Comparative Animal Anatomy and Physiology

Mammalian physiology and anatomy

Mammalian physiology and anatomy covers the organ systems of mammals, the structures that make up each system, and the specific cell types that carry out each function. Exam questions usually ask you to link a structure to its location, a cell type to its tissue of origin, or an organ to its function, so learn each structure together with where it lies and what it does. The organ of Corti is the sensory receptor for hearing. It lies on the basilar membrane inside the cochlea of the inner ear, and its hair cells convert sound-induced vibration into nerve impulses. It should not be confused with the olfactory epithelium of the nasal cavity, which detects smell, or with any sensory structure of the heart or kidney.

Skeletal muscle growth and repair depend on satellite cells. Mature muscle fibres are multinucleated and do not divide, so when athletic training enlarges a muscle or an injury heals, the work is done by satellite cells that lie between the sarcolemma and the basal lamina. When activated, these stem-like cells proliferate, differentiate into myoblasts, and fuse with existing fibres. Myelin sheaths are made by Schwann cells and oligodendrocytes, gastric oxyntic cells secrete hydrochloric acid, and sponge choanocytes drive water currents, so none of them contributes to muscle regeneration.

Lung volumes are defined by the spirometric measurements of breathing. Tidal volume (TV) is the air moved in a normal breath, inspiratory reserve volume (IRV) is the extra air that can be inhaled after a normal breath, and expiratory reserve volume (ERV) is the extra air that can be exhaled after a normal breath. Vital capacity is the sum of these three, so VC = TV + IRV + ERV. If VC is 4000 mL, TV is 500 mL and IRV is 2500 mL, then ERV is 4000 - 500 - 2500 = 1000 mL.

The cell types that line or form the tissues of different organs are a common point of confusion. Microglia are the resident phagocytic cells of the brain, Leydig cells in the testis secrete testosterone, retinal ON bipolar cells are neurons of the visual pathway in the eye, and pneumocytes line the alveoli of the lung, where gas exchange occurs. Each cell type should be learnt with its tissue and its main role, because the same role (for example, phagocytosis or hormone secretion) can be carried out by cells in several organs.

Hormones are classified by their source gland and their target tissue. Epinephrine is released by the chromaffin cells of the adrenal medulla and acts on the heart to increase its rate and force of contraction. Prolactin is made by the anterior pituitary and acts on the mammary glands in mammals and on the crop in pigeons, where it stimulates the production of crop milk. Calcitonin is secreted by the thyroid and lowers blood calcium by acting on bone. Thyrotropin-releasing hormone is produced by the hypothalamus and acts on the anterior pituitary.

Sweat glands are exocrine glands in the dermis that secrete sweat onto the skin surface. When sweat evaporates, it removes heat from the body, so the sweat glands are the main effectors of human thermoregulation. This function must be kept apart from other skin structures: touch is detected by mechanoreceptors such as Meissner and Pacinian corpuscles, skin colour depends on melanin produced by melanocytes, and fat is stored in adipocytes of the subcutaneous tissue.

Glucose handling by the kidney is a classic physiology topic. Glucose is filtered freely at the glomerulus and then reabsorbed almost completely in the proximal convoluted tubule by sodium-glucose cotransporters. Glucose appears in urine (glucosuria) only when plasma glucose exceeds the renal threshold, which is about 180 mg/dL, because the tubular transport capacity is then saturated. At a blood glucose of 100 mg/dL in a healthy person, urinary glucose is essentially absent, that is, below 1 mg/dL.

Endocrine hormone classes and glands

Hormones are grouped chemically into peptides and proteins, glycoproteins, steroids, and amines derived from amino acids. Knowing the chemical class of a hormone helps you predict where it is made, how it acts, and whether it can cross the cell membrane. Peptide hormones, such as glucagon and insulin, are water-soluble and bind to surface receptors. Glucagon is a peptide produced by the alpha cells of the pancreatic islets, and it raises blood glucose by stimulating glycogenolysis in the liver.

Glycoproteins are proteins with covalently attached carbohydrate chains. Follicle-stimulating hormone (FSH) is a glycoprotein made by the gonadotrophs of the anterior pituitary, and it is the same class as LH and TSH. Do not confuse glycoprotein with peptide: the carbohydrate side chains are the distinguishing feature.

Steroid hormones are lipid-soluble and are synthesised from cholesterol. Pregnenolone is the first steroid in the synthetic pathway and is produced in the adrenal cortex, as well as in the gonads, from cholesterol by the side-chain cleavage enzyme. Pregnenolone is a precursor for other steroids such as cortisol, aldosterone and sex steroids.

Melatonin is an indoleamine derived from the amino acid tryptophan through serotonin. It is secreted by the pineal gland and follows a day-night rhythm, being high in darkness. Thyroid hormones and catecholamines are also amine-type hormones, but they are derived from tyrosine.

A simple way to answer matching questions is to check the chemical class first, then the gland. For example, a tryptophan derivative points to the pineal gland, a steroid precursor from the adrenal cortex, and a pancreatic islet hormone that is a peptide points to the alpha cells.

Respiratory lung volumes

Lung volumes are the basic spirometric measurements of breathing, and lung capacities are sums of two or more volumes. The four primary volumes are tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV) and residual volume (RV). TV is the air inhaled and exhaled in a quiet breath, IRV is the extra air inhaled beyond TV, ERV is the extra air exhaled after a normal expiration, and RV is the air left in the lungs after maximal expiration. RV cannot be measured by simple spirometry, which is why it is treated separately from the other volumes.