VIII. Animal Physiology

Key focus of this chapter: digestion hormones

This chapter reviews respiration, circulation, digestion, osmoregulation, excretion, endocrine signaling, and nutrients.

A. Respiration

: process of gas exchange in the body.

Internal respiration

– gas exchange between systemic capillary blood and body tissues.

– Cellular respiration is a separate process that produces ATP; aerobic respiration uses oxygen as the final electron acceptor.

External respiration

– gas exchange between alveoli and pulmonary capillary blood.

– exchange of O₂ and CO₂ between lung and blood of the pulmonary capillaries.

– Ventilation moves air into and out of the lungs; circulation transports gases between lungs and tissues.

– Systemic capillary–tissue gas exchange is internal respiration.

Figure of external respiration

1. Respiratory tract

a. Air Passage

b. Function of each part

Classification	Features and roles
Pharynx	• Muscular tube for passageway of digestion and respiration
Larynx	• Cartilage connective tissue for protection of the trachea • Sound generating part • Located between the pharynx and trachea, anterior to the esophagus
Epiglottis	• Flap tissue to prevent food from entering the respiratory tract
Trachea	• Rigid structures composed of C-shaped ring of cartilage to keep air flow • Wind pipe
Bronchi	• Lead air to each lung
Bronchioles	• Branches of bronchi • Small and finer tubes
Alveoli	• Air sac for gas exchange between lung and capillary

2. Inspiration and expiration

a. Inspiration

The diaphragm moves downward and contracts
Thoracic volume increases and alveolar pressure falls below atmospheric pressure
Air moves into alveoli

b. Expiration

During quiet expiration, the diaphragm relaxes and moves upward
Elastic recoil reduces lung volume and raises alveolar pressure above atmospheric pressure
Air moves out from alveoli

B. Cardiovascular System

1. Animal circulatory systems

Animal	Number of Chambers of heart	Number of Circuits
Fish	Two	One
Amphibians and Reptiles (Lizards, turtles, snakes)	Three	Two
Birds and mammals	Four	Two

2. Heart of mammal

a. The pathway of circulation

b. Anatomy of heart

Classification		Features and roles
Atrioventricular valves		• Keep blood moving from atrium and to ventricle • Prevent backflow into atrium • Flap composed of connective tissue
Semilunar valves		• Keep blood moving from ventricle and to arteries (aorta, pulmonary artery) • Prevent backflow into ventricle • Flap composed of connective tissue
Intercalated disks		• Contain gap junctions and mechanical junctions between cardiac muscle cells • Allow electrical coupling and mechanical attachment between neighboring cardiac muscle cells
Atrium		• Chamber for receiving blood
Ventricle		• Chamber for pumping blood out
Conduction system	Sino-atrial node (SA node)	• Functions as pacemaker • Located in the wall of the right atrium • Initiates the heartbeat; conduction pathways coordinate atrial followed by ventricular contraction
	Atrioventricular node (AV node)	• Located in the wall between right atrium and right ventricle • Delays conduction to allow ventricular filling before ventricular contraction

c. Cardiac pressure

Systole – phase of ventricular contraction
Diastole – phase of ventricular relaxation
Cardiac output

– volume of blood pumped by a ventricle per minute; the left ventricle ejects into the aorta

– stroke volume × heart rate

Stroke volume

– volume of blood ejected by a ventricle per beat

3. Blood vessels

a. Blood circulation in the blood vessels

b. Functions of each part

Blood vessels	Features and functions
Artery	• Thick muscular wall and large vessel • Transports blood from heart to arteriole • Highest pressure
Arteriole	• Small vessel • Transports blood from artery to capillary • Branched from artery • Determines the distribution of blood
Capillary	• Thin and smallest vessels with only endothelium tissues • Highest area and lowest blood velocity • Transports blood from arteriole to each body tissue • Exchanges gases, nutrients, and wastes to body cells
Venule	• Thin and small vessel • Collects blood from capillary • The movements of skeletal muscles help return blood from capillary to heart in venule and vein
Vein	• Large vessel with low blood pressure • Transports blood from venules to heart • Has valves to prevent back flow

** Portal venous system

Transports blood from the capillaries to other capillaries via veins
Eg/ hepatic portal vein

c. Blood pressure

Main force for the circulatory blood movement

d. Starling forces

Driving forces for the fluid movement into and out of capillaries
Capillary hydrostatic pressure and capillary oncotic pressure

Starling forces	Features and functions
Capillary hydrostatic pressure	• Driving force of fluid from capillary to interstitial fluid. • Net fluid movement depends on opposing hydrostatic and oncotic pressures and capillary permeability.
Capillary oncotic pressure (Osmotic pressure)	• Driving force of fluid from interstitial fluid into capillary. • Proteins of blood plasma in capillary pull water from interstitial fluid. • The classical model predicts venous-end reabsorption; in many tissues at steady state, net filtration persists and lymphatics return the filtered fluid.

