The internal environment of the body is tissue fluid, which bathes all cells making up the body. The composition of tissue fluid must remain constant if cells are to remain alive and healthy. Tissue fluid is nourished and purified when molecules are exchanged across thin capillary walls. Tissue fluid remains constant only if the composition of blood remains constant.
In practical terms, we can think of the systemic circuit as a means to conduct blood to and away from the capillaries, because only here does exchange with tissue fluid take place. Nutrient molecules leave the capillaries to be taken up by the cells, and waste molecules given off by the cells are received by the capillaries to be transported away. Capillaries abound in all parts of the body, and no cell is more than a few micrometers from a capillary.
Blood is composed of two parts: formed elements and plasma. All of the formed elements contribute to homeostasis, as outlined in table 1. Oxygen is utilized during cellular respiration, a process that provides energy for metabolic activities. Fighting infection keeps the body intact and prevents it from succumbing to disease caused by viruses and bacteria. Clotting of blood when a vessel has been cut prevents the loss of this vital fluid.
|Red blood cells||Transport oxygen and hydrogen ions|
|White blood cells||Fight infection|
|Platelets||Assist blood clotting|
Plasma, too, contributes to homeostasis, as noted in table 2. The nutrients needed and wastes given off by cells are carried in plasma. Nutrients leave plasma at the capillaries and wastes enter plasma at the capillaries. Blood pressure created by the pumping of the heart forces water out of a capillary at the arteriole end and osmotic pressure maintained largely by proteins draws water back in at the venous end of a capillary. Plasma proteins not only maintain osmotic pressure, but also buffer the blood; a function they share with the salts, as we shall discuss in more detail later.
|Water||Provides fluid environment|
|Proteins||Create osmotic pressure, aid clotting, and help buffer blood|
|Nutrients||Required for cellular metabolism|
|Wastes||Produced by cellular metabolism|
|Salts||Aid metabolic activity and help buffer blood|
Special lymph capillaries, called lacteals, are found within the villi. They absorb the products of fat digestion.
This manner of regulating normalcy results in a fluctuation between two extreme levels. Not until body temperature drops below normal do receptors stimulate the regulating center and effectors act to raise body temperature. Regulating centers are located in the central nervous system, consisting of the brain and spinal cord (fig. 3a, 3b). The hypothalamus is a portion of the brain particularly concerned with homeostasis; it influences the action of the medulla oblongata, a lower part of the brain, the autonomic nervous system, and the pituitary gland.
The nervous system has two major portions: the central nervous system and the peripheral nervous system (table 3). The peripheral nervous system consists of the cranial and spinal nerves. The autonomic nervous system is a part of peripheral nervous system and contains motor neurons that control internal organs. It operates at the subconscious level and has two divisions, the sympathetic and parasympathetic systems. In general, the sympathetic system brings about those results we associate with emergency situations, often called fight or flight reactions, and the parasympathetic system produces those effects necessary to our everyday existence.
|Cerebrum||Consciousness, creativity, thought, morals, memory|
|Lower portions||Reception of sensory data, coordination of muscular activity, homeostasis|
|Spinal cord||Automatic reflex actions|
|Cranial nerves, spinal nerves||Carry sensory information to motor impulses from the CNS|
|Autonomic system||Those cranial and spinal motor nerves that control internal organs|
The reflex arc is the action unit of the nervous system. In this arc, a sense receptor initiates nerve impulses that travel by way of a sensory fiber to the central nervous system where integration takes place. Following this, nerve impulses travel by way of motor neurons to either a gland or muscle that then reacts.
The nerve impulse is an electrochemical change that is propagated along the length of a neuron from dendrite to axon. The nerve impulse is the same in all neurons; the specific effect that results is dependent on the organ being stimulated. For example, each part of the cerebrum has a different function: stimulation of the occipital lobes results in vision; stimulation of the temporal lobe produces a sensation of sound. Sensations are the prerogative of the cerebrum since only the cerebrum is responsible for consciousness.
A region of close proximity between neurons is called a synapse. At a synapse, one neuron ends at the presynaptic membrane and the next neuron begins at the postsynaptic membrane. The small gap between is the synaptic cleft. Transmission across a synapse is by means of neurotransmitter substances which are stored in small synaptic vesicles on the axon side of the synapse. Nerve impulses cause the release of neurotransmitter substances, which diffuses across the synaptic cleft to be received by the postsynaptic membrane. If stimulation results, nerve impulses begin in the next neuron (fig. 4).
