Glossary of Biology - Homeostasis

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What is homeostasis?
It is the process by which a stable internal environment within an organism is maintained
What are some important homeostatic mechanisms?
The maintenance of a water and solute balance, osmoregulation, the removal of metabolic waste products, excretion, the regulation of blood glucose levels, and the maintenance of a constant internal body temperature, thermoregulation
What are the primary homeostatic organs in humans?
The kidneys, liver, the large intestine, and the skin
What are kidneys?
They regulate the concentration of salt and water in the blood through the formation and excretion of urine.
What are they shaped like?
Where are they located?
Behind the stomach and liver
How many nephrons are in each kidney?
One million units
What are the three regions of the kidney?
The cortex, the medulla, and the pelvis
Where does blood enter the kidney?
Through the renal artery
What does the renal artery divide into?
Many afferent arterioles that run through the medulla and into the cortex
What does each afferent arteriole branch into?
A convoluted network of capillaries called a glomerulus
What do the capillaries converge into?
They converge into an efferent arteriole, as opposed to converging directly into a vein
What does the efferent arteriole divide into?
It divides into a fine capillary network known as the vasa recta
What does the vasa recta do?
It enmeshes the nephron tubule and then converges into the renal vein
What is this arrangement of tandem capillary beds known as?
A portal system
What does a nephron consist of?
A bulb called Bowman’s capsule
What does it do?
It embraces a glomerulus and leads into a long coiled tubule that is divided into five units
What are those five units?
The proximal convoluted tubule, the descending limb of the loop of Henle, and ascending limb of the loop of Henle, the distal convoluted tubule, and the collecting duct
What is important about how the nephron are positioned?
They are positions so that the loop of Henle runs through the medulla, while the convoluted tubules and Bowman’s capsule are in the cortex
What are the three processes that regulate salt and water balance in blood?
Filtration, secretion, and reabsorption
What happens during filtration?
Blood pressure forces 20 percent of the blood plasma entering the glomerulus into the surrounding Bowman’s capsule
What are the fluid and small solutes entering the nephron called?
The filtrate
How is the filtrate compared to the blood plasma?
It is isotonic with blood plasma
What does this mean?
Their metabolism, heart rate, and respiration rate greatly decrease during these months
What does the nephron secrete?
The nephron secretes substances such as acids, bases, and ions from the interstitial fluid into the filtrate by both passive and active transport
What does secretion maintain?
It maintains blood pH, potassium concentration in the blood, and nitrogenous waste concentration in the filtrate
How does reabsorption work?
Essential substances such as glucose, salts, and amino acids and water are reabsorbed from the filtrate and returned to the blood
What does this result in?
It results in the formation of concentrated urine, which is hypertonic to the blood
How do the nephron maintain the bloodstream’s solute concentration?
Through the selective permeability of its walls and the maintenance of an osmolarity gradient, the nephron reabsorbs nutrients, salts, and water from the filtrate and returns them to the body. This maintains the bloodstream’s solute concentration
Are the walls of the proximal tubule and the descending limb of the loop of Henle permeable to water?
Yes they are
What about the walls of the lower ascending limb?
They are only permeable to salt
What about when ADH is around?
The walls of the collecting duct are permeable to water and urea, but only slightly permeable to salt
How is an osmolarity gradient established?
The selective permeability of the tubules establishes one in the surrounding interstitial fluid
How do solutes create an osmolarity gradient?
By exiting then reentering at different segments of the nephron
What does this do to tissue osmolarity?
It increases from the cortex to the inner medulla
What are the solutes that contribute to the maintenance of the gradient?
Urea and salt (Na+ and Cl-)
Where does urea go?
It diffuses out of the collecting duct and eventually reenters the nephron by diffusing into the ascending limb
Where is salt cycled?
It is cycled between the two limbs of the loop of Henle
Where do Na+ and Cl- go?
They diffuse out of the lower half of the ascending limb, while the upper half actively pumps out Na+ (Cl- passively follows)
What does the combination of passive diffusion and active transport of solutes accomplish?
It maximizes water conservation and the excretion of urine hypertonic to the blood
How does filtrate flow?
Filtrate enters the Bowman’s capsule and flows into the proximal convoluted tube
What happens there?
There is an increasing ionic concentration in the interstitial fluid, which causes more water to diffuse out of the nephron
What happens to the majority, 60-70%, of the Na+ in the filtrate?
It is reabsorbed by both active and passive mechanisms
What happens to water and Cl-?
They both passively follow
Where does the filtrate then flow?
It flows down the descending limb into the renal medulla
Where does the filtrate flow then?
Through the ascending limb, which is impermeable to water, and then into the distal convoluted tube
Where does the filtrate go from there?
It continues through the collecting duct, where water reabsorption is under hormonal control
What hormone controls its reabsorption?
What happens to the remaining filtrate, urine?
It is hypertonic to the blood and highly concentrated in urea and other solutes
What are the two hormones that regulate water reabsorption?
Aldosterone and ADH
Where is aldosterone produced?
It is produced in the adrenal cortex
What does it stimulate?
It stimulates the reabsorption of Na+ from the collecting duct, and the secretion of K+
What does Na+ reabsorption do?
It increases water reabsorption, leading to a rise in blood volume, and hence a rise in blood pressure
What happens in a person suffering from Addison’s disease?
Aldosterone is produced insufficiently or not at all
What does this cause?
It causes excess excretion of dilute urine and dehydration
What is ADH also known as?
Where is it formed?
It is formed in the hypothalamus
Where is it stored?
In the posterior pituitary
What is it?