4. Blood

a. Cellular elements (45 % of blood)

Cell types		Features and functions
Erythrocytes (Red blood cells)		• Have hemoglobin to transport oxygen and carbon dioxide • Mature mammalian red blood cells lack nuclei and mitochondria
Leukocytes (White blood cells) - cells of immune system - have nuclei - larger than red blood cells - can move into the lymphatic system from the blood vessels	Basophils	• Secrete chemicals (histamine and anticoagulant heparin) to allergic reactions or damaged tissues
	Eosinophils	• Minor phagocytic activity • Release enzymes to kill parasites • Sometimes discharges toxins from enzyme damage to tissues
	Lymphocytes	• Helper T cells – secrete cytokines to activate B cells and cytotoxic T cells • B cells – form plasma cells to secrete antibodies • Cytotoxic T cells – secrete molecules (perforin) to kill infected or tumor cells • Natural killer cells (Null cells) - release factors to kill infected or tumor cells
	Monocytes	• Become macrophages for phagocytic activity
	Neutrophils	• Phagocytosis of microorganism or foreign particles
Platelets		• Hemostasis - process of blood clotting • Lack nuclei

b. Plasma elements (55 % of blood)

Classification		Features and functions
Ion		• Regulation of osmotic balance, controlling of membrane permeability, pH buffering • Eg/ Bicarbonate, chloride, sodium, potassium, calcium, magnesium
Plasma proteins	Albumin	• Regulation of osmotic balance and pH buffering
	Fibrinogen	• Formation of blood clotting
	Immunoglobulins	• Antibodies for immune response
Water		• Solvent for transporting other materials
Other substances		• Nutrients, hormones, waste products

c. Transport oxygen

Hemoglobin

– consists of a tetrameric protein in red blood cells.

– an allosteric protein.

– each subunit protein has a heme group, which contains iron.

– heme group binds O₂to transport.

– binding affinity of CO is greater than O₂

** Myoglobin

– bind O₂to store in muscles.

– similar structure with hemoglobin but with monomer shape.

Oxygen – binding curve of hemoglobin

– increased O₂ binding affinity means increased percent O₂ saturation at the same partial pressure of O₂

– cooperativity with oxygen binding to hemoglobin.

– Bohr effect means that O₂ binding affinity of hemoglobin decreases with low pH (high CO₂).

d. Transport carbon dioxide

Most CO₂ (70%) from respiring cells is transported to lungs as bicarbonate ion (HCO₃^–) mainly in plasma after bicarbonate is formed in red blood cells
Small amount of CO₂ (23%) is bound to hemoglobin and transported
The rest of CO₂(7%) is dissolved in plasma
Blood pH is buffered in part by the carbonic acid–bicarbonate system (HCO₃^–)
Exchanging O₂ and CO₂ by passive diffusion

e. States of red blood cells

f. Blood clotting

① The coagulation cascade, involving clotting factors, Ca²⁺, and platelet surfaces, converts prothrombin to thrombin. Vitamin K supports hepatic synthesis of several clotting factors.
② Thrombin enzyme converts an inactive fibrinogen to fibrin.
③ Fibrin forms framework of clot at damaged tissue.

Scientific schematic — Correction to the simplified sequence: red blood cells are produced in red bone marrow and circulate for about 120 days. Aged cells are removed by macrophages mainly in the spleen and liver; destruction is not confined to the liver.

C. Digestion

1. Mouth

Mechanical breakdown of food by teeth.
Chemical breakdown by salivary amylase, which breaks starch into maltose and shorter carbohydrate fragments.
Moistening of food to form a bolus.

2. Esophagus

Conducts food from mouth to stomach by waves of muscular contraction called peristalsis.
Controls the passage of food by muscular ring valves called sphincters.

3. Stomach

Functions for storage and digestion.
Mechanical breakdown by churning movements.
Gastric glands secrete mucus from mucous cells, pepsinogen from chief cells, and hydrochloric acid (HCl) from parietal cells.
Pepsin, which is converted from pepsinogen by HCl, digests proteins.