A neuromuscular junction has the same components as a synapse. In this case, however, the postsynaptic membrane is the membrane of a muscle fiber. Again a neurotransmitter substance diffuses across the synaptic cleft but this time the action potential that travels along the T system of a muscle fiber causes the release of calcium, which triggers a muscle contraction. When a skeletal muscle contracts, the actin filaments slide past the myosin filaments, thereby shortening sarcomeres and therefore, the muscle. While at first it may seem that the muscular system does not play a role in homeostasis, voluntary muscles very definitely do play a role because by their contraction the individual can take the necessary actions to bring about a more favorable external environment.
|Hypothalamus||Hypothalmic-releasing and release-inhibiting hormones||Regulate anterior pituitary hormones|
|Anterior pituitary||Thyroid-stimulating||Stimulates thyroid|
|Adrenocorticotropic||Stimulates adrenal cortex|
|Posterior pituitary||Antidiuretic||Promotes water reabsorption by kidney|
|Thyroid||Thyroxin||Increases metabolic rate|
|Parathyroid||Parathyroid||Maintains blood calcium and phosphorus levels|
|Adrenal cortex||Glucocorticoids (e.g., cortisol)||Promotes gluconeogenesis|
|Mineralocorticoids (e.g., aldosterone)||Promotes sodium reabsorption by kidneys|
|Adrenal medulla||Epinephrine and norepinephrine||Stimulates fight or flight reaction|
|Pancreas||Insulin||Lowers blood sugar level|
|Glucagon||Raises blood sugar level|
|Gonads||Androgens (male) Estrogens and progesterone (female)||Promotes secondary sex characteristics|
The respiratory center, located in the medulla oblongata automatically discharges nerve impulses to the diaphragm and the muscles of the rib cage. In its relaxed state, the diaphragm is dome-shaped, but upon stimulation, it contracts and lowers. Also the rib cage moves upward and outward. As the thoracic cavity increases in size, air pressure within the expanded lungs lowers and is immediately rebalanced by air rushing in through the nose. This is why it can be said that humans breathe by negative pressure. When the respiratory center stops sending out stimulatory nerve impulses the diaphragm and rib cage resume their original positions and exhalation occurs.
There are chemoreceptors adjacent to the respiratory center in the medulla oblongata that are sensitive to the carbon dioxide content of the blood, and chemoreceptors in aorta and carotid arteries that are sensitive to both the carbon dioxide content and the pH of the blood. When the carbon dioxide concentration rises or when the pH lowers the respiratory center is stimulated and the breathing rate increases. It is interesting to observe that the oxygen content of the blood does not directly affect the activity of the respiratory center.
Within the digestive tract the food is broken down to nutrient molecules small enough to be absorbed by the villi of the small intestine. Digestive enzymes are produced by the digestive tract and by the pancreas. In addition the liver produces bile, an emulsifier that plays a role in the digestion of fats. Bile, which is stored in the gallbladder, enters the small intestine along with the pancreatic enzymes. Following the absorption of nutrients, blood passes from the region of the small intestine to the liver by way of the hepatic portal vein.
The liver, which monitors the blood, is a very important organ of homeostasis. The liver breaks down toxic substances like alcohol and other drugs, and it produces urea, the end product of nitrogenous metabolism. The liver produces the plasma proteins and stores glucose as glycogen after eating. In between eating it releases glucose, thereby keeping the blood glucose concentration constant. The liver destroys old blood cells and breaks down hemoglobin--hemoglobin breakdown products are excreted in bile.
The regulatory center for body temperature, located in the hypothalamus, is sensitive to temperature changes in arterial blood flowing through it. Depending on the body temperature, the regulatory center brings about the adaptive responses listed in table 5, and body temperature then increases or decreases.
|Structures||When Body Cools||When Body Warms|
|Superficial blood vessels||Constricts||Dilates|
Regulation of the size of superficial arterial blood vessels and the activity of sweat glands is an important means by which body heat can be either conserved or dissipated. We can liken these activities to either closing or opening the windows of a house. The autonomic nervous system controls these reactions; the sympathetic system brings about the effects that conserve heat, and the parasympathetic acts to release heat. The body cools when blood vessels lying in the skin are dilated and the warm blood passing through them loses heat to the environment by radiation. Sweating also cools the body because as perspiration evaporates, the body loses heat. Evaporation is more efficient on dry days than on humid days; humidity, then, does affect our ability to cool off.
If body temperature falls too low, shivering in addition to vasoconstriction will occur. Shivering requires that nerve impulses be sent to the skeletal muscles.
Humans contribute to the regulation of body heat by wearing appropriate clothing. In cold climates, humans wear clothing that traps an insulating layer of warm air next to the body to compensate for a lack of body hair. The formation of "goose bumps" is an ineffective attempt to raise the now absent hairs of the body to achieve a layer of trapped air naturally. In warm climates, clothing is worn to protect the body against the burning rays of the sun, but such clothing should be loose so that heat may still be lost by radiation.
Arterial blood pressure will rise whenever blood volume increases or whenever there is a decrease in the cross-sectional area of the arteries. Sympathetic neurons under the control of regulatory centers (called cardiac and vasomotor centers) located in the medulla oblongata of the brain, can increase the heartbeat and constrict the arteries. A faster heartbeat temporarily increases the amount of blood within the arteries, and constriction of blood vessels, usually those of the skin and intestines, reduces their cross-sectional area.
The vasomotor center can be activated by impulses received from pressoreceptors located in the aorta and carotid arteries. When pressoreceptors are stimulated by a decrease in blood volume, as when we stand up suddenly after lying down, nerve impulses are sent to the vasomotor center and then blood pressure rises. The vasomotor center can also be effective when blood volume suddenly decreases, as when hemorrhaging occurs. At these times, it causes the blood reservoirs of the body (i.e., the veins, spleen, and liver) to contract and send more blood into the arteries.
