Homeostasis and Waste Removal

• Homeostasis maintains relatively constant conditions inside the body.
• The kidneys play a key role in homeostasis and waste removal (excretion).
• The kidneys filter blood, removing substances and regulating water and solute concentrations.

Homeostasis

• The body automatically balances water intake and loss.
• Drinking water is a primary source of water gain, but water is also in food and produced by cell respiration.
• Water is lost mostly through urine, but also through sweat, feces, and exhaled air.
• Daily water gain and loss are roughly equal, maintaining a stable total body water content.
• Homeostasis maintains a 'steady state' of internal body conditions.
• The 'internal environment' refers to the surroundings of cells inside the body, mainly blood and tissue fluid.
• Tissue fluid is a watery solution containing salts, glucose, and other solutes.
• It surrounds cells, enabling nutrient transfer between blood and cells.
• It is formed by leakage from blood capillaries and similar to blood plasma but lacks plasma proteins.
• The body regulates water, salts, carbon dioxide levels, blood pH, glucose concentration, and body temperature.
• Proper cell function depends on optimal conditions provided by tissue fluid.
• Cells lose water (dehydrate) in tissue fluid containing too many solutes.
• Cells can swell with water if tissue fluid is too dilute.
• Both scenarios prevent efficient cell function and potentially cause damage.
• Incorrect tissue fluid pH affects enzyme activity, as does a variance in body temperature from 37°C.
• Waste products like urea are toxic to cells and must be removed.

Urine

• Adult humans produce about 1.5 dm³ of urine daily.
• Urine volume varies based on water intake and loss through other means like sweat.
• Every liter of urine contains approximately 40 g of waste products and salts.
• Urine contains ions such as sodium (Na+), chloride, potassium, phosphate (HPO42-), and ammonium (NH4).
• Kidneys act as both homeostatic and excretory organs, regulating water/salt concentration and concentrating nitrogenous waste for elimination.

Nitrogenous Waste

• Nitrogenous waste contains the element nitrogen.
• Urea and ammonia are examples of nitrogenous waste.
• All animals excrete nitrogenous waste products.
• Excess carbohydrates and fats can be stored, while excess proteins cannot.
• Proteins are broken down into carbohydrates (stored as glycogen) and urea.
• Urea enters the bloodstream and is filtered by the kidneys to form urine.
• Excretion is the removal of metabolic waste, opposed to removal of undigested food and dead cells.

Urinary System

• The kidney is supplied by blood through the renal artery, branching directly from the aorta for high-pressure blood entry.
• The 'cleaned' blood exits each kidney through the renal vein into the vena cava.
• Urine exits each kidney through the ureters & is stored in the bladder.
• Urine exits the bladder through the urethra, which contains sphincter muscles.
• Sphincter muscles control the flow of urine; the lower one is voluntary, and the upper one is involuntary.

The Kidneys

• The outer region of the kidney is the cortex, containing tiny blood vessels branching from the renal artery and nephrons.
• Nephrons/kidney tubules are the filtering units.
• The medulla is the middle layer of the kidney through which the tubules run.
• Pyramids are bulges in the medulla that point inward, towards the concave side of the kidney.
• Tubules join at the tips of pyramids, emptying urine into the pelvis.
• The pelvis connects to the ureter and carries urine to the bladder.

Structure of the Nephron

• Each kidney contains about a million nephrons.
• The Bowman's capsule is the hollow cup at the nephron's start.
• Glomerulus describes the ball of capillaries surrounded by the Bowman's capsule; it is where blood is filtered.
• The renal artery divides into smaller arteries (arterioles) that supply the glomerulus's capillaries.
• Proximal and distal convoluted tubules are other names for the first and second coiled tubules.
• Proximal means near the tubule's start, and distal means near the end.

Ultrafiltration in the Bowman's Capsule

• Blood enters the kidney through the renal artery and into the glomerulus.
• Resistance to flow caused by the glomerulus creates high blood pressure in the incoming arteriole.
• This pressure forces fluid through the capillary walls and Bowman's capsule into the capsule space.
• Blood in the glomerulus and the capsule space are separated by the capillary wall, the wall of the capsule, and the basement membrane.
• Water, ions, and small molecules (glucose, urea) pass through the filtration process, while blood cells and large molecules (proteins) are retained.
• Glomerular filtrate is the fluid that enters the capsule space.
• Ultrafiltration is the process of separating different-sized molecules under pressure.
• The kidneys produce about 125 cm³ (0.125 dm³) of glomerular filtrate per minute or 180 dm³ per day.
• Only 1.5 dm³ of urine is lost daily, with 99% of the glomerular filtrate reabsorbed back.

Changes to the Filtrate in the Rest of the Nephron

• Scientists have analyzed fluid samples from inside the nephron. Analysis is done by piercing the microscopic tubules with glass pipettes.
• There are two-coiled regions of the tubule in the cortex, separated by the loop of Henle, which runs into the kidney's medulla.
• After the second coiled tubule, several nephrons join to form a collecting duct that drains the urine into the renal pelvis.
• In the blood plasma (sample 1), proteins, glucose, urea, and salts are present.
• Glomerular filtrate (sample 2) does not contain proteins because they are too big to pass through, but it contains glucose, urea, & salts.
• By the end of the first coiled tubule (sample 3), 80% of the water has been reabsorbed, resulting in a flow rate of 20%.
• Glucose is fully reabsorbed for use by the body; 80% of the sodium is also reabsorbed.
• By the time the fluid is in the collecting duct (sample 4), almost all of the water (99%) has been reabsorbed & urea is more concentrated.
• Other solutes are concentrated in the urine, and some, like ammonium ions, are secreted into the fluid.

