Hemodynamic factor
This mechanism is based on the disruption of the delicate balance of the so-called Starling forces directly within the microvasculature. The main causal factors here are elevated venous pressure and hypervolemia (pathological increase in circulating blood volume).
How does this work at the microvessel level?
- In arterioles and precapillaries: the effective hydrostatic pressure significantly exceeds the effective oncotic reabsorbing force of the plasma. As a result, there is a sharp increase in the filtration of the fluid part of the blood through the vascular wall into the interstitial space.
- In venules and postcapillaries: the effective hydrostatic pressure becomes equal to or even exceeds the oncotic reabsorbing force. Due to this physiological shift, the resorption (reabsorption) of fluid from the interstitium back into the vascular bed is slowed down or completely halted.
The outcome of these two parallel processes is a massive accumulation of free fluid in the tissues and the formation of pronounced edema.
Lymphogenic (lymphatic) factor
This type of edema occurs when adequate lymph drainage from peripheral tissues is impaired. There are two major causes for this condition: the appearance of a mechanical barrier in the path of the lymph, or its excessive formation, leading to the overload of the entire lymphatic system.
The etiology of the lymphogenic factor involves several groups of causes:
- Developmental anomalies: for example, congenital hypoplasia of lymph nodes and vessels, due to which the system is initially incapable of pumping the required volume of fluid.
- Mechanical disorders: external compression of vessels or obstruction of their lumen. Obstruction can be caused by malignant tumor metastases or parasitic infestations (e.g., filariasis).
- Functional and systemic failures: spastic reactions of lymphatic vessel walls, significant hypoproteinemia, and elevated central venous pressure. High pressure in the veins purely physically impedes the normal drainage of lymph into the venous system.
Pathogenetically, two variants of lymphatic insufficiency are distinguished:
- Mechanical: lymph flow is blocked by a physical barrier (compression or obstruction), leading to lymph stasis and, consequently, edema.
- Dynamic: lymphatic pathways are anatomically completely intact, but the volume of lymph formed in the tissues is so large that the drainage function cannot cope with the overload. Outflow is critically slowed down. Clinically, this is often observed in pronounced hypoproteinemia, in patients with renal failure, or severe nephrotic syndrome.
Oncotic factor
In medical literature, this mechanism is also referred to as the hypoproteinemic or hypoalbuminemic factor. Its main characteristic is the development of edema against the background of a drop in blood oncotic pressure and its simultaneous (or isolated) increase directly in the intercellular fluid.
The activation of the oncotic factor is based on two key pathogenetic processes:
- Hypoproteinemia (hypoalbuminemia): a critical decrease in plasma protein concentration. First and foremost, this concerns albumins, which play the primary role in retaining water inside blood vessels.
- Tissue changes: protein molecules located in the interstitial fluid acquire increased hydrophilicity (the ability to bind water). This condition is called interstitial hyperonchia.
The implementation mechanism is extremely simple and logical: due to the combination of protein deficiency in the vessels and its excessive activity in the tissues, the effective oncotic reabsorbing force of the plasma catastrophically decreases. Fluid can no longer be retained inside the vascular bed and begins to be actively drawn into the tissues. As a result of this redistribution, persistent edema is formed.
Briefly: answers to topical questions
What pathogenesis factors of edema exist besides hemodynamic, lymphogenic, and oncotic?
In addition to hemodynamic, lymphogenic, and oncotic factors, membranogenic and osmotic factors of edema pathogenesis are distinguished.
- Membranogenic factor — based on a substantial increase in the permeability of microvascular walls, leading to the exit of fluid and proteins from microvessels into the interstitium.
- Osmotic factor — develops due to an increase in osmotic pressure in the intercellular fluid caused by the accumulation of ions and osmo-active metabolites.
Which mediators cause increased vascular wall permeability in membranogenic edema?
Increased vascular wall permeability in membranogenic edema is caused by biologically active substances that increase interendothelial gaps.
The main mediators include:
- Histamine — causes vasodilation and a sharp increase in permeability.
- Bradykinin — along with histamine, promotes the exit of exudate into tissues.
- Serotonin — increases interendothelial gaps in microvessels.
- Prostaglandins — not only directly increase interendothelial gaps but also potentiate the effect of histamine and bradykinin on vascular permeability.
Which hormones cause sodium and water retention in systemic edema?
Sodium and water retention in systemic edema is caused by aldosterone, antidiuretic hormone, and deoxycorticosterone.
- Aldosterone — acts on the renal tubules, significantly enhancing the reabsorption of sodium ions and water.
- Antidiuretic hormone (vasopressin) — produced by the hypothalamo-hypophyseal system in response to hyperosmolality or hypovolemia, stimulates water reabsorption in the distal renal tubules.
- Deoxycorticosterone — upon hypersecretion, promotes the retention of sodium ions in tissues, which secondarily causes water retention.
For quick memorization of the three main factors of edema development, use the abbreviation HLO: Hemodynamic (blood pressure), Oncotic (plasma proteins), Lymphogenic (lymph drainage).
Class and exam questions
What is the essence of the hemodynamic factor?
Through an increase in hydrostatic blood pressure. Because of this, fluid filtration into tissues increases in arterioles, and its reabsorption is inhibited in venules.
How do mechanical and dynamic lymphatic insufficiencies differ?
In mechanical insufficiency, there is a physical obstacle to outflow (compression, metastases, parasites). In dynamic insufficiency, the vessels are intact, but they cannot cope with the huge volume of locally formed lymph.
Why do edemas occur in nephrotic syndrome?
Due to protein loss, hypoproteinemia develops. This triggers the oncotic factor (plasma reabsorbing force drops) and dynamic lymphatic insufficiency due to the overload of lymph drainage.
What else to study on this topic
- Changes in Starling forces at the level of precapillaries and postcapillaries
- The role of central venous pressure in blocking lymph drainage
- The effect of tissue hyperonchia and protein hydrophilicity on water retention
- Etiology of lymphatic pathway obstruction (metastasis, parasitic infestations)
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