Homeostasis and the human kidney

Inhalt

1. What is homeostasis and why is it important?

2. Describe the kidney as a homeostatic organ

3. Nephron – structure and function
Ultrafiltration
Reabsorption in the proximal convoluted tubule
The role of the loop of Henle
Regulation in the distal convoluted tubule
Water reabsorption in the collecting duct

4. Hunger and thirst

5. Antidiuretic hormone (ADH) and aldosterone

1. What is homeostasis and why is it important?

Homeostasis is the ability of an organism to maintain a relatively constant internal environment despite changes in and exchanges with the external environment. The importance of a stable internal environment was emphasised by the French physiologist Claude Bernard as early as 1859. By maintaining a relatively stable internal environment, complex multicellular animals are able to live freely in changing external environments. The American physiologist Walter Cannon (1871-1945) called this stable state of the internal environment homeostasis, from the Greek words homeo (same) and stasis (staying). Homeostasis is dynamic and it is the result of compensating regulatory responses performed by homestatic control systems.

Reasons for the importance of homeostasis:

- If for example the pH of the body and the body temperature are kept constant, then enzymes can be maintained at their optima.
- It maintains an equilibrium between various substances in the body.
- It allows a considerable degree of independence from abiotic factors such as temperature and it also enables organisms such as mammals to live in areas ranging from the arctic to the tropics.
- It is necessary for osmoregulation in order to keep a balance of salts and water.
- It maintains the supply of hormones e.g. thyroid.
- It maintains the supply of nutrients such as glucose.

The homeostatic control systems are feedback mechanisms. The term feedback is used for a variety of situations in which the rate of a process is affected by its end products or consequences. Negative-feedback control systems are the most common homeostatic mechanisms in the body. Like all of the many different physiological control systems the negative-feedback system incorporates the following features: The regulated variable is monitored by sensors or receptors that pass information into an integrator or detector which then compares the sensor’s input with the setpoint. If there is a difference between the two, the integrator generates an error signal that is usually proportional to the magnitude of the difference. The effector’s response completes a negative feedback loop that runs from the regulated variable through the sensor to the integrator and back to the regulated variable by the way of the effector. This means that information is fed back to the integrator from the effector. Such systems are sometimes referred to a closed loop systems. The feedback is called ‘negative’ not only because it stops the effector doing one thing and stimulating it to do the opposite, but also because it often involves doing the opposite to what is happening in the external environment e.g. a very cold environment leads to the response of an increased internal temperature. In physiological control systems the sensors may be nerve or gland cells, the integrators may be at almost any place in the body. The error signal may take the form of nerve impulses or chemical substances carried in the blood over long distances (hormones) or diffusing over short distances between cells (paracrine and autocrine agents). The effectors may be muscles or exocrine glands – those whose secretions pass to the physiological exterior. Examples of the negative-feedback system include the control of the heart rate, blood pressure, water balance and blood glucose levels.

Negative-feedback systems stabilize variables near their setpoints because the response of the effector minimizes the error signal. In positive-feedback, another homeostatic mechanism, a change in the regulated variable causes the effector to drive it even further away from the initial value. Thus positive-feedback does not favour stability and often abruptly displaces a system away from its normal setpoint. Destructive positive feedback commonly occurs in disease, where it results in very rapid deterioration of homeostasis, but positive feedback is not always abnormal. Typically responses driven to an extreme by positive feedback are either self terminating (like labour contractions) or are brought back into control by separate negative feedback systems.

It has to be noted that homeostatic control systems cannot maintain complete constancy of any given feature of the internal environment. Therefore, any regulated variable will have a more-or-less narrow range of normal values depending on the external environmental conditions. Also the setpoint of some variables regulated by homeostatic control systems can be reset – that is, physiologically raised or lowered. It is not possible for everything to be maintained relatively constant by homeostatic control systems. There is a hierarchy of importance, such that the constancy of certain variables may be altered markedly to maintain others at relatively constant levels.

2. Describe the kidney as a homeostatic organ.

The kidneys are the major excretory and osmoregulatory homeostatic organs of the mammalian body. Their functions include:

- Osmoregulation – regulation of water and inorganic-ion balance. They do this by excreting just enough water and inorganic ions to keep the amounts of these substances in the body relatively constant.
- Removal of metabolic waste products from the blood and their excretion in the urine. This prevents waste products, which can be toxic, from accumulating in the body. These metabolic wastes include urea from the catabolism of protein, uric acid from nucleic acids, creatinine from muscle creatine, the end products of haemoglobin breakdown (which give urine much of its colour), and many others.

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