Regulation of Body Temperature and Body Fluids: Kidneys and Liver

As an integrated study of homeostasis, we will examine body temperature regulation and the maintenance of body fluids by the kidneys and liver. These systems are an integrated example of biological chemistry.

Basic Knowledge

Body temperature regulation (center = hypothalamus):
・When cold: The sympathetic nervous system causes constriction of skin blood vessels and contraction of arrector pili muscles (goosebumps), while shivering by skeletal muscles produces heat; metabolism is promoted by thyroid thyroxine and adrenal medulla adrenaline.
・When hot: The sympathetic nervous system promotes sweating, and vasodilation of skin blood vessels releases heat.
Kidneys:
・Structure: nephron (renal unit) = glomerulus + Bowman's capsule + renal tubule. There are approximately 2 million in both kidneys.
・Functions: ① filtration (produces glomerular filtrate from blood in the glomerulus, approximately 180 L/day), ② reabsorption (reabsorbs glucose, amino acids, water, and salts in the renal tubules and collecting ducts), ③ urine concentration (produces approximately 1.5 L/day of urine).
・Regulation: vasopressin (antidiuretic hormone) = increases water reabsorption (during dehydration), mineralocorticoids = increase Na⁺ reabsorption.
Liver (also called a chemical factory):
・Detoxification: Breaks down alcohol, drugs, and harmful substances (a storehouse of chemical reactions). Converts ammonia into urea (urea cycle).
・Bile production: Helps digest and absorb fats.
・Glycogen storage: A reserve source of blood glucose.
・Plasma protein synthesis: Albumin, fibrinogen, and others.
・Body temperature regulation: Produces heat through its large metabolic activity.

Further Study

The urea cycle (ornithine cycle) is a sophisticated metabolic pathway that detoxifies ammonia. Ammonia (NH₃) produced by protein metabolism is highly neurotoxic, and its accumulation in the blood can be fatal. In the liver, it is converted into urea by the reaction 2NH₃+CO₂→(NH₂)₂CO+H₂O and excreted by the kidneys. This cycle was discovered by Krebs and Henseleit in 1932. It is a classic metabolic pathway and a chemically elegant example involving ATP consumption and the cooperation of multiple enzymes.
Alcohol metabolism: Ethanol→acetaldehyde (alcohol dehydrogenase ADH)→acetic acid (acetaldehyde dehydrogenase ALDH)→water and CO₂. About half of Japanese people have weak ALDH2 activity (genotype ALDH2*2), so acetaldehyde cannot be broken down efficiently, causing the face to become red. This is a well-known topic in chemistry and toxicology and a typical example of drug metabolism.
Kidney filtration mechanism: The glomerulus is a cluster of capillaries. Small molecules in the blood (water, salts, glucose, amino acids, urea, and creatinine) are all filtered, whereas red blood cells and proteins cannot pass through (a filter based on molecular mass). Approximately 180 L of glomerular filtrate is produced each day, and 99% is reabsorbed in the renal tubules, concentrating the urine to approximately 1.5 L. Glucose and amino acids are normally reabsorbed 100%, but when blood glucose is excessively high in diabetes, some leaks through and is excreted as glucose in the urine. This is the origin of the term diabetes mellitus.
Water balance: In a 70 kg person, total body water is approximately 42 L. Daily water balance is nearly equal between intake (food + fluids = approximately 2.5 L) and excretion (urine 1.5 L + sweat 0.5 L + exhaled air 0.5 L, and so on). During strenuous exercise or fever, large amounts of sweat are produced, so water must be replenished together with salt (to prevent heatstroke).
All of these processes are connected through a network of negative feedback controlled by the hypothalamus, which maintains the internal environment at a constant level. This is the integrated picture of “homeostasis.”

Important Notes

① The hypothalamus (diencephalon) is the command center for body temperature, water balance, blood glucose, the autonomic nervous system, and hormones. Remember it as the “integrated center.” ② Filtration in the kidney occurs in the glomerulus, while reabsorption occurs in the renal tubule. Do not reverse them. ③ Vasopressin is an antidiuretic hormone secreted by the posterior pituitary gland. It promotes water reabsorption during dehydration. Alcohol suppresses vasopressin secretion, causing frequent urination after drinking. ④ Do not confuse the liver with the kidneys. The liver is a chemical processing factory (urea synthesis, detoxification, and metabolism), while the kidneys are filtration devices (removing urea from the blood and excreting it). They work together to maintain body fluids.

Practice

1. What is the functional unit of the kidney (glomerulus + Bowman's capsule + renal tubule) called?
2. What is the metabolic pathway in which ammonia is converted into urea in the liver called?
3. Which part of the brain serves as the command center for homeostasis, including body temperature, blood glucose, and water balance?