MCAT Biology Review

Chapter 10: Homeostasis


Two main organ systems were discussed in this chapter: the excretory system and the skin. Both of these systems play an essential role in homeostasis. However, this chapter also covered a very MCAT-worthy concept: no system works alone. Multiple systems participate in homeostasis. While the kidneys are the major players in salt, water, and acid–base balance, their function depends on the endocrine system, circulatory system, and respiratory system. While the skin is an important immune organ, its function as a thermoregulatory organ is dependent on the nervous system; it also sends sensory signals to the nervous system. As you move onto the last organ system in the next chapter—the musculoskeletal system—notice how each system interacts with other systems in order to produce a fully functioning organism. The MCAT is far more focused on how you conceptualize the big picture, and not on how many trivial details you memorize. As you study, focus on understanding these systems and how each system influences the rest of the body.

Concept Summary

The Excretory System

·        The excretory system serves many functions, including the regulation of blood pressure, blood osmolarity, acid–base balance, and removal of nitrogenous wastes.

·        The kidney produces urine, which dumps into the ureter at the renal pelvis. Urine is then collected in the bladder until it is excreted through the urethra.

·        The kidney contains a cortex and a medulla. Each kidney has a hilum, which contains a renal artery, renal vein, and ureter.

·        The kidney contains a portal system with two capillary beds in series.

o   Blood from the renal artery flows into afferent arterioles, which form glomeruli in Bowman’s capsule (the first capillary bed).

o   Blood then flows through the efferent arteriole to the vasa recta, which surround the nephron (the second capillary bed), before leaving the kidney through the renal vein.

·        The bladder has a muscular lining known as the detrusor muscle, which is under parasympathetic control. It also has two muscular sphincters.

o   The internal urethral sphincter consists of smooth muscle and is under involuntary (parasympathetic) control.

o   The external urethral sphincter consists of skeletal muscle and is under voluntary control.

·        The kidney participates in solute movement through three processes:

o   Filtration is the movement of solutes from blood to filtrate at Bowman’s capsule. The direction and rate of filtration is determined by Starling forces, which account for the hydrostatic and oncotic pressure differentials between the glomerulus and Bowman’s space.

o   Secretion is the movement of solutes from blood to filtrate anywhere other than Bowman’s capsule.

o   Reabsorption is the movement of solutes from filtrate to blood.

·        Each segment of the nephron has a specific function.

o   The proximal convoluted tubule (PCT) is the site of bulk reabsorption of glucose, amino acids, soluble vitamins, salt, and water. It is also the site of secretion for hydrogen ions, potassium ions, ammonia, and urea.

o   The descending limb of the loop of Henle is permeable to water but not salt; therefore, as the filtrate moves into the more osmotically concentrated renal medulla, water is reabsorbed from the filtrate. The vasa recta and nephron flow in opposite directions, creating acountercurrent multiplier system that allows maximal reabsorption of water.

o   The ascending limb of the loop of Henle is permeable to salt but not water; therefore, salt is reabsorbed both passively and actively. The diluting segment is in the outer medulla; because salt is actively reabsorbed in this site, the filtrate actually becomes hypotonic compared to the blood.

o   The distal convoluted tubule (DCT) is responsive to aldosterone and is a site of salt reabsorption and waste product excretion, like the PCT.

o   The collecting duct is responsive to both aldosterone and antidiuretic hormone and has variable permeability, which allows reabsorption of the right amount of water depending on the body’s needs.

·        The kidney is under hormonal control. When blood pressure (and volume) are low, two different hormonal systems are activated.

o   Aldosterone is a steroid hormone regulated by the renin–angiotensin–aldosterone system that increases sodium reabsorption in the distal convoluted tubule and collecting duct, thereby increasing water reabsorption. This results in an increased blood volume (and pressure), but no change in blood osmolarity.

o   Antidiuretic hormone (ADH or vasopressin) is a peptide hormone synthesized by the hypothalamus and released by the posterior pituitary. Its release is stimulated not only by low blood volume but also by high blood osmolarity. It increases the permeability of the collecting duct to water, increasing water reabsorption. This results in an increased blood volume (and pressure) and a decreased blood osmolarity.