The Loop of Henlé

• The loop of Henle concentrates the fluid in the tubule for more water reabsorption.
• Mammals with long loops of Henle produce more concentrated urine and conserve water (desert animals), while ones with short loops (otters/beavers) have easy access to water.
• Part of the plasma (water and small molecules) leaves the blood in the Bowman's capsule and enters the nephron
• As the fluid passes the nephron, all of the glucose & most of the sodium/chloride ions are reabsorbed into the blood.
• More water and ions get reabsorbed in in the rest of the tubule & some solutes like ammonium ions are secreted into the tubule.
• The urine at the end contains urea at a much higher concentration than in the blood.
• The urine contains controlled quantities of water and ions.

Control of Body Water Content

• The kidneys regulate blood water content by controlling urine concentration.
• If the blood is too concentrated, the kidneys produce less urine that's more dilute, and vice versa.
• These changes are controlled by antidiuretic hormone (ADH) produced by the pituitary gland.
• The hormone ADH is produced by the pituitary gland.
• 'Diuresis' refers to the flow of urine, while 'antidiuresis' means producing less urine.
• ADH is released when the body loses too much water.
• Receptor cells in the hypothalamus detect increased blood concentration and signal the pituitary gland to release ADH.
• ADH causes the collecting ducts in the kidney to become more permeable to water, leading to increased water reabsorption into the blood.
• The urine becomes more concentrated, and the blood becomes more dilute.
• ADH exhibits negative feedback: a change is detected, a process begins to return conditions to normal, and the corrective process switches off when conditions return to normal.
• If the blood becomes too dilute (after drinking lots of water), ADH is not released, the kidney tubules become less permeable, and more water passes out of the body in the urine.

Control of Body Temperature

• Homeothermic animals (mammals and birds) maintain a constant body temperature.
• Humans maintain a body temperature of about 37 °C.
• Cold-blooded (other) animals have body temperatures matching their surroundings.
• Homeotherms use physiological changes to generate or lose heat.
• Endotherms generate heat from internal chemical reactions and regulate heat loss via sweating and blood flow through the skin.
• Behavioral adaptations manage temperature, such as putting on warmer clothing.
• Maintaining a stable body temperature of 37 °C allows chemical reactions to occur at a steady, predictable rate.
• High temperatures destroy enzymes via denaturation; endotherms maintain body temperatures around 40°C where enzymes function best.

Monitoring Body Temperature

• The thermoregulatory center in the hypothalamus monitors core body temperature in humans & mammals.
• Temperature receptors in the skin send signals to the hypothalamus, stimulating the brain to alter behavior when in hot/cold environments.
• The thermoregulatory center detects blood temperature changes & sends signals to regulate temperature by physiological means.
• A thermostat is a device used in electrical appliances which is turned on or off by change in temperature to keep their temperature steady.

The Skin and Temperature Control

• The skin functions for mechanical damage resistance, pathogen barrier, prevention of water loss, sensory organ for temperature changes, and to control heat loss.

• The skin has 3 layers: the epidermis, dermis, and hypodermis.

  • The outer epidermis prevents water loss/microorganism invasion.
  • The hypodermis insulates against heat loss and stores energy.
  • The dermis contains sensory receptors, sweat glands, and blood vessels (involved in temperature control).

• The hypothalamus sends signals to the skin to correct temperature changes.

• The hypothalamus detects and responds to increases in core body temperature.

  • Sweat glands produce more sweat that evaporates from the skin, cooling the body.
  • The hair erector muscles relax, and hairs lie flat, so that heat is lost.
  • Arterioles leading to capillary loops in the dermis dilate, increasing blood flow to the skin's surface (vasodilation).

• Erector muscles attached to hair follicles pull the hairs upright in the cold, trapping a layer of air for insulation.

• Birds 'fluff out' their feathers in cold weather.

• Arterioles constrict in cold conditions, reduces blood flow to the skin's surface, and reduces heat loss (vasoconstriction).

• Vasoconstriction/dilation are controlled by tiny muscles (sphincter muscles) in the arteriole walls.

• In cold conditions, the body's metabolism speeds up to generate more heat.

• Shivering occurs as muscles contract and relax to generate more heat.

• Sweating, vasodilation, vasoconstriction, hair erection, shivering, behavioral actions, and changes to metabolism regulate body temperature.

• A body temperature difference greater than a few tenths of a degree of 37°C may indicate illness.

• A temperature of 39°C might be due to an illness.

Test your knowledge of homeostasis, waste removal, and the functions of the kidneys. This quiz covers the regulation of internal conditions, urine production, protein breakdown, and water balance in the human body. Questions also address the composition of tissue fluid and urine.