Renal physiology focuses on how the kidneys function to maintain homeostasis in the body. The kidneys filter blood to remove waste products and excess substances, regulate fluid and electrolyte balance, and maintain acid-base balance. Key processes include glomerular filtration, tubular reabsorption, and tubular secretion.
Table of Contents
Structure of Kidney:
A frontal section through the kidney reveals an outer region called Renal cortex and an inner region called the Renal medulla.
Renal Hilum: Renal hilum is the entry and exit site or point where the renal artery, renal vein, nerves and ureter are connected.
Renal Pelvis: Emerging from the hilum, a funnel-shaped structure is the renal pelvis. collects urine from the renal calyces (major & minor calyce) and channels it into the ureter.
Renal Column: The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of medulla- renal pyramids and renal papillae.
Renal Pyramid: A renal pyramid is a cone-shaped tissue in the kidney, which is responsible for urine formation, and channels urine into the renal pelvis.
Renal Papillae: The renal papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion.
Kidney lobe: The renal pyramid and renal column together constitute the kidney lobe.
Urine Formation
Kidneys excrete the unwanted substances or metabolic wastes through urine. Normally about 1 to 1.5 L of urine is formed every day.
The mechanism of urine formation includes 3 process:-
(I) Glomerular filtration (Il) Tubular Reabsorption and (Ill) Tubular Secretion
(I) Glomerular filtration:
Glomerular filtration is the process by which the blood passing through glomerular capillaries is filtered through filtration membranes. It is the first process of urine formation.
Filtration membrane formed by three layers-
1. Glomerular Capillary Membrane: It is formed by a single layer of endothelial cells which has many pores called fenestra or filtration pores.
2. Basement membrane: Separates the endothelium of glomerular capillary and visceral layer of Bowman’s capsule.
3. Visceral Layer of Bowman’s capsule: It is composed of a single layer of flattened epithelial cells connected with cytoplasmic extensions called pedicles or feet. The pedicles are arranged in an interdigitating manner leaving small clefts or spaces called slit pore. Filtration takes place through these slit pores. The epithelial cells with pedicles are called podocytes.
Process of glomerular filtration : Blood passes through glomerular capillaries, the plasma filtered & into Bowman’s capsule except plasma proteins.
Glomerular Filtration Rate (GFR):
- GFR is defined as the amount of filtrate formed in all nephrons of both kidneys in the given unit time.
- Normal GFR is – 125 ml/min or 180 L/day.
Pressure determining filtration:
- Glomerular capillary pressure: Pressure exerted by the blood in glomerular capillaries about 60 mm Hg.
- Colloidal Osmotic Pressure: Pressure exerted by the plasma proteins in glomeruli. These proteins develop colloidal, osmotic pressure. which is about 25 mm Hg.
- Hydrostatic Pressure in Bowman’s Capsule : Pressure exerted by filtrate in Bowman’s capsule, also called capsular pressure about 15 mm Hg.
Net Filtration Pressure (NFP): Net filtration pressure is the balance between pressure favouring filtration and measures.
Factors Regulating GFR:
- Renal Blood flow
- Tubuloglomerular feed back
(ll) Tubular Reabsorption:
Tubular reabsorption is the process by which water and other substances are reabsorbed by renal tubules and back to the blood. When glomerular filtrate flows through the tubular portion (PCT, Henlee’s loop, DCT and Collecting tubule) of nephron, a large quantity of water, electrolytes and other substances are reabsorbed.
Selective Reabsorption: Tubular reabsorption is known as selective reabsorption because the tubular cells reabsorb only the substances necessary for the body. Essential substances- Glucose, amino acids, vitamins, water.
Reabsorption through parts of Tubule of Nephron.
1. Proximal Convoluted Tubule: Glucose, amino acid, vitamin, water (65%), Na+, Mg2+, Ca2+, H+, PO3–, hormone etc.
2. Henlee’s Loop:
- Descending part :- Maximum H₂O
- Ascending part :- Na+ (maximum), Cl–, K+
3. Distal Convoluted Tubule (DCT):
4. Collecting Tubules: Na+, H₂O, Urea, Cl–, HCO3–.
(lll) Tubular Secretion:
Tubular secretion is the process by which the unwanted substances are transported from blood to renal tubules. (Tubular excretion)
- PCT: Urea, organic acids, creatine, H+, NH4+
- Henlee’s Loop:
- DCT: H+, K+, NH4+
- Collecting Tubule: H+, K+,NH4
Micturition:
- Micturition is a process by which urine is voided/empty out from the urinary bladder.
- Reflex process. Controlled voluntarily to some extent.
- Functional anatomy and nerve supply of the urinary bladder is essential for the process of micturition.
Nerve supply to Urinary bladder:
Urinary bladder and the internal sphincters are supplied by Sympathetic and Parasympathetic divisions of ANS, whereas the external sphincter is supplied by Somatic nerve fibres.
- Sympathetic Nerve: Relaxation of Detrusor muscle ; Constriction of Internal sphincter.
- Parasympathetic Nerve: Contraction of Detrusor muscle ; Relaxation of internal sphincter.
- Somatic Nerve: Constriction of external sphincter.