How does the nervous system help the excretory system maintain homeostasis

Review

. 2015 Apr 20;100(5):479-84.

doi: 10.1113/expphysiol.2014.079889. Epub 2015 Jan 20.

Affiliations

• PMID: 25599970
• DOI: 10.1113/expphysiol.2014.079889

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Review

The crosstalk between the kidney and the central nervous system: the role of renal nerves in blood pressure regulation

Erika E Nishi et al. Exp Physiol. 2015.

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Abstract

What is the topic of this review? This review describes the role of renal nerves as the key carrier of signals from the kidneys to the CNS and vice versa; the brain and kidneys communicate through this carrier to maintain homeostasis in the body. What advances does it highlight? Whether renal or autonomic dysfunction is the predominant contributor to systemic hypertension is still debated. In this review, we focus on the role of the renal nerves in a model of renovascular hypertension. The sympathetic nervous system influences the renal regulation of arterial pressure and body fluid composition. Anatomical and physiological evidence has shown that sympathetic nerves mediate changes in urinary sodium and water excretion by regulating the renal tubular water and sodium reabsorption throughout the nephron, changes in the renal blood flow and the glomerular filtration rate by regulating the constriction of renal vasculature, and changes in the activity of the renin-angiotensin system by regulating the renin release from juxtaglomerular cells. Additionally, renal sensory afferent fibres project to the autonomic central nuclei that regulate blood pressure. Hence, renal nerves play a key role in the crosstalk between the kidneys and the CNS to maintain homeostasis in the body. Therefore, the increased sympathetic nerve activity to the kidney and the renal afferent nerve activity to the CNS may contribute to the outcome of diseases, such as hypertension.

© 2014 The Authors. Experimental Physiology © 2014 The Physiological Society.

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Learning Outcomes

• Explain how different organ systems relate to one another to maintain homeostasis

Each organ system performs specific functions for the body, and each organ system is typically studied independently. However, the organ systems also work together to help the body maintain homeostasis.

Water Levels

For example, the cardiovascular, urinary, and lymphatic systems all help the body control water balance. The cardiovascular and lymphatic systems transport fluids throughout the body and help sense both solute and water levels and regulate pressure. If the water level gets too high, the urinary system produces more dilute urine (urine with a higher water content) to help eliminate the excess water. If the water level gets too low, more concentrated urine is produced so that water is conserved.

Internal Temperatures

Similarly, the cardiovascular, integumentary (skin and associated structures), respiratory, and muscular systems work together to help the body maintain a stable internal temperature. If body temperature rises, blood vessels in the skin dilate, allowing more blood to flow near the skin’s surface. This allows heat to dissipate through the skin and into the surrounding air. The skin may also produce sweat if the body gets too hot; when the sweat evaporates, it helps to cool the body. Rapid breathing can also help the body eliminate excess heat. Together, these responses to increased body temperature explain why you sweat, pant, and become red in the face when you exercise hard. (Heavy breathing during exercise is also one way the body gets more oxygen to your muscles, and gets rid of the extra carbon dioxide produced by the muscles.)

Conversely, if your body is too cold, blood vessels in the skin contract, and blood flow to the extremities (arms and legs) slows. Muscles contract and relax rapidly, which generates heat to keep you warm. The hair on your skin rises, trapping more air, which is a good insulator, near your skin. These responses to decreased body temperature explain why you shiver, get “goose bumps,” and have cold, pale extremities when you are cold.

Case Study: Fevers

So what happens when you have a fever? Does this mean your body is unable to maintain its homeostasis, in the same way your house will get too hot if your air conditioner is broken?

In extreme cases, a fever can be a medical emergency; but fever is an adaptive physiological response of our body to certain infectious agents. Certain chemicals called pyrogens will trigger your hypothalamus to shift the set point to a higher value. This is more like you programming the thermostat in your house to a higher temperature to save energy on a hot day when you are not going to be home during the day. These pyrogens can come from microorganisms that infect you, or they can be produced by your body cells in response to an infection of some sort.

