Kaplan MCAT Biology Ch 10 - Homeostasis (Diff. creator)

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from Kaplan MCAT Premier Program 2008-2009

Homeostasis

- a process by which a stable internal environment within an organism is maintained

Examples of Homeostasis Mechanism

a) osmoregulation
b) excretion
c) thermoregulation

Osmoregulation

- maintenance of a water and solute balance

Excretion

- the removal of metabolic waste products

Thermoregulation

- the regulation of blood glucose levels
- the maintenance of a constant internal body temperature

Primary Homeostatic Organs
(in mammals)

a) kidneys
b) liver
c) large intestine
d) skin

Kidneys

- regulate the concentration of salt and water in the blood through the formation and excretion of urine
- are bean shaped
- are located behind the stomach and liver

Nephron

- unit of kidneys

Kidney Structure

a) cortex (outer)
b) medulla (inner)
c) pelvis

Blood Movement in the Kidney
(portal system)

renal artery --> afferent arterioles --> glomerulus --> efferent arterioles --> vasa recta --> renal vein

Glomerulus

- network of capillaries

Vasa Recta

- fine capillary network

Nephron's Structure

a) Bowman's capsule
b) proximal convoluted tubule
c) descending limb of loop of Henle
d) ascending limb of loop of Henle
e) distal convoluted tubule
f) collecting duct
- loop of Henle goes through the medulla, convoluted tubules and bowman's capsules are in the cortex

Kidney Function

a) filtration
b) secretion
c) reabsorption

Filtration

- filtrate: fluid and small solutes entering the nephron, isotonic to blood plasma
- molecules too large to filter through the glomerulus (blood & albumin) remain in the circulatory system

Secretion

- nephron secretes acids, bases and ions from the interstitial fluid into the filtrate by both passive and active transport
- secretion maintains blood pH, potassium concentration in the blood and nitrogenous waste concentration in the filtrate

Reabsorption

- glucose, salts, amino acids and water are reabsorbed from the filtrate and returned to the blood
- this is to concentrate urine, which is hypertonic to the blood

Nephron Function

a) selective permeability
b) osmolarity gradient
c) flow of filtrate

- reabsorbs nutrients, salts and water from the filtrate and returns them to the body
- thus maintaining the bloodstream's solute concentration

Nephron Function:
Selective Permeability

- walls of proximal tubule and descending limb of the loop of Henle are permeable to water
- walls of the lower ascending limb are permeable only to salt
- in presence of ADH, the walls of the collecting duct are permeable to water and urea, but only slightly permeable to salt

Nephron Function:
Osmoloarity Gradient

- is established by selective permeability of the tubules
- tissue osmolarity increases from cortext to inner medulla
- solutes that contribute to the maintenance of the gradient are urea and salt
- urea diffuses out of the collecting duct and reenters through the ascending limb
- salt diffuses out of the lower half of ascending limb, but actively pumped out in the upper half

Nephron Function:
Flow of Filtrate

- all glucose, amino acids and important organic molecules are reabsorbed in the proximal convoluted tubule via active transport
- 60-70% of Na⁺ is reabsorbed (active and passively), water and Cl⁻ follow passively
- due to higher ionic concentration in medulla, water diffuse out of the nephron
- in collecting duct, water reabsorption is under hormonal control (ADH)

Hormonal Regulation:
Hormones that regulate water reabsorption

a) aldosterone
b) ADH (antidiuretic hormone)

Aldosterone

- produced by adrenal cortex
- stimulates both the reabsorption of Na⁺ from the collecting duct and the secretion of K⁺
- reabsorption of Na⁺ increases water reabsorption, increase blood volume & increase blood pressure
- secretion is regulated by the renin-angiotensin system

Addison's Disease

- aldosterone is insufficiently produced or not at all
- overexcretion of urine with high Na⁺ concentration
- cause a considerable drop in blood pressure

ADH (antidiuretic hormone)

- also known as vasopressin
- formed in the hypothalamus
- stored in the posterior pituitary
- increases water reabsorption
- acts directly on the collecting duct, increasing its permeability to water
- amount produced depends on plasma osmolarity
- high solute concentration in blood casues increased ADH secretion & vice versa
- alcohol & caffeine inhibit ADH (dilute urine + dehydration)

