Osmoregulation

Types of osmoregulation

Organisms are divided into two groups based on the type of their osmoregulation. These two groups are osmoconformers and osmoregulators.

Osmoconformers include marine invertebrates. They adjust their body’s osmolality to match their environment even though the ionic composition inside their body may be different from that of their surroundings.

On the other hand, osmoregulators include mammals, fish, and most animals in general. Osmoregulators tightly regulate an internal osmolality that is different from their environment. These organisms have specialised organs that actively control the uptake and excretion of salt to maintain their body’s constant osmolality.

Osmoregulation in humans

About 60% of the human body is composed of fluids. This amount may slightly vary between individuals based on their gender, age, and lean muscle mass.

The fluids in the human body are separated between two main compartments, inside and outside of the cells. Approximately 2/3 of our body’s water content is in our intracellular fluids (ICF); the remaining 1/3 forms our extracellular fluid (ECF). ECF consists of the fluid between cells (interstitial fluid) and the blood plasma. A disruption in the osmotic pressure of any of these compartments can result in an imbalance in the movement of water between them and hence alter the concentration of their electrolytes. Furthermore, a fall in the plasma volume can lead to low blood pressure with severe consequences.

Fig. 2 - Hypothalamus location

Osmoreceptors detect changes in the osmotic pressure of the blood in the hypothalamus. These changes are then relayed to the control centre in the hypothalamus. If the blood is too concentrated, the osmoreceptors detect this, and the hypothalamus responds by stimulating thirst and increasing the release of the antidiuretic hormone (ADH). ADH is an endocrine hormone (a messenger that is released straight into the bloodstream) that targets the kidneys and increases water reabsorption from the urine. If the blood is detected to be too diluted, the hypothalamus decreases ADH release, allowing more water to be excreted in the urine.

This mechanism is controlled by negative feedback. As soon as the osmotic pressure in the blood is restored to its optimum value, the response from the hypothalamus returns to its baseline value as well. This process allows the osmolality of the blood to be maintained at a relatively constant value.

Fig. 3 - Regulation of body water levels by negative feedback driven by the antidiuretic hormone (ADH)

Structure and role of kidneys

Mammals have two kidneys located in the rear of the abdominal cavity on either side of the spinal cord. Kidneys are essential organs that have four main functions:

1. Osmoregulation: Regulating water content of the blood.
2. Excretion: Kidneys excrete metabolic waste products, such as urea, as well as substances that are in excesses in the body, such as sodium or potassium ions.
3. pH regulation: By controlling the excretion and reabsorption of bicarbonate, kidneys regulate the blood pH.
4. Endocrine secretion: Kidneys are also endocrine glands. They release erythropoietin (EPO) hormone, which acts on the bone marrow and increases the number of red blood cells.

Structure of kidneys

The kidney comprises several structures, as seen in the schematic diagram below.

Fig. 4 - Anatomy of the kidney and the structures within it

These structures and their descriptions are summarised in the table below.

Table 1. Structures of the kidney.

Key:

• Red - An artery with oxygenated blood
• Blue - A vein with deoxygenated blood
• Yellow - Other structures

The nephron: definition and structure

The nephron is the functional unit of the kidney. It consists of a 14 mm tube with a narrow radius closed at both ends. The nephron consists of different regions, each with different functions. These structures include:

• The Bowman’s capsule: The beginning of the nephron is surrounded by a dense network of blood capillaries called the glomerulus. The inner layer of Bowman’s capsule is lined with specialised cells called podocytes which prevent the passage of large particles such as cells from the blood into the nephron.
• Proximal convoluted tubule: The continuation of the nephron from the Bowman’s capsule. This region contains highly twisted tubules that are surrounded by blood capillaries. Furthermore, the epithelial cells lining the proximal convoluted tubules have microvilli to enhance the reabsorption of substances from the filtrate.
• Loop of Henle: A long U-shaped loop that extends from the cortex deep into the medulla and back into the cortex again. This loop is surrounded by blood capillaries and plays an important part in establishing the corticomedullary gradient.

• Distal convoluted tubule: The continuation of the loop of Henle lined with epithelial cells. Fewer capillaries surround the tubules in this region than the proximal convoluted tubules.
• Collecting duct: A tube which multiple distal convoluted tubules drain into. The collecting duct carries urine and eventually drains into the renal pelvis.

Various blood vessels are associated with different regions of the nephron. The table below shows the name and description of these blood vessels.

Table 2. Blood vessels are associated with the nephron.