Excretion

Excretion is the process of removing waste products that are produced during metabolic activities in the body. These waste products include carbon dioxide from respiration and photosynthesis, water or metabolic water from respiration, urea from deamination (the conversion of amino acids to ammonia and then to urea for safe elimination), salts, and oxygen released during photosynthesis.

Anabolism

Anabolism involves the synthesis of new substances, represented by the equation A + B → C. Catabolism, on the other hand, involves the breakdown of complex compounds into simpler ones, such as in the process of digestion, represented by the equation C → A + B.

Egestion refers to the elimination of undigested or indigestible food materials from the body.

The Kidneys

The kidneys play a vital role in excretion. The renal artery carries oxygen-rich blood to the kidneys and blood with a high concentration of urea. In contrast, the renal vein carries low-oxygen blood without urea. Inside the kidneys, nephrons, Bowman's capsule, and proximal convoluted tubules are involved in the filtration process.

Nephrons filter blood from the glomerulus capillaries, and the waste enters Bowman's capsule before moving into the proximal convoluted tubules. In these tubules, glucose is selectively reabsorbed through active transport. Diabetic individuals may excrete some glucose in their urine, indicating improper pancreatic function. Large protein molecules in the urine may be a sign of high blood pressure or damaged glomerular walls. The reabsorption of salts depends on the body's needs.

The kidneys selectively reabsorb the necessary amount of water needed by the body. When plasma leaks out, it becomes tissue fluid, which acts as a medium for excretory products. Water is reabsorbed based on the body's needs, while all amino acids and urea are reabsorbed. Fine-tuning occurs within the kidneys.

The collecting ducts collect urine from the nephrons and transport it through the pelvis, ureter, and bladder. The glomerulus, a cluster of blood vessels, allows small molecules, wastes, and fluid to pass through the tubule, while preventing larger substances like blood proteins from passing through. The loop of Henle helps in the reabsorption of filtered sodium ions (Na+) to deliver only small amounts to downstream sites.

Homeostasis

Homeostasis involves maintaining a constant internal environment, including temperature, pH, water concentration, and glucose concentration in the blood. Osmoregulation refers to the body's ability to maintain the water concentration in the blood and tissue fluid. Anti-diuretic hormone (ADH) influences the permeability of collecting ducts and tubules.

When a person drinks excessive water, the blood becomes diluted, which can lead to cell swelling and bursting. The hypothalamus detects this change and signals the pituitary gland to release less or no ADH, resulting in less water re-entering and leaving the body. This leads to dilute and clearer urine. Inadequate water intake, on the other hand, leads to concentrated blood. The hypothalamus detects this condition and signals the release of more ADH, causing the collecting duct walls to become permeable, allowing water to re-enter the system. This leads to concentrated urine with urea, resulting in a brighter yellow color.

Dialysis is a treatment used when the kidneys are not functioning properly. It involves using a dialysis solution that contains the necessary nutrients and proportions to facilitate filtration through osmosis and diffusion.

The skin serves various functions, including protection against injuries, sensory perception, temperature regulation, and vitamin D synthesis in the presence of sunlight. It consists of the epidermis, which has a continuously renewing cornified layer and a pigmented Malpighian layer containing melanin for UV protection. The dermis houses hair follicles, while receptors for pressure, pain, and temperature are present.

Temperature regulation is essential for maintaining homeostasis. In cold conditions, voluntary actions such as wearing warm clothing, physical movement, increasing environmental temperature, and consuming hot beverages help regulate body temperature. Involuntary actions include shivering and vasoconstriction, reducing heat loss. In hot conditions, voluntary actions include consuming cold drinks, adjusting clothing, seeking cooler environments, decreasing environmental temperature, and taking baths. Involuntary responses involve vasodilation and sweating, aiding heat dissipation through evaporation.

Maintaining normal blood glucose levels is crucial to prevent cellular damage. High blood glucose levels stimulate the release of insulin from the pancreas, converting glucose to glycogen stored in the liver or muscles, reducing blood glucose levels. Low blood glucose levels trigger the release of glucagon, which converts stored glycogen back into glucose. Adrenaline also promotes glycogen breakdown.

To summarize, excretion involves eliminating waste products generated through metabolic activities. The kidneys, through nephrons and tubules, play a significant role in waste filtration and reabsorption. Homeostasis and osmoregulation maintain a stable internal environment. Dialysis is used when kidney function is impaired. The skin performs multiple functions, including protection, sensation, temperature regulation, and vitamin D synthesis. Temperature regulation involves both voluntary and involuntary responses. Regulating blood glucose levels is achieved through the actions of insulin, glucagon, and adrenaline.