Our body is made of 2/3 water which is carefully distributed between different compartments. Any disruption to the balance between these fluids can cause a problem. Osmoregulation is the homeostatic process for preserving the balance between these compartments.
Osmoregulation and its importance
Before talking about osmoregulation, we need to define osmolality and osmotic pressure.
Osmolality refers to the measurement of the number of dissolved particles in moles per litre of fluid.
Osmotic pressure is the pressure that needs to be applied to a solution to prevent the inward movement of water from another, less concentrated solution when both solutions are separated with a semipermeable membrane, such as a cell membrane.
Osmotic pressure is determined by osmolality. A higher osmolality of a solution results in higher osmotic pressure.
Fig. 1 - The concept of osmotic pressure
Osmoregulation is an active homeostatic regulation of the osmotic pressure of the body fluids within organisms. Since the osmotic pressure determines water movement, osmoregulation in effect allows maintenance of the fluid balance and the concentration of electrolytes in the body. As mentioned in our homeostasis article, homeostatic mechanisms, including osmoregulation, requires four elements to be functional. These include a sensor, a control centre, an effector, and a feedback system.
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.
Electrolytes are essential minerals that carry an electric charge. Electrolytes will help your body regulate pH levels, keep you hydrated and others. Examples of electrolytes include chloride, magnesium, calcium etc. You have probably seen sports drinks such as Lucozade advertising electrolytes in their drinks to give you a boost. But don’t worry, you don’t need Lucozade to have enough electrolytes; a healthy diet will provide you with all essentials.
However, if you start getting low on electrolytes, it will impair your body function. It can cause acid imbalances, muscle contractions, blood clotting and others. The symptoms include fast heart rate, fatigue, nausea and others.
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:
- Osmoregulation: Regulating water content of the blood.
- 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.
- pH regulation: By controlling the excretion and reabsorption of bicarbonate, kidneys regulate the blood pH.
- 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
Structure | Description |
Fibrous capsule | A protective membrane that surrounds the kidney. |
Cortex | The light-coloured outer region of the kidney. The cortex is made of the Bowman’s capsules, convoluted tubules, and blood vessels. |
Medulla | The darker coloured inner region of the kidney is the shape of multiple pyramids. The medulla consists of loops of Henle, collecting ducts and blood vessels. |
Renal pelvis | A funnel-shaped cavity where the collecting ducts end into. The urine collects here before entering the ureter. |
Ureter | A urinary tube that carries the urine from the kidney to the bladder. |
Renal artery | The renal artery is a direct branch of the abdominal aorta. It supplies the kidney with oxygenated blood. |
Renal vein | The renal vein returns blood from the kidney and drains directly into the inferior vena cava. |
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.
Blood vessels | Description |
Afferent arteriole | A small artery arising from the renal artery. The afferent arteriole enters the Bowman’s capsule and forms the glomerulus. |
Glomerulus | A very dense network of capillaries arising from the afferent arteriole where fluid from the blood is filtered into the Bowman’s capsule. The glomerular capillaries merge to form the efferent arteriole. |
Efferent arteriole | A small artery arising from the recombination of glomerular capillaries. The narrow diameter of the efferent arteriole increases the blood pressure in the glomerular capillaries allowing more fluids to be filtered. The efferent arteriole gives off many branches forming the blood capillaries. |
Blood capillaries | These blood capillaries originate from the efferent arteriole and surround the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. These capillaries allow the reabsorption of substances from the nephron back into the blood and the excretion of waste products into the nephron. |
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