Plus-Hex CLINICAL
Introduction
Fluid therapy can help restore and maintain fluid levels within the body. It is commonly used in practice, in a number of disciplines, to help support patients through surgical procedures, during in-patient care and in medical emergencies.
Fluid therapy is an important element of veterinary treatment, with different types of formulations of fluids used in different scenarios, depending on the desired outcome of the treatment.
This article will explore the different elements of fluid therapy, including intravenous fluid therapy( IVFT), and will also provide calculations for use in practice.
Physiology of fluid movement around the body
The circulatory system plays a substantial role in transporting water throughout the body. In healthy animals, water makes up between 50 % and 80 % of the bodyweight, depending on the age of the animal [ 1 ]. The water is distributed around the body and is found within intracellular and extracellular spaces, which are separated by semi-permeable membranes [ 2 ].
The semi-permeable membranes allow the movement of water through the process of osmosis. Osmosis moves water molecules from areas levels of low solute concentration to areas of higher solute concentration, helping to dilute the solute concentration to restore an equilibrium( balance). Osmosis continues until the two concentrations are equal [ 3 ].
In some instances, such as certain diseases, the body can struggle to transfer water between spaces. The semi-permeable membranes can allow fluid to leak from an area, moving water out of the circulation, so it collects in areas of the body. The fluid can then sit in the interstitium, in which case it is known as oedema, or it can collect in body cavities, where it is known as ascites [ 1 ].
Intracellular fluid, or the fluid found within cells, accounts for around 40 % of a patient ' s bodyweight. Extracellular fluid, the fluid located outside cells, makes up 20 % of the bodyweight [ 1 ]. Water within the body is constantly moving, transferring between cells and into the tissues. However, water is also continually lost and needs to be replaced to maintain homeostasis [ 3 ].
Water is lost from the body through different functions, such as respiration, urination and passing faeces, and is replaced by drinking fluids and extracting water from food [ 3 ]. When fluid losses exceed fluid intake, dehydration can occur. When fluid losses occur due to disease or blood loss, hypovolaemic shock can develop due to a reduction in the circulating volume [ 3 ].
A sound understanding of water movement in the body can help veterinary professionals decide which type of fluids could be prescribed to the patient and understand the effect they may have on the body.
Different types of fluid therapy
Different types of fluid formulations can be prescribed by a veterinary surgeon( VS), depending on the outcome that is required. Each formulation will have a different effect on the patient ' s body and will be processed by the body systems in different ways.
Crystalloid solutions
Different types of crystalloid solutions can be administered to patients, depending on the diagnosis they have received. A crystalloid solution contains electrolytes; these are small molecules and are similar to the solutes that can be found within normal body fluids. Sodium is often found in crystalloid solutions as it aids osmolality, helping the fluid to move throughout the body [ 1 ].
Crystalloid solutions can be divided into three different categories: isotonic, hypotonic and hypertonic.
Isotonic crystalloid fluids are the most commonly used fluids for IVFT within small animal veterinary practice. They include saline, Hartmann ' s and Ringer ' s solutions. They can be used in the treatment of a number of conditions, such as hypovolaemia, shock and dehydration. Isotonic crystalloid solutions can be used to replace fluid deficits, and large volumes can be given over a short space of time.
Hartmann ' s solution contains a number of different electrolytes, including sodium, potassium, calcium, sodium and chloride, making it a good supportive treatment for patients with an electrolyte imbalance. Most general cases requiring fluids are likely to receive Hartmann ' s solution, including patients undergoing surgery or patients with diarrhoea, vomiting or dehydration.
The formulation of isotonic solutions is similar to the composition of extracellular fluid, with Hartmann ' s solution containing electrolytes and a lactate buffer that helps regulate the pH. Isotonic solutions also have a similar osmolality to plasma, so when administered they do not change the intracellular volume [ 1 ].
Hypotonic crystalloid fluids have a lower osmolality than plasma and, when administered, can result in water moving into the interstitial and intracellular spaces [ 1 ]. They are usually used in cases where water loss and dehydration are suspected, without hypoperfusion [ 1 ]. These solutions allow water to enter the cells of the body, so intracellular water accumulates [ 4 ].
Volume 41( 1) • February 2026
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