It is an antidiuretic, it causes increased water reabsorption
Where does it work?
It acts directly on the collecting duct, increasing its permeability to water
What is the amount of ADH produced dependent on?
The plasma osmolarity
What does a high solute concentration in the blood cause?
Increased ADH secretion
What does a low solute concentration in the blood cause?
A reduced ADH secretion
What do Alcohol and caffeine do to ADH secretion?
They inhibit ADH secretion
What happens by the time the filtrate exits the nephron?
Most of the water has been reabsorbed
What does the remaining fluid, composed of urea, uric acid, and other wastes do next?
The remaining fluid leaves the collecting tubule and exits the kidney via the ureter
What is the ureter?
It is a duct leading to the bladder
What happens in the bladder?
Urine is stored there until it is excreted from the body through the urethra
What happens concerning the nephron and glucose in a healthy individual?
The nephron reabsorbs all of the glucose entering it, producing glucose-free urine
What happens in the urine of a diabetic?
It is not glucose-free
What does the high glucose concentration in a diabetic do to the nephron’s active transport system?
It overwhelms it, leading to the excretion of glucose in the urine
What does the liver do?
It helps regulate blood glucose levels and produces urea
What happens to glucose and other monosaccharides absorbed during digestion?
They are delivered to the liver via the hepatic portal vein
What happens to glucose-rich blood in the liver?
It is processed, which converts excess glucose to glycogen for storage
What happens if the blood has a low glucose concentration?
The liver will convert glycogen into glucose and release it into the blood
What does this accomplish?
It restores blood glucose levels to normal
How does the liver synthesize glucose from non-carbohydrate precursors?
Via the process of gluconeogenesis
What controls glucose metabolism?
Hormones and the nervous system
What else is the liver responsible for processing?
Nitrogenous wastes
What happens to excess amino acids?
They are absorbed in the small intestine and transported to the liver via the hepatic portal vein
What happens to amino acids in the liver?
They undergo a process called deamination
What is deamination?
It is when the amino group is removed from the amino acid and converted into ammonia, a highly toxic compound
What happens to the ammonia?
The liver combines the ammonia with carbon dioxide to form urea, a relatively nontoxic compound
What happens to the urea?
It is released into the blood and eventually excreted by the kidneys
What else is the liver responsible for?
Detoxification of toxins, storage of iron and vitamin B12, destruction of old erythrocytes, synthesis of bile, synthesis of various blood proteins, defense against various antigens, beta-oxidation of fatty acids to ketones, and interconversion of carbohydrates, fats, and amino acids
What does the large intestine do?
It absorbs water and sodium not previously absorbed in the small intestine
What else does it function as?
An excretory organ for excess salts
What happens to excess calcium, iron, and other salts?
They are excreted into the colon and them eliminated with feces
What is the largest organ of the body?
The skin
How much of a person’s body weight does skin comprise?
16% of total body weight
What are the two major layers of skin?
The epidermis and the dermis
What lies beneath the dermis?
Subcutaneous tissues, sometimes called the hypodermis
What is the epidermis?
The outermost epithelial layer
What are the five cellular layers of the epidermis?
The stratum basalis or stratum germinativum, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum
What is the deepest layer?
The stratus basalis
What does it continually do?
It continually proliferates, pushing older epidermal cells outward
What happens as the older cells reach the outermost layer of the stratum corneum?
They die, lose their nuclei, and transform into squames of keratin
What happens to the keratinized cells?
They are tightly packed, serving as a protective barrier against microbial attach
What projects above the surface of the epithelium?
What type of pores open to the surface?
What are the two layers of the dermis called?
The layer of loose connective tissue is called the papillary layer, and the layer of dense connective tissue is known as the reticular layer
What else is within the dermis?
The sweat glands, the sense organs, blood vessels, and the bulbs of hair follicles
What is the hypodermis composed of?
It is composed of loose connective tissue
What is the hypodermis abundant in?
Fat cells
What does it bind?
It binds the outer skin layers to the body
What is the function of the skin?
The skin protects the body from microbial invasion and from environmental stresses such as dry weather and wind
What do specialized epidermal cells called melanocytes do?
They synthesize the pigment melanin
What does melanin do?
It protects the body from ultraviolet light
What types of stimuli is the skin a receptor of?
Pressure and temperature
What does the skin excrete?
Excess water and salts from the body
How is the skin a thermoregulatory organ?
It helps control both the conservation and release of heat
What do sweat glands do?
They secrete a mixture of water, dissolved salts, and urea via sweat pores
What happens as sweat evaporates?
The skin is cooled
What does this imply?
Sweating has both an excretory and a thermoregulatory function
What type of control is sweating under?
What does subcutaneous fat in the hypodermis do?
It insulates the body
What does hair do?
It entraps and retains warm air at the skin’s surface
How is epinephrine involved?
It an increase the metabolic rate, thereby increasing heat production
How can muscles generate heat?
They can contract rapidly
How can heat loss be inhibited?
Through the constriction of blood vessels in the dermis
What does dilation of these same blood vessels do?
It dissipates heat
What are some alternative mechanisms used by some mammals to regulate their body temperature?
Panting is a cooling mechanism that evaporates water from the respiratory passages
Most mammals have a layer of fur, what does fur do?
It traps and conserves heat
What do some mammals exhibit varying states of in the winter months in order to conserve energy?
What is hibernation?
It is a type of torpor during which the animal remains dormant over a period of weeks or months with body temperature maintained below normal
What are animals with a constant body temperature referred to as?
Homeotherms or endotherms

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