a series of patients with CVM of the lower extremity (1). Herein we report the clinical presentation, radiographic imaging, and management of 15 patients with CVM of the upper limb.
We identiﬁed patients with capillary malformations(CMs) and prominent veins of the upper extremity by culling the Vascular Anomalies Center data base at Boston Children’s Hospital from 1999 to 2013.Patients with clinical or radiographic evidence of arteriovenous or lymphatic malformation were excluded. For each patient, we collected data on age, sex, appearance and location of the lesion, associated features, radiographic ﬁndings, and management. The Boston Children’s Hospital Committee on Clinical Investigation approved this study.
Fifteen patients fulﬁlled our inclusion criteria (11girls, 4 boys). The mean age at initial presentation to the Vascular Anomalies Center was 11.1 years. Nine patients were evaluated for a median period of43 months; six patients were assessed on only one occasion. Radiographic studies included magnetic resonance imaging (MRI) (n=10), ultrasonography(n=1), and ascending venography (n=3). Five patients did not have imaging studies. Clinical data and radiographic ﬁndings are listed in Table 1. Eleven patients (73%) had a large, blotchy CMthat was not well demarcated from normal skin(Fig. 1); the other four (27%) had darker, conﬂuent, more sharply marginated stains. Prominent veins were most often noted in the forearm and upper arm. Sixty percent of patients had bony or soft tissue overgrowth of the aﬀected limb. Imaging studies most commonly showed dilatation of superﬁcial (n=10) or intramuscular (n=6) veins. Most patients were asymptomatic; only ﬁve (33%) reported pain.
We describe a series of 15 patients with regional CM of the upper extremity, phlebectasias, and minorhypertrophy or hypotrophy—the counterpart ofCVM in the lower extremity. Most patients withCVM of the upper limb had large, blotchy, ill-deﬁned CMs, but a few had dark, homogeneous,well-circumscribed capillary stains, a pattern notseen in CVM of the lower extremity (1). These darkupper extremity lesions resembled those seen inKlippel–Trenaunay syndrome (KTS) (often called“geographic stains”), which are deep red or violace-ous, sharply deﬁned, and associated with lymphaticanomalies and a high incidence of complications (2).The presence of dark, conﬂuent CMs solely in CVMof the upper limb and not the lower limb may reﬂectdiﬀerences in vascular density and blood vesseldiameter. Limb length discrepancy in CVM of theupper extremity was less common than in CVM of the lower extremity. This may reﬂect a lower rate of formal assessment since length diﬀerences are lessclinically obvious and important in the upper limb.
The diﬀerential diagnosis of CVM of the upper extremity includes Parkes Weber syndrome (PWS)and KTS. Robertson described “gigantism” associated with diﬀuse staining and congenital arteriovenous ﬁstulas in the upper extremity (3), an entity now subsumed by the eponym PWS (4). Capillary stains in CVM and PWS can look similar, but they can be diﬀerentiated by palpation. Patients with PWS have a warm limb, occasionally associated with a thrill; hand-held Doppler or ultrasonography will reveal fast ﬂow (5). Patients with KTS, like those with PWS, have a vascular stain, varicosities, and limb hypertrophy. Capillary stains in KTS are typically more localized, darker in color, well demarcated, and studded with lymphatic vesicles. Furthermore, now that most patients once thought to have KTS have been reclassiﬁed as having congenital lipomatous overgrowth with vascular, epidermal, and skeletal anomalies (CLOVES syndrome) (6), KTS is far less common than PWS in the upper limb (5). In most cases a diagnosis of CVM of the upper extremity can be made clinically; however, the presence of dilated veins on ultrasonography may be helpful in conﬁrming the diagnosis.
Diﬀuse capillary malformation with overgrowth(DCMO) is another recently described vascular disorder with overgrowth (7). Its scattered distribution throughout the body, rather than a regional vascular stain, distinguishes DCMO from CVM. Although these entities are clinically distinct, the molecular basis for DCMO and CVM has not been determined. It is possible that postzygotic mutations in the same signaling pathway may cause both conditions; the alteration in DCMO may occur early in embryogenesis, causing widespread involvement, whereas the mutation in CVM may occur later in development, resulting in regional involvement .
Many patients with CVM of the upper limb are asymptomatic. The most common complaint in our patients was pain, which is usually related to ectatic veins and likely due to vessel turgidity or formation of microthrombi. A minority of patients in our series were interested in therapy. Two of our patients were treated with pulsed dye laser, with improvement(follow-up was not available for the third patient)but with minimal change in the hand. In general, we found that pulsed dye laser treatment of capillary stains involving the upper limb was more eﬀective proximally than distally.
In conclusion, we describe a small series of patientswith CVM of the upper limb consisting of regionalCMs, ectatic veins, and minor overgrowth andundergrowth. These patients, like those with CVMof the lower extremity, have minor clinical ﬁndingsand few complications.
- 1. Uihlein LC, Liang MG, Fishman SJ et al. Capillary-venous malformation in the lower limb. Pediatr Dermatol 2013;30:541–545.
- 2. Maari C, Frieden IJ. Klippel-Trenaunay syndrome: the importance of “geographic stains” in identifying lymphatic disease and risk of complications. J Am AcadDermatol 2004;51:391–398.
- 3. Robertson DJ. Congenital arteriovenous ﬁstulae of theextremities. Ann R Coll Surg Engl 1956;18:73–98.
- 4. Young AE. Combined vascular malformations. InYoung AE, Mulliken JB, eds. Vascular birthmarks:hemangiomas and malformations. Philadelphia: W.B.Saunders, 1988:264.
- 5. Mulliken JB, Young AE. Vascular malformations with overgrowth. In: Mulliken JB, Burrows PE, Fishman SJ,eds. Mulliken & Young’s vascular anomalies: hemangio-mas and malformations, 2nd ed. New York: Oxford University Press, 2013:603–636.
- 6. Alomari AI. Characterization of a distinct syndromethat associates complex truncal overgrowth, vascular, and acral anomalies: a descriptive study of 18cases of CLOVES syndrome. Clin Dysmorphol 2009;18:1–7.
- 7. Lee MS, Liang MG, Mulliken JB. Diﬀuse capillary malformation with overgrowth: a clinical subtype of vascular anomalies with hypertrophy. J Am Acad Dermatol 2013;69:589–594.
- Lily Changchien Uihlein, M.D., J.D.*
- Marilyn G. Liang, M.D.†Steven J.
- Fishman, M.D.‡
- Ahmad I. Alomari, M.D.§
- John B. Mulliken, M.D.–
- *Dermatology Division, Department of Medicine, Loyola University Medical Center, Maywood, Illinois,
- †Dermatology Program, ‡Department of Surgery,
- §Department of Interventional Radiology, and Plastic and Oral Surgery, Boston Children’s Hospital, Boston, Massachusetts
Address correspondence to Marilyn G. Liang, M.D., Boston Children’s Hospital, 300 Longwood Avenue, Boston, MA 02115, ore-mail: firstname.lastname@example.org.
- DermNet NZ – New Zealand
- ENTO KEY | OTOLARYNGOLOGY & OPHTHALMOLOGY INSIGHT
- RADY’S CHILDREN HOSPITAL – USA, CA, San Diego
- MOUNT SINAI
- BOSTON CHILDREN’S HOSPITAL
- MAYO CLINIC, USA, Minnesota, Rochester
- JOHN HOPKINS HOSPITAL – USA, Maryland
- NTERNATIONAL SOCIETY FOR STUDIES OF VASCULAR ANOMALIES
- CLOVES SG
- PROS SG
- M-CM SG – Netherlands
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