Food mixed with gastric juice becomes acid chyme.
Mucus lubricates and protects the surface of stomach walls.
Cardiac orifice (Gastroesophageal sphincter) regulates back flow of food from stomach to esophagus.
Pyloric sphincter controls the passage of acid chyme from stomach to small intestine.

4. Small intestine

: most chemical digestion for all nutrients occurs in small intestine, which is composed of duodenum, jejunum, and ileum.

a. Secretion to duodenum

Pancreas – secretes pancreatic juice, which contains the digestive enzymes and bicarbonate for neutralizing the acid chyme, to duodenum.
Liver – secretes bile, which contains bile salts that emulsify fats and aid their digestion, to duodenum.
Epithelium of small intestine – secretes digestive enzyme.

b. Absorption

Villi – highly folded mucosal projection to increase surface area for absorption.
Microvilli – microscopic projection on individual cells.
Long-chain fatty acids and monoglycerides enter intestinal epithelial cells, are reassembled into triglycerides, and leave in chylomicrons that enter lacteals. Short-chain fatty acids enter portal blood directly.
Glucose and amino acid are absorbed by blood capillary and carried to liver via hepatic portal vessel.

5. Liver

a. Secretion for digestion

Bile production and storage in gallbladder.
Bile contains bile salts and bicarbonate for helping lipid digestion as a function of the emulsifying agent but does not contain enzyme.

b. Multiple functions

Destruction of aged red blood cells.
Glycogen synthesis, breakdown, and storage.
Synthesis of plasma proteins such as albumin, clotting proteins, and angiotensinogen.
Conversion of ammonia to urea for excretion.
Elimination of waste or bacteria from blood.

6. Pancreas

a. Exocrine (secretion of pancreatic juice into the duodenum)

Secrete amylase to digest carbohydrate.
Secretes trypsinogen, which is activated to trypsin in the small intestine to digest proteins.
Secrete lipase to digest fat.
Secrete bicarbonate ions to neutralize the acid chyme.

b. Endocrine (secretion of hormones into the bloodstream)

α cells secrete glucagon to convert glycogen to glucose.
β cells secrete insulin to convert glucose to glycogen.

7. Large intestine (Colon)

Composed of four major parts, ascending colon, transverse colon, descending colon, and sigmoid.
Absorption of water, ions, and other minerals.
Feces transformation and storage of feces.
Absorption of vitamin K produced by resident gut bacteria.

8. Overview of digestion

9. Hormonal control of digestion

D. Osmoregulation and excretion

Osmoregulation – regulates the concentration of water and solute.
Excretion – eliminates the nitrogen containing waste product from metabolism.

1. Animal osmoregulation and excretion

Classification of animal		Osmoregulation and excretion
Flatworms		• Protonephridia with flame cells
Earthworm		• Metanephridia
Insect		• Malpighian tubules
Bony fish	Freshwater fish	• Excreting a large amount of water by kidneys for water balance in the hypotonic environment
	Saltwater fish	• Marine bony fish drink seawater and excrete excess salts mainly through gill ionocytes; kidneys also eliminate some ions
Bird		• Marine birds use specialized salt glands to excrete excess salt; kidney function also contributes to water balance

2. Mammal osmoregulation and excretion

a. Basic renal processes of mammal kidney

Filtration – hydrostatic pressure drives water and small solutes from glomerular blood into Bowman’s capsule; blood cells and most plasma proteins are retained.
Secretion – transport toxins and excess salts from body fluids (peritubular capillaries) to renal tubules by active transport.
Reabsorption – transport filtrated substances from renal tubules to body fluids (peritubular capillaries) by active or passive transports.
Excretion – elimination of urine from the body; excreted amount = filtered amount − reabsorbed amount + secreted amount.