Practice Questions

1. As the level of pyrogens increases in your blood, and the set point resets higher, chemoreceptors now stimulating the hypothalamus are responding to ________ as the variable, rather than thermoreceptors responding to body temperature as the variable.
   1. temperature
   2. pyrogens
   3. heart rate
   4. blood pressure
2. The control center is the _________.
   1. skeletal muscle
   2. sweat glands
   3. blood vessels
   4. hypothalamus
3. Because the set point has been increased, you now feel cold even though you have what would normally be a body temperature within the healthy range. This produces the “chills” you feel when you get a fever. In response, the hypothalumus will work to increase body temperature. Which response will do this?
   1. The hypothalamus will stimulate sweat glands and dilating blood vessels as effectors to cool off the body.
   2. The hypothalamus will stimulate skeletal muscles to shiver and constricting blood vessels.

Although the evidence is only indirect, fever is believed to enhance the body’s immune response. The increased temperature may actually impair the replication of infecting bacteria and viruses that are adapted to survive best at your normal homeostatic body temperature range. This can give your immune cells a chance to destroy the microorganisms before they can rapidly multiply and spread in the body. There is also some indirect evidence that increased body temperature slightly modifies several metabolic reactions in ways that also allow the immune system to function more efficiently.

Practice Questions

1. Once the new higher set point is reached, the thermoreceptors stimulate the _________ as the control center.
   1. skeletal muscle
   2. sweat glands
   3. blood vessels
   4. hypothalamus
2. In response to the increasing set point, the sweat glands and blood vessels (effectors) are stimulated to _________.
   1. secrete sweat for evaporation and dilate vessels for increased heat loss from blood near the surface of the skin.
   2. shiver to create heat and constrict vessels to conserve heat by keeping blood away from the surface of the skin.

Unfortunately during some infections, pyrogen levels come in “waves.” This adjusts your temperature set point up and down. When pyrogen levels dip, you get the other part of the fever experience: “the sweats” and feeling flushed. As long as the pyrogen levels continue to increase and decrease you will feel like you are swinging back and forth.

Practice Question

1. Once the pyrogen level is reduced because the infection is under control, the ________ (control center) will reset the higher set point to normal.
   1. thermoreceptors
   2. chemoreceptors
   3. hypothalamus

Your body will continue to swing back and forth between the body’s normal upper and lower temperature limits, but because it is now within your “normal” temperature range, you probably won’t even notice that your body is still at work, maintaining the homoeostasis of this variable.

Practice Question

1. Patients often get a fever after an operation. Which of the following would not be a reasonable cause of such a response?
   1. Tissue trauma from the operation has stimulated body cells to release pyrogens.
   2. Despite precautions, some bacteria have infected the person during the operation.
   3. The operation has damaged the thermoreceptors
   4. Post-operative medications have impacted the immune system, causing the release of pyrogens.

Homeostasis of Ions

Body functions such as regulation of the heartbeat, contraction of muscles, activation of enzymes, and cellular communication require tightly regulated calcium levels. Normally, we get a lot of calcium from our diet. The small intestine absorbs calcium from digested food.

The endocrine system is the control center for regulating blood calcium homeostasis. The parathyroid and thyroid glands contain receptors that respond to levels of calcium in the blood. In this feedback system, blood calcium level is the variable, because it changes in response to the environment. Changes in blood calcium level have the following effects:

• When blood calcium is low, the parathyroid gland secretes parathyroid hormone. This hormone causes effector organs (the kidneys and bones) to respond to increase calcium levels. The kidneys prevent calcium from being excreted in the urine. Osteoclasts in bones reabsorb bone tissue and release calcium.
• When blood calcium levels are high, the thyroid gland releases calcitonin. Calcitonin causes the kidneys to reabsorb less calcium from the filtrate, allowing excess calcium to be removed from the body in urine. Calcitonin also suppresses the formation of active vitamin D in the kidneys; without vitamin D the small intestines don’t absorb as much dietary calcium. Osteoblasts, stimulated by calcitonin, use calcium in the blood to add to bone tissue.

Practice Questions

Based on the above description of calcium homeostasis, try to answer these questions:

Calcium imbalance in the blood can lead to disease or even death. Hypocalcemia refers to low blood calcium levels. Signs of hypocalcemia include muscle spasms and heart malfunctions. Hypercalcemia occurs when blood calcium levels are higher than normal. Hypercalcemia can also cause heart malfunction as well as muscle weakness and kidney stones.

Practice Question

What problem(s) is/are associated with calcium homeostasis dysfunction?

1. heart disease
2. bone disease
3. both
4. neither

Watch this video for another discussion on homeostasis and organ systems:

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