Homeostasis

The human body can function within a range of conditions: a range of temperatures, a range of levels of exertion, a range of levels of nutrition, etc. But within the body there are a number of parameters which must be maintained within a narrow range. Remarkably, the body's interior fluids are normally kept close to "set point" values of temperature, pressure, and chemical composition. Within the body are a number of control processes that maintain the body within an acceptable range around the set points, and maintaining this overall dynamic equilibrium is called homeostasis. Historically, the French physiologist Claude Bernard recognized this dynamic equilibrium as the "constancy of the interior milieu" in the mid nineteenth century. The term "homeostasis"was coined by Walter B. Cannon in the 1920s. Cannon's book "The Wisdom of the Body" has inspired continuing discussions and literature about homeostasis.

The regulatory processes that keep the physical and chemical parameters within the narrow ranges required for cells to function are generally classified as "feedback systems". Most of these processes are negative feedback systems that counteract changes in the internal environment. Such systems require a sensor to compare current conditions to the set point value, a communication system to provide that information to a control center, and an operational mechanism (effector) that influences the system toward that set point. There are a few positive feedback systems that reinforce changes when it meets a physiological need.

• Sensors
  • Temperature Regulation: Nerve endings in the hypothalamus, abdomen, spinal cord, skin, and large veins.
  • Blood Supply Regulation:
    • Blood Pressure Regulation: The main pressure receptors are specialized stretch receptors in the sinuses (small cavities) within the aorta and the carotid arteries.
    • Blood Distribution:
    • Blood Perfusion into Cells:

• Control Centers
  • Temperature Regulation: Hypothalamus
  • Blood Supply Regulation: the medulla oblongata of the brain with three components.
    • the cardioaccelerator center
    • the cardioinhibitor center
    • the vasomotor center

• Effectors
  • Temperature Regulation: Muscles, which may be signaled to shiver, which generates heat.
    • Vasoconstriction of the blood vessels supplying the skin which reduces heat loss.
    • Increase in metabolic rate to generate heat.
  • Blood Supply Regulation:
    • the cardioaccelerator center stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve.
    • the cardioinhibitor center slows cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve.
    • the vasomotor center controls vessel tone or contraction of arterial walls, mainly by release of the neurotransmitter norepinephrine from sympathetic neurons.

Temperature Regulation

The set point for the temperature of the interior of the human body is 98.6°F (37°C) and the body manages to keep the temperature within about 1°F of that set point value. It is remarkable that even in conditions of extreme discomfort due to cold or heat, the body's interior is kept very close to this set point. The control center which maintains the set point for temperature is the hypothalamus. Nerve endings in the hypothalamus, abdomen, spinal cord, skin, and large veins act as temperature sensors and transmit information to the hypothalamus.

If the body temperature drops, the hypothalamus activates various effector mechanisms to raise the temperature back toward the set point. One type of effector is in the muscles, where shivering will generate heat from muscular activity. Another effector is vasoconstriction of the blood vessels supplying the skin which reduces heat loss. A third effector involves increasing the metabolic rate to produce heat. When normal body temperature is restored, the hypothalamus stimulation ceases.

Because of the thermal energy produced by metabolism, the average human body needs to give off thermal energy at a rate of about 90 watts to maintain equilibrium. This cooling of the body can be accomplished by the physical mechanisms of conduction, convection and radiation as well as by the evaporation of perspiration.

Blood Supply Regulation

Homeostasis for the blood is directed toward ensuring adequate blood flow for the body's needs over a considerable range of circumstances. This requires regulation of blood pressure, distribution, and perfusion into the cells. There are neural, endocrine, and autoregulatory mechanisms. Reference: ER Services.

Blood Pressure Regulation

The standard measurement of blood pressure is in mmHg, and normal blood pressure is about 120/80 mmHg where the top number is the systolic pressure and the bottom the diastolic pressure. Those are typical effective resting maximum and minimum pressures at the height of the heart. For the purpose of regulation of the pressure, there are specialized stretch receptors in the thin-walled section of the sinuses in the aorta and carotid arteries. They respond the the degree of stretch caused by the pressure of the blood. and send nerve signals to the medulla oblongata of the brain.

When the blood pressure is too high, these pressure receptors fire at a higher rate and trigger parasympathetic stimulation of the heart, causing its output to fall. The degree of sympathetic stimulation of the peripheral arterioles will also decrease, resulting in vasodilation. When the blood pressure drops too low, the rate of firing of the pressure receptors drops and the cardiac accelerator portion of the medulla oblongata will increase sympathetic stimulation of the heart to increase its output. Also sympathetic stimulation of the peripheral arterioles will cause them to constrict, also contributing to a rise in blood pressure.

Blood Distribution

Homeostasis for the cardiovascular system involves a flexible and highly organized distribution system.

Blood Perfusion into Cells

Blood Water Balance Regulation

The regulation of the water balance in the blood is a major function of the kidneys. This homeostatic function of the kidneys is regulated by the antidiuretic hormone (ADH) which is released by the pituitary gland in response to signals from the hypothalamus. ADH acts to increase the permeability of the tubules in the nephrons of the kidney to release more water back into the bloodstream. This is a good example of homeostasis by use of a hormone to switch on permeability to retain water in the blood, and then ceasing the ADH when water needs to be excreted by the kidneys.

Blood Oxygen Content Regulation

Blood Sugar Regulation

Regulation of the pH of the Blood

Ion Content of the Blood

Positive Feedback: Uterine Contraction

In a positive feedback system, a change produces a response that intensifies the original change. The change proceeds in the direction of the original change to carry a process quickly to completion. Such processes do not have a set point like negative feedback regulatory processes. The classic example is that of the uterine contractions that lead to the birth of a baby. The early contractions of labor press the baby's head agains the cervix, which has stretch receptors that send signals to the hypothalamus. The hypothalamus responds by producing the hormone oxytocin, released by the pituitary gland, that stimulates more and stronger uterine contractions. The stronger contractions put more pressure on the cervix, prompting more oxytocin release, and the process repeats until the baby is born.

Positive Feedback: Blood Clotting

In a positive feedback system, a change produces a response that intensifies the original change. The change proceeds in the direction of the original change to carry a process quickly to completion. In response to a tear in the lining of a blood vessel, blood platelets cling to the injured site and release chemicals that attract more platelets. The blood platelets accumulate until a clot forms to block the blood flow. Fortunately, the platelet accumulation then stops; self-termination is a common characteristic of positive feedback mechanisms in the body.

Positive Feedback: Sneeze!

In a positive feedback system, a change produces a response that intensifies the original change. The change proceeds in the direction of the original change to carry a process quickly to completion. A sneeze starts with a small stimulus that intensifies rapidly to its conclusion.