b. Function of each part

Classification of kidney process		Features and functions
Filtration (pressure-driven)	Glomerulus	• Ball shaped capillary tubes • Glucose in protein-free plasma is filtered completely
Filtration (pressure-driven)	Bowman’s capsule	• Cup shaped renal tubes • Surround glomerulus • Filter water glucose and small solutes (sodium and potassium ions, amino acids, urea) • Cannot filter large molecules such as blood cells or plasma proteins
Secretion (active transport) and Reabsorption ( active or passive transport)	Proximal tubule	• Secretion and reabsorption of salt, nutrients (glucose and amino acid), and water • Most of the reabsorption of water and salt occurs • pH regulation
	Distal tubule	• Secretion and reabsorption of salt and water • Control K+ and NaCl • pH regulation • Aldosterone increases Na⁺ reabsorption and K⁺ secretion; water follows osmotically when the epithelium is water-permeable
Reabsorption ( active or passive transport)	Descending loop of Henle	• Reabsorption of water
Reabsorption ( active or passive transport)	Ascending loop of Henle	• Reabsorption of NaCl
Excretion	Collecting duct	• Collects tubular fluid; ADH regulates water reabsorption and urine concentration

c. Passage of waste products

E. Endocrinology

1. Hormone

: intercellular chemical signals produced by endocrine glands.

a. Hypothalamus hormones

Hypothalamic releasing and inhibiting hormones regulate the anterior pituitary through portal blood. ADH and oxytocin are synthesized in the hypothalamus and released from the posterior pituitary.

b. Steroid hormones

Lipid-soluble and poorly soluble in water.
Cross plasma membranes and bind intracellular receptors in the cytoplasm or nucleus.
Eg/ Androgens, Estrogens, Progesterone, cortisol, aldosterone

c. Peptide hormone

Hydrophilic and dissolved in water.
Cannot cross plasma membranes and bind to receptors on the surface of a target cell.
Eg/ Most hormones

d. Tropic hormones

Act on another endocrine gland to regulate its hormone secretion.
Eg/ FSH, LH, TSH, ACTH, hypothalamus hormones

e. Nontropic hormones

Eg/ Endorphin, MSH, prolactin

2. Hormones of endocrine glands

Hypothalamus Hormones	Pituitary	Hormone	Target	Functions of pituitary hormones
GHRH (stimulates GH) GHIH (inhibits GH)	Anterior Pituitary	GH (growth hormone)	Liver and bone	• Stimulates cell division and increasing cell size
TRH (stimulates TSH)		TSH (thyroid stimulating hormone)	Thyroid	• Controls homeostatic functions and cellular metabolism (digestion, heart rate, blood pressure, reproductive functions) • Stimulates production of T3 and T4
CRH (stimulates ACTH)		ACTH	Adrenal cortex	• Responds to long term stress • Primarily stimulates glucocorticoid secretion; aldosterone is mainly regulated by angiotensin II and plasma K⁺
GnRH (stimulates LH and FSH)		LH (luteinizing hormone)	Testis and ovary	• Causes the follicle to undergo ovulation • Stimulates the Leydig cells for testosterone secretion in testes
GnRH (stimulates LH and FSH)		FSH (follicle stimulating hormone)	Testis and ovary	• Stimulates ovaries for primary follicle development • Stimulates the Sertoli cells for spermatogenesis in testes
PRH (stimulation) PIH (inhibition)		Prolactin	Mammary glands	• Stimulates mammary glands for milk production and secretion
		MSH	Melanocytes	• Stimulates color changes of skin in reptiles and amphibians
	Posterior Pituitary	ADH (antidiuretic hormone, vasopressin)	Kidney tubules	• Stimulates the kidney for retention of water from renal tubules to body capillaries and increases blood pressure • Causes concentrated urine • Insufficient ADH causes Diabetes insipidus
	Posterior Pituitary	Oxytocin	Mammary glands, uterine muscles	• Initiates milk release • Induces labor

3. T3 and T4 from Thyroid Gland

: thyroid gland consists of two lobes sited on trachea and controls homeostatic functions and cellular metabolism such as digestion, heart rate, blood pressure, and reproductive functions.

Hypothalamic TRH stimulates pituitary TSH, which stimulates thyroid secretion of T3 and T4

Triiodothyronine (T3)

– has three iodine atoms.

– more powerful activation than T4.

Thyroxine (T4)

– has four iodine atoms.

– higher rate of production than T3.

– converted to T3 in kidney, liver, or target cells.

Metabolic disorders of the thyroid gland

Metabolic disorders	Causes and symptoms
Hyperthyroidism	• Excessive secretion of thyroid hormone • High rate of metabolism with high temperature, poor emotional control, high blood pressure, irritability, profuse sweating, and weight loss • Eg/ Graves’ disease (protruding eyes)
Hypothyroidism	• Insufficient secretion of thyroid hormone • Low rate of metabolism with cold intolerance, lethargy, and weight gain • Eg/ Myxedema, Cretinism in infants

4. Regulation of blood calcium levels

a. Parathyroid hormone (PTH)

Secreted by parathyroid gland when Ca²⁺ levels in blood are decreased.
Raises blood calcium by indirectly stimulating osteoclast-mediated bone resorption.
Increases renal calcium reabsorption and activates vitamin D, which increases intestinal calcium absorption.

b. Calcitonin

Secreted by thyroid gland when Ca²⁺ level in blood is increased.
Can lower blood calcium by inhibiting osteoclast-mediated bone resorption; it has a limited role in normal adult calcium homeostasis.
Can increase renal calcium excretion.