·        The kidney can regulate pH by selective reabsorption or secretion of bicarbonate or hydrogen ions.


·        The skin acts as a barrier, protecting us from the elements and invasion by pathogens.

·        The skin is composed of three major layers: the hypodermis (subcutaneous layer), dermis, and epidermis.

o   The epidermis is composed of five layers: the stratum basalestratum spinosumstratum granulosumstratum lucidum, and stratum corneum. The stratum basale contains stem cells that proliferate to form keratinocytes. Keratinocyte nuclei are lost in the stratum granulosum, and many thin layers form in the stratum corneum.

o   Melanocytes produce melanin, which protects the skin from DNA damage caused by ultraviolet radiation; melanin is passed to keratinocytes.

o   Langerhans cells are special macrophages that serve as antigen-presenting cells in the skin.

o   The dermis is composed of two layers: the papillary layer and the reticular layer.

o   Many sensory cells are located in the dermis, including Merkel cells (deep pressure and texture), free nerve endings (pain), Meissner’s corpuscles (light touch), Ruffini endings (stretch), and Pacinian corpuscles (deep pressure and vibration).

o   The hypodermis contains fat and connective tissue and connects the skin to the rest of the body.

·        The skin is important for thermoregulation, or the maintenance of a constant internal temperature.

o   Cooling mechanisms include sweating, which absorbs heat from the body through evaporation of water from sweat, and vasodilation. Sweat glands are innervated by postganglionic cholinergic sympathetic neurons.

o   Warming mechanisms include piloerection, in which arrector pili muscles contract, causing hairs to stand on end (trapping a layer of warmed air around the skin), vasoconstriction, shivering, and insulation provided by fat.

·        The skin also prevents dehydration and salt loss from the body.

Answers to Concept Checks

·        10.1

1.    Bowman’s space → proximal convoluted tubule → descending limb of the loop of Henle → ascending limb of the loop of Henle → distal convoluted tubule → collecting duct → renal pelvis → ureter → bladder → urethra

2.    Renal artery → afferent arteriole → glomerulus → efferent arteriole → vasa recta → renal vein

3.    The parasympathetic nervous system causes contraction of the detrusor muscle.

4.    Filtration is the movement of solutes from blood into filtrate at Bowman’s capsule. Secretion is the movement of solutes from blood into filtrate anywhere besides Bowman’s capsule. Reabsorption is the movement of solutes from filtrate into blood.

5.    Bowman’s capsule is the site of filtration, through which water, ions, amino acids, vitamins, and glucose pass (essentially everything besides cells and proteins). The proximal convoluted tubule controls solute identity, reabsorbing vitamins, amino acids, and glucose, while secreting potassium and hydrogen ions, ammonia, and urea. The descending limb of the loop of Henle is important for water reabsorption using the medullary concentration gradient. The ascending limb of the loop of Henle is important for salt reabsorption and allowing dilution of the urine in the diluting segment. The distal convoluted tubule is also important for solute identity by reabsorbing salts while secreting potassium and hydrogen ions, ammonia, and urea. The collecting duct is important for urine concentration; its variable permeability allows water to be reabsorbed based on the needs of the body.

·        10.2

1.    Keratinocytes are the primary cells of the epidermis.

2.    Stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale

3.    The papillary layer and the reticular layer

4.    The body can cool itself through sweating and vasodilation. The body can warm itself through vasoconstriction, piloerection, and shivering.

Shared Concepts

1.    Behavioral Sciences Chapter 2

1.    Sensation and Perception

2.    Biochemistry Chapter 8

1.    Biological Membranes

3.    Biology Chapter 6

1.    The Respiratory System

4.    Biology Chapter 7

1.    The Cardiovascular System

5.    Biology Chapter 8

1.    The Immune System

6.    General Chemistry Chapter 9

1.    Solutions