The kidneys are also involved in monitoring blood pressure because of the role they play in regulating blood volume. When blood pressure decreases, the kidneys release renin, an enzyme that leads to the formation of angiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Under the influence of aldosterone, the kidneys retain sodium. As sodium is reabsorbed, water follows passively and both blood volume and blood pressure rise. In the presence of high blood pressure, the heart releases atrial natriuretic hormone, which has the opposite effect on the kidneys. This illustrates that homeostasis is often regulated by the contrary actions of hormones.
Proteins are effective chemical buffers both within cells and within blood. Hemoglobin is the most active protein buffer within blood, and it absorbs excess hydrogen ions when it is not carrying oxygen.
There are two other types of chemical buffers in body fluids--the carbonate and phosphate buffer systems. The phosphate system (NaH2PO4 and Na2HPO4) effectively buffers urine and cytoplasm. The carbonate system (a mixture of carbonic acid, H2CO3, and sodium bicarbonate, NaHCO3) is present in both tissue fluid and blood. The concentration of buffering substances is regulated by the lungs and/or kidneys. For example, when carbon dioxide is exhaled by the lungs or the bicarbonate ion is excreted by the kidneys, the concentration of the buffering substances can return to their most effective levels.
If the hydrogen ion concentration of the blood remains high, the respiratory center of the medulla oblongata is stimulated and the breathing rate increases. As carbon dioxide is excreted the pH shifts toward normal. This respiratory regulation of the acid-base balance is a physiological type of buffer system that is an important adjunct to the chemical systems discussed.
The kidneys are a powerful mechanism by which the pH may be regulated. The kidneys may form either an acid or alkaline urine, bringing the hydrogen ion concentration back toward normal. When the kidneys form an acid urine, they excrete H+, and when the kidneys form an alkaline urine, they excrete the bicarbonate ion. However, the full effect of the kidneys is not realized for ten to twenty hours.
Notice that the pH of the body is regulated in three ways. Chemical buffers both within cells and within body fluids react immediately to regulate the hydrogen ion and hydroxide ion concentrations. The pulmonary system requires a few minutes to bring about its effects while the kidneys take from ten to twenty hours. The kidneys, however, are the most powerful of the three.
If the supply of glycogen should run out and the blood glucose level remains low, both thyroxin and glucocorticoids stimulate gluconeogenesis, or the conversion of amino acids and glycerol to glucose by the liver.
Regulation of body temperature, blood pressure, pH, and glucose concentration are four examples of how the body maintains homeostasis. The hypothalamus is involved to a degree in each of these regulations. The hypothalamus contains a regulatory center for body temperature but is also involved in regulation of blood pressure and breathing rate through its control over the medulla oblongata. Through the production of hypothalamic-releasing factors and release-inhibiting factors, the hypothalamus directly controls the pituitary gland and indirectly controls the secretions of other glands, such as the thyroid and the adrenal cortex.
The body has both short-term and long-term measures to control bodily conditions. In regard to temperature control, the short-term measures include shivering and constriction of arteries to conserve body heat, and dilation of arteries along with sweating to lose body heat. A significant long-term measure to increase body temperature is an increase in thyroxin. Thyroxin raises the metabolic rate.
A rapid elevation in blood pressure occurs when the vasomotor center stimulates the constriction of abdominal blood vessels and increases the heartbeat. A longer lasting effect occurs when the kidneys secrete renin leading to a reabsorption of sodium and water. The resulting increase in blood volume increases blood pressure.
The pH of the body is immediately regulated by chemical buffers, while the excretion of carbon dioxide must wait until blood moves through the lungs. The kidneys are also involved in regulating blood pH, but the effect may not noticed for up to twenty hours. The blood glucose level is usually regulated by insulin and glucagon. But other hormones can also have an effect since thyroxin and glucocorticoids promote gluconeogenesis.
A feedback mechanism is often involved in maintaining homeostasis. The temperature-regulating center is activated when the body temperature rises above or falls below a certain level. Once the temperature is within a normal range, the center stops sending out stimulatory nerve impulses. The vasomotor center promotes a rise in blood pressure, but once this has been attained the center is no longer active. If the pH becomes too acidic, the chemoreceptors in the aortic and carotid arteries signal the respiratory center and the breathing rate increases. Once the pH is within a normal range, these bodies no longer signal the respiratory center and breathing rate returns to normal. When glucose concentration is high, insulin is secreted; but once the glucose level falls, insulin is not secreted. These examples make it clear that feedback is a self-regulating mechanism.
2. Use regulation of body temperature to illustrate maintenance of homeostasis by a feedback mechanism.
3. Use regulation of normal blood pressure to illustrate how the nervous system and the endocrine systems are both involved in maintaining homeostasis.
4. Use the regulation of blood pH to illustrate that there are short-term and long-term mechanisms for maintaining homeostasis.
5. Use the regulation of a normal blood glucose concentration to illustrate that contrary actions of hormones help maintain homeostasis.