Aldosterone vs. ADH

- ultimately do the same thing (increase water reabsorption in the kidney)
- have different mechanisms of action
- ADH directly increases water reabsorption from the nephron's collecting duct
- aldosterone indirectly increases reabsorption by increasing sodium reabsorption from the collecting duct

Excretion from the Kidneys

- by the time filtrate exits the nephron, most of the water has been reabsorbed
- remaining fluid is composed of urea, uric acid and other waste
- leaves collecting tubule via ureter, then into the bladder and later excreted through the urethra

Excretion from the Kidneys:
for the Diabetic

- in a healthy individual, the nephron reabsorbs all the glucose entering it, producing glucose-free urine
- for a diabetic, ghigh blood glucose concentration overwhelms the nephron's active transport system, leading to the excretion of glucose in the urine

The Liver

- helps regulate blood glucose levels and produces urea
- glucose and other monosaccharides absorbed during digestion are delivered to the liver via the hepatic portal vein
- glucose-rich blood is processed by the liver
- excess glucose is converted to glycogen for storage
- reversed if blood has a low glucose concentration to restore blood glucose levels to normal
- can synthesize glucose from noncarbohydrate precursors via gluconeogenesis

The Liver continued

- glycogen metabolism is under both hormonal and nervous control
- liver is also responsible for processing nitrogenous wastes
- deamination of amino acids, amino group removed and amino acid becomes urea (highly toxic)
- liver combines ammonia with CO to form urea (nontoxic
- urea is released into the blood and eventually excreted by the kidneys

The liver is also 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
- interconversion of carbohydrates, fats and amino acids

The Large Intestine

- absorbs water and sodium not previously absorbed in the small intestine
- also functions as an excretory organ for excess salts
- excess calcium, iron and other salts are excreted into the colon and then eliminated with the feces

The Skin:
Structure

- largest organ of the body
- 16% of total body weight
- two major layers are epidermis and dermis, the underneath are the subcutaneous tissues/hypodermis

Epidermis

- the outermost epithelial layer

Five Cellular Layers of Epidermis

a) stratum basalis (stratum germinativum)
b) stratum spinosum
c) stratum granulosum
d) stratum lucidum
e) stratum corneum

- as epidermal cells get older, they are pushed outward toward stratum corneum. They loose their nuclei, die and become scales of keratin

Layers of Dermis

a) papillary layer (loose connective tissue)
b) reticular layer (dense connective tissue)

Hypodermis

- "subcutaneous tissues"
- composed of loose connective tissue
- is abundant in fat cells
- binds the outer skin layers to the body

The Skin:
Functions

- osmoregulation and thermoregulation
- protects the body from microbial invasion and from environmental stresses (dry weather & wind)
- is receptor of stimuli (pressure & temperature)
- is an excretory organ (remove excess water & salts)
- a thermoregulatory organ (control both conservation & release of heat)

Melanocytes

- specialized epidermal cells
- synthesize the pigment melanin, which protects the body from ultraviolet light

Sweat Glands

- secrete a mixture of water, dissolved salts and urea via sweat pores
- as sweat evaporates, skin is cooled
- sweating has both an excretory & a thermoregulatory function
- sweating is under nervous control

Regulating Body Temperature

- subcutaneous fat in the hypodermis insulates the body
- hair entraps and retains warm air at the skin's surface
- hormones (epinephrine) increase the metabolic rate, therefore increase heat production
- muscles can generate heat by contracting rapidly (shivering)
- heat loss can be inhibited by constricting blood vessels in the dermis (or dilation to dissipate heat)
- panting
- hibernation

Panting

- a cooling mechanism
- evaporates water from the respiratory passages

Torpor

- a state to conserve energy
- metabolism, heart rate and respiration rate greatly decrease
- ex: hibernation

Hibernation

- a type of torpor
- the animal remains dormant for a period of weeks or months with body temperature maintained below normal

Homeotherm

- or "endotherm"
- an animal that keeps their body temperature constant

Thyroid Gland

- a way for thermoregulation
- produces thyroid hormones (primarily thyroxine)
- controls the basal metabolic rate

Basal Metabolic Rate

- contributes a great deal to the warmth or coolness of the body

Hyperthyroidism

- overproduction of thyroid hormones
- the person will feel excessively warm

Hypothyroidism

- a decrease of thyroid hormones' level
- the person will feel cold

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