5. Regulation of blood glucose levels

a. Glucagon

Secreted by alpha cells in pancreas when glucose levels in blood are decreased.
Conversion of glycogen to glucose.
Stimulates liver to release glucose into blood stream.

b. Insulin

Secreted by beta cells in pancreas when glucose levels in blood are increased.
Lowers blood glucose by promoting glucose uptake in skeletal muscle and adipose tissue, promoting glycogen synthesis, and suppressing hepatic glucose production.
Conversion of glucose to glycogen.

** Islet of Langerhans: composed of Alpha Cells, Beta cells, Delta cells, and F cells in pancreas

6. Stress response

Regulated in part by adrenal glands located above the kidneys

a. Adrenal medulla

Stimulated from hypothalamus via spinal cord.
Secretes epinephrine and norepinephrine to response short term stress.
Raises blood glucose, heart rate, and blood pressure during the acute stress response.

b. Adrenal cortex

ACTH primarily stimulates glucocorticoid secretion; aldosterone is mainly regulated by angiotensin II and elevated plasma K⁺.
Cortisol supports longer-term stress responses; aldosterone regulates sodium, potassium, and extracellular fluid volume.
Cortisol increases glucose availability and suppresses inflammatory and immune responses.
Aldosterone increases renal Na⁺ reabsorption and K⁺ secretion, supporting extracellular fluid volume and blood pressure.
Aldosterone induces the reabsorption of water and sodium in the kidneys.

7. Other chemical messengers

a. Pheromones

Chemical substances communicated by odors outside the body.
Functions in marking territories, courtship behavior, classifying social order between same species.

b. Renin

Proteolytic enzyme that regulates arterial blood pressure.
Often called as hormone.

c. Melatonin

Amino-acid-derived hormone secreted from pineal gland in brain.
Functions in circadian rhythms.
Secreted at night.

d. Endorphin

Neuropeptide of chemical signal.
Decreases pain perception in brain.

F. Nutrients

1. Vitamin

Organic compounds
Needed in small amounts, but problems with deficiencies
Must generally be obtained from the diet in adequate amounts; synthesis varies by vitamin and species (for example, skin can synthesize vitamin D).

Vitamins	Types	Functions
Water-soluble Vitamins	Vitamin B1 (thiamine)	• Participates in cellular respiration by removing CO2 from organic materials • Contained in grains, peanuts, pork, and legumes
	Vitamin B2 (riboflavin)	• Component of coenzymes FAD and FMN • Metabolism for energy • Contained in grains, vegetable, and meats
	Vitamin B3 (niacin)	• Precursor of the coenzymes NAD⁺ and NADP⁺ • Contained in grains, nuts, and meats • Insufficient amount results in pellagra disease
	Vitamin C (L-ascorbic acid)	• Involved in collagen synthesis • Related with skin of body • Insufficient amount results in scurvy disease • Contained in fruits and vegetables
Fat-soluble Vitamins	Vitamin A (retinol)	• Precursor of retinal, the light-absorbing component of photoreceptor visual pigments • Insufficient amount results in vision problems • Contained in green vegetables or orange
	Vitamin D	• Involved in bone growth and immune system • Made by skin from sunlight • Contained in egg yolk • Insufficient amount results in rickets disease
	Vitamin E (tocopherol)	• Involved in antioxidant, which is to protect cell membrane against oxidation reaction • Deficiency can cause hemolytic anemia and neurologic dysfunction • Contained in nuts, seeds, and vegetable oils
	Vitamin K (phylloquinone)	• Important component for blood clotting • Dietary vitamin K is absorbed mainly in the small intestine; gut bacteria also produce vitamin K • Contained in green vegetables

2. Minerals

Inorganic substances
Needed in small amounts, but cause problems in large amounts
Phosphorus, magnesium, calcium – important in bone and tooth formation
Sodium and potassium – function in muscle contraction and nerve conduction
Iron – binds oxygen in hemoglobin and components